C&ds Unit-i

C & DS for I MCA

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Program: A computer program is a set of instructions which are executed in a sequential order. In a C program, the instructions (statements) execute in the order in which they appear until unless the order is not specifically changed by control structures in C. History of C: C is originally developed by Dennis Ritchie at AT and T’s Bell Laboratories in 1972. C was evolved from ALGOL, BCPL and B and uses many concepts from these languages and added the concept of data types and other powerful features. One of the most popular operating systems, UNIX is developed in C. Many version of C were proposed and to assure that the C language remains standard, ANSI (American National Standards Institute) appointed a technical committee in 1983 to define a standard for C. The committee approved a version in 1989 and is referred to ANSI C. It is later approved by ISO (International Standards Organization). Features of C: The increasing popularity of C is due to its many desirable qualities: • It is a highly portable language and can be easily implemented in different operating systems. • It is robust language; with its rich set of built in functions and operators we can write any complex programs easily. • Programs written in C are efficient and fast than other languages like BASIC • C language is well suited for structured programming, in which the programmer thinks of a problem in terms of function modules or blocks. A proper collection of these modules (blocks) make a complete program. That is why a C program can termed as a collection of different functions. One of those functions is compulsory and is it the main() function. C compiler starts execution of the program by making a call to main function. • C has ability to extend itself; we can continuously as add our own functions to the C Library. Executing a C program: The following are series of steps involved: • Creating the program (using a editor) • Compiling the program • Linking the program with functions that are needed from the C library; • Executing the program We can use any text editor to create a C program. C Compiler is the software tool (or component) that is responsible for converting the high level C program into machine understandable code (machine level language). This code is called

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object code and is stored in file with the extension “.obj”. The Linker is used to link the various functions called in program. If the functions are user defined, the definition of the function should be in the range of the program. The operating system handles the execution of the programs and all input and output operations are channeled through the operating system. Commenting in C: Comments are generally written to explain the logic of the program, sometimes to specify the problem statement and sometimes to specify the details of the author of the program. These comments are purely for better understandability of the user (programmer) and do not make any sense to the C compiler. The compiler ignore the comments while generating the object code. Therefore the file with extension .obj doesn’t contain the conversions for these comments. Commenting in C can be done in 2 ways: Multi-Line Commenting (/*…..*/) – the text placed between /* and */ will be commented (ignored by compiler) and it also supports multiple lines of the commented text. Single Line Commenting (//) – when a sequence of two forward slashes appears in the program, the entire text following it in the same line will be commented. “\\” can comment only a single line of text. Algorithm and Flow Chart: Since we will be solving lot of logical problems while writing C programs, it is necessary to understand the components (namely Algorithm/ Pseudo code and Flow Chart) that can be used to represent solutions for complex problems. Algorithm/ Pseudo code: An algorithm is a method of representing the step-by-step procedure for solving a problem. An algorithm is very useful for finding the right solution for a problem by breaking the problem into simple cases. An algorithm is also called Pseudo code. For example a simple algorithm for finding a large among three numbers can be written like this. Begin 1. Read 3 numbers x,y,z from the user. 2. if x>y and x>z 3. then x is the largest 4. else if y>z and y>z 5. then y is the largest among the 3 numbers 6. else 7. Z is the largest of the three. 8. End-if End.

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Flow Chart: Flow Chart is a diagrammatic (pictorial) representation of an algorithm. It is built using different types of shapes (rectangle, oval, etc) and symbols. Different shapes are selected to represent different types of operations.
Flow chart helps to understand the program easily.
As different shapes are used to specify different operations it is easy to understand the complex programs.
In the simplest form the flow chart consists of a series of interconnected rectangle in which each rectangle specifies a specific instruction. The following are the most commonly used symbols in a flow chart Start/End Block: To indicate the start and end of the flow chart Process Block: To place individual statements those are to be processed?

Decision Block: To place decision making conditions

Output statements (displaying output)

Function calls

Connector: used to connect different paths which are merging.

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Flow chart for the above algorithm will be as follows: Start

Read values for x,y,z

Yes

x>y & x>z

X is largest. Output x

No

y>x & y>z

Yes

y is largest. Output y

No

z is largest. Output z

End

Getting started with C: Learning C language is similar to learning the English language. The classical method of learning English to learn the set of alphabets (character set in C), words (tokens) and sentences (sentences) and paragraphs (program). It is just that they are termed differently in C. Character Set: The character set of C language contains the following: Alphabets – A, B, C, …….X, Y Z a, b, c……….. x, y, z Digits – 0, 1, 2, 3…….9 Special symbols: ~ ` ! @ # % $ & * ( ) _ - + = | \ { } [ ] : ; “ ‘ < > , . ? /

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C Tokens: In a C program, the basic element recognized by the compiler is the "token." A token is source-program text that the compiler does not break down into component elements. Token are formed with the combinations of characters in character set of C. The keywords, identifiers, constants, string literals, and operators are examples of tokens. Punctuation characters such as brackets ([ ]), braces ({ }), parentheses ( ( ) ), and commas (,) are also tokens.

Constants: A constant is a quantity that doesn’t change during the execution of the program. Constants are also called literals. C supports various kinds of constants (different kind of constants for different data types). Illustration:

Integer Constants: Integer constant refers to a sequence of digits. Decimal integer consists of a set of digits, 0 through 9. Integer can negative or positive. Negative integers are indicated by using the unary operator minus (-). Eg: 867 -124 Staff: Janaki Ram N.

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Real Constants: are also called floating point constants. These can be represented in 2 forms fractional form or exponential form. In fraction form a decimal point is used. Eg: 23.456 768.45 -945.45 The exponential form is usually used for when the value of real constant is either too small or too large. The real constant is represented in 2 parts namely mantissa and exponent. Mantissa appears before ‘e’ and exponent appears after ‘e’. Range of real constants expressed in exponential form is -3.4e38 to 3.4e38. Eg: 3.2e-5 4.1e8 +467e31 Note: ANSI C supports unary plus (+) which was not supported earlier. Character Constants: A character constant is either a single alphabet or a single digit or a single special symbol enclosed within single inverted commas (apostrophes). Eg: ’A’ ’g’ ’9’ ’*’ Each character has an integer value known as ASCII (American Standard Codes for Information Interchange) code. For example the ASCII code associated with ’a’ is 97. To print the ASCII code of a character we can use the format specifier %d instead of %c while in the printf statement. String Constants: A string Constant is a sequence of characters enclosed in a pair of double quotes. The character may be alphabets, digits, special characters and blank spaces. Eg: “INDIA” “Indian Engineer” “What a pleasant day!” “?_)…-?” Backslash Character Constants (Escape Sequences): C supports special kind of characters which are formed by combining the normal characters with the escape character (\). These characters have special meaning. Each represents a single character although it has 2 characters. Some of Escape Sequences: ’\b’ ’\f’ ’\n’ ’\r’ ’\t’ ’\v’

blank space form feed new line carriage return horizontal tab vertical tab

’\’’ ’\”’ ’\?’ ’\\’ ’\0’ ’\a’

single quote Double Quote Question Mark back slash Null audible alert

Keyword and Identifiers Keywords are the word in C which has fixed meaning and these meanings cannot be changed. They serve as the building blocks of the program. The following is the list of 32 keywords in C.

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auto case const default

double enum float goto

Unit-1

int register short sizeof

struct typedef unsigned volatile

break char continue do

VVIT

else extern for if

long return signed static

switch union void while

Identifiers are user defined names formed with different combinations of alphabets, digits and underscore in character set with certain rules. Identifiers are used to name the variables, functions, arrays, etc. Rules to forming identifiers (user defined name):    

Identifiers must start with an alphabet or underscore. Identifiers cannot contain special character other than underscore. Maximum length should be 31 characters. Should not be a keyword

Variables: Variable is data name that may be used to store the data value. Unlike constants (literals) variables may have different value during the execution of program. In general a meaningful name is chosen for a variable depending upon the context on the program. Variable names are case sensitive and therefore variable AMOUNT is different from variable amount. Since the variable name is identifier, the above mentioned rules need to be followed. Eg: x, sum, Avg, Root1, Nearest_Number. Declaring of variables: Before using the variables in a program, they should be declared. Declaration tells the compiler what the variable name is and what type of data it can store. Undefined usage of variables results in error during compilation. Syntax: ; Separate statements need to be used to declare variables of different kinds. Multiple variables of same kind can be defined in a single statement by separating them with commas (separate statements are also allowed). Eg: int a,b,c; float x; char str; Initializing and assigning value to Variables: To initialize or assign a value to a variable we use the assignment operator “=”. The variable can be initialized during declaration or can be assigned a value anytime later. Eg: int a=3; int sum=0; int x; x=45; If a variable is not initialized; unpredicted values will be stored, until any value is assigned. Data Types: Data Types are used to specify what type of data can be stored, exchanged or returned by a variable or a function. C supports a variety of data types which can be classified into two groups: o Primary (Fundamental or Primitive) Data Types

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o User Defined Data Types (Derived Data Types) Primary Data Types: C compiler supports 5 fundamental data types namely int, float, double, char and void. Many compilers also support extended data types such as long int, long double, unsigned int, short int, unsigned long int, unsigned short int, unsigned char, etc. Integer types (int): Integers are the natural numbers (1, 2, 3….) including 0 and their negatives (-1, -2, -3, ….). int is the keyword used to declare variable or functions of type integer. On a 16 bit computer when an integer is declared it is allocated 2 bytes of memory. Eg: int x; The range of integers that can be stored depends on the word length of the computer. If we use a 16 bit word length, the size of the integer is limited to the range -32768 to -32767 (i.e 215 to 215-1). A signed integer uses the first bit for sign and 15 bit for the magnitude of the integer. If the

first bit is 1 then it indicated negative numbers. If it is Zero then it is positive number. A 32 bit word length computer can store integers ranging from -2147483648 to 2147483647. The range of the integers can be restricted or increased by qualifying “int” with the proper combination keywords short, long and unsigned. “short” decreases the range where as “long” and “unsigned” increases the range. The use of qualifier “signed” is optional because by default integers are signed integers. Note: To distinguish long integer literals from regular literals they are appended by a qualifier L (or l). Similarly U (or u) is appended for unsigned integers. For L and U any case can be used. Eg: 56789U (unsigned int) 987654321ul (unsigned long integer) 987.34567784787L (long double) Floating Point Data Type: Floating point numbers are real numbers which can have fractional values (decimal point). Floating Point numbers are stored with 6 digits of precision and uses 32 bits (4 bytes) of memory. The keyword used is float. “double” can be used to increase the range real numbers and also to increase the accuracy with higher precision. Double (double precision numbers) uses 64 bits of memory (8 bytes) giving a 14 digit precision. To increase the precision even more we can use long double which uses 80 bits. Eg: float avg;

double discriminant;

long double acc_value;

Character Type: A single character type can be declared using the key word “char”. Character uses 8 bits (1 byte) of memory for storage. While storing the chars the corresponding ASCII value is stored in binary format. The qualifiers signed and unsigned can be used to change the range of chars. By default char is signed char whose range is -128 to 127, where as the range of unsigned char is 0 to 255. Void Types: The void has no values. It is generally used to specify the type of the function and a function is said to be void when it does not return any value. There will not be any void variables. The size and range of different data types on a 16 bit computer are listed here:

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User-defined Data Types (Derived Data Types): These data types are created by the user (programmer), which are either derived from primitive data types or formed by different combination of the primitive data types. Derived data type can be a structure, union, enum, pointer or it can be type defined using typedef keyword. typedef usage: C allows the programmer to define a new data type and give it a user friendly name which originally represents the existing data type. In other words the we can rename a existing data type using typedef. Syntax: typedef ; Where, type can be any of the fundamental data type or a derived data type (such as structure) which is available before execution of this statement. Eg:

typedef int units; Once the above statement is executed, a new data type units is available whose behavior exactly same as int. So we can now create variable, functions, etc of type units. units a,b; units u=9; Other user defined data types will be discussed later.

Constant Variables: A variable can be made constant throughout the execution of the program by declaring it using the key word const.

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Eg: const int size=100; In the above case, the value of the variable size is always 100. It variable size can be used anywhere in the program but the value cannot be altered. Constant variable must compulsorily be initiated while they are declared. They cannot be initialized in a separate statement as we can do in case of normal variables. Reading and Displaying Data: As we have seen earlier, variables can be assigned values with the help of the assignment operator. Another way of assigning a value to variable is by using scanf function. The general format of scanf is:

scanf(“control string”, &variable1,&variable2,…);

The control string can be a format specifier or a collection of format specifiers which specify the type of the values that can be read into the variables (variable1, variable2,..etc). The ampersand symbol (&) before each variable is a pointer operator; which indicates the address of the variable. This means that the values read are stored in the address of the variable. More on this will be understood when we learn pointers. Eg: scanf(“%d”, &var1); %d is the format specifier which specifies that a integer value must be read from the user console (or terminal). To display any text (or string) or the value of any variables, we can use the printf function. General format: printf(“Simple String”); printf(“Simple String + Control String”, variable1, variable2,…); printf(“Control String”, variable1, variable2,…); Control string specifies the type of the value that is displayed from the variables. Eg: printf(“Hello World”); printf(“Value of the variable x is %d”, x); printf(“%f”, avg); The following are the various format specifier that we can used in scanf and printf functions Format specifier

Used to Represent

Comment

%d

signed decimal integer

used in both scanf and printf

%i

signed decimal integer (same

used in both scanf and printf

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as above) %c

character

%s

string (character array)

In scanf multi word strings cannot be read.

%f

Floating Point Number

By default displays the 6 precision digits when used in printf

Limits the number of number of digits after decimal point to 2 (can be any value less than 6)

Used in printf to to define the accuracy needed. For example while dealing with cost of any items, we need only two digits (that represent paise) after decimal point.

%0.2f

%u

Unsigned int

%%

Used to display the percent symbol

used only in printf not in scanf

Operators: C supports a huge variety of operators. An operator is use to ask the computer to perform certain mathematical or logical manipulations. Operators are used to manipulate data and variables. Following are the categories of operators in C:  Arithmetic Operators  Relational Operators  Logical Operators  Assignment Operator  Increment and Decrement Operators  Conditional Operators  Bitwise Operators  Special Operators Arithmetic Operators: These operators are used to perform the basic Arithmetic operations such as addition, subtraction, multiplication and division.
Unary minus operator in effect multiplies its single operand by -1 there by changing the sign of the operand.
Integer Division truncates (does not round off) any fractional part
The modulo division operator gives the reminder after performing the integer division.
Modulo Division cannot be performed on floating point.

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Operator Description

Cardinality

+

This is used to perform addition on integers or real numbers. This can also be used as unary plus (optional)

Binary/Unary (2 or 1)

-

This used to perform subtraction on integers or real numbers. This is also used as unary minus

Binary/Unary (2 or 1)

*

This is called asterisk and is used to perform multiplication.

Binary (2)

/

Forward slash operator is used to perform division

Binary (2)

%

Modulo Division. Can be used only for integers and produces reminder of integer division

Binary (2)

If both operands of an arithmetic operation are integers then it is called integer expression and the result of the integer expression will be an integer. Eg: 2+3  the result is 5 5/2  the result is 2 and not 2.5 (decimal part is truncated in integer division) 5%2 the result is 1 (% returns the reminder of integer division of 5 by 2) If an expression has both real operands (float or double) then it is called real expression and result will be a real number. 5.0/2.0  the result is 2.5 4.3/8.45 0.508876 An expression involving one integer operand and one real operand can be called as mixed mode operation and the result of mixed mode operation is a real number. 5.0/2  2.5 (even if 2 is integer 5.0 is real and so result is real). Relational Operators: These operators are used to compare two quantities to determine the relation between those quantities. An expression that contains relational operators is called relational expression. The result of the relational expression is either one or zero (true or false in advanced languages). Zero corresponds to false and one corresponds to true. Eg: a=34 etc Following are the 6 relational operations: Operator Description Cardinality <

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used to check if the left hand operand is less than right hand operand

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Binary(2)

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<=

less than or equal to

Binary

>

used to check if the left hand operand is greater than right hand operand

Binary

>=

greater than or equal to

Binary

==

This is used to check if the value of left hand operator is equal to right hand operator. Note that this is different from assignment operator (=)

Binary

!=

not equal to

Binary

Relational operations can be performed on multiple arithmetic expressions. In such cases the arithmetic expression are evaluated first and then the results are compared. Eg: (a+2) < (b-c); Relational expressions are used to form conditions in the decision statements (will be explained later). Logical Operators: Logical Operators are used to perform logical AND, logical OR and NOT. Logical operators are used to check more than one condition. && -- Logical AND || -- Logical OR ! -- Logical NOT “&&” and “||” are binary operations and “!” is a unary operator. An expression which combines 2 or more relational expressions by using logical operations is called a compound relational expression. Eg: a>b && x==10 d<=f || r>=e !(d!=f)

Op1

Truth Table Op2

Value of the Expression Op1 && Op2 Op1 || Op2

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Non-Zero (1) Non-Zero (1)

1

1

Non-Zero (1)

0

0

1

0

Non-Zero (1)

0

1

0

0

0

0

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NOT (!) has only operand and can represented as !Op1. If Op1 is a Non-Zero value then the result of !Op1 is zero. If the value of Op1 is zero then the result of !Op1 is 1. Assignment Operators: Assignment operators are used to assign the result of an expression to a variable. The operator “=” is used for assignment. Eg: v1= 20; v2= v1+v3; C also supports shorthand assignment operators of the form v op= exp; where v is a variable, exp is an expression and op is a binary arithmetic operation. The operator op= is known as the shorthand assignment operator. The statement v op= exp is equivalent to v = v op exp. The operation op can be +, -, %, / or *. Eg: a *=b (a= a*b) x += 2 (x= x+2) a /= n+2 [a= a/(n+2)] Shorthand assignment makes the statement more concise and easy to read and makes it more efficient. Increment and Decrement Operators: Both the operators are unary and operate on only one operand. Increment operator is ++ and increases the value of the operand by 1. Decrement operator is – and subtracts 1 from the operand. Based the place the operator is placed w.r.t operand there can post increment and pre decrement. Similarly post decrement and pre decrement also exists. If the operator is placed before the operand then it is pre (increment or decrement) else if it placed after the operand then it called post (increment or decrement). Eg: i++ ++i --j j-i++ and ++i are equivalent to i=i+1 and --j and j-- are equivalent to j=j-1. ++i and i++ mean the same when they are independent statements. But when used in a expression they behave differently. In an expression when ++i is used, the value of i is incremented before it is used in the evaluation of the expression, where as when i++ is used, the previous value i is first used for evaluating the expression and later incremented. Sample code for illustration: int a=5,b,c; a++; //increment value of a by 1 so a value is 6 --a; //decrements the value of a by one and so value of a will be 5 b=a++; //since post increment is used, the value of a (5) is assigned to b and later //incremented. Therefore b will be 5 and a will be 6 c=++a; //since pre-increment is used, the value of a is incremented before assigning it to c // Therefore value of c will be 7 and value of a will be 7

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printf(“%d %d %d”,a,b,c); Output: 7 5 7 Conditional Operator: The conditional operator “?:” is used to form conditional expressions of the form exp1 ? exp2 : exp3; where exp1, exp2 and exp3 are expressions. exp1 is evaluated first and if it is nonzero (true), then the expression exp2 is evaluated and becomes the value of the expression. If exp1 is zero (false) then exp3 is evaluated and it becomes the value of the expression (can be assigned to any variable). Eg: q = (f>g)?f:g The above statement makes the variables to be assigned with greatest of both f and g. If f=5 and g=6 then f>g condition returns false and so q=g i.e. q=6. Logically this is equivalent to using if else block. if(f>g) q=f; else q=g; Bitwise Operators: These operators are used to manipulate the data at bit level. They can be used for testing the bits, shifting them right or left. Bitwise operators may not be applied to float or double. Bitwise AND “&” is used to perform AND operation on individual bits of the operands. It is a binary operation and needs two operations. Eg: c=2 & 3; First the two operands are converted into binary bits and then AND operation is applied on individual bits. If assume the word length of a computer as 16bit we will have 16 digits in the binary conversion. 2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 3 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 ---------------------------------------2 & 3  0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0  2 (result) Bitwise OR “|” (single pipeline) is used to perform OR operation on individual bits of the operands. This is again a binary operations and it needs 2 operands. Eg: j= 5 | 9; First 5 and 9 are converted into binary system and later on OR operation is performed on each bit of the numbers.

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5 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 1 9 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 1 ---------------------------------------5 | 9  0 0 0 0 0 0 0 0 0 0 0 0 1 1 0 1  13 (result) Bitwise exclusive OR “^” – this is similar to the operations except that it performs exclusive OR operation on Shift left or left shift “<<” is used to shift the significant bits of the operand to left. Eg: c= a<>” is used to shift the significant bits of the operand to right. Eg: c= a>>b; Where “a” is operand whose bits has to be shifted right and “b” is number of positions that they should be shifted. The result of the operation is assigned to “c”. Let value of “a” be 4 and value of “b” be 1. This means the significant bits of the binary conversion of 4 should be shifted left by one position. 4 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 4>>1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0  2 In the above illustration the underlined bits are the significant bits as all other digits before them are zero’s. They are shifted right and gap formed on the left is filled with zero. Notice that right most digit (0 in this case) is eliminated.

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Note: Right shifting a number by one position will half the value (in above case result of 4>>1 is 2). Therefore x=x>>1 will simply reduce the value of “x” to half. Similarly x=x>>2 will make it reduced to 1/4th. One’s Complement “~” (can also be referred to as bitwise NOT) is a unary operation that applies on a single operand and performs logical NOT operation on individual bits of the binary conversion of the operand. Eg: c= ~a; where “a” is the operand on which one’s complement is applied and the value is assigned to “c”. Let value of a be 6. 6 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0 ~6 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 1 Special Operators: C supports some special operators such as comma (,) operator, sizeof operator, pointer operator (& and *) and member selection operators (, and ->). Comma Operator “,” is used to link the related expressions together. A comma-linked expression is evaluated left to right. The value of the right most expression is the value of the combined expression. Eg: x = (a = 34, b = 4, a+b); In the above statement first the value 34 is assigned to “a” and then 4 us assigned to “b” and the addition (a+b) is performed and the result of a+b is stored in “x”. Since comma operator has lowest precedence use of parenthesis is compulsory. Some applications of comma operator are as follows for( i=0, j=10; i<=j; i+=2, j++) Notice that both i and j are initialized using comma operator and the “;” is used to specify the condition and in the last section both i and j are incremented using comma operator. while(c=getch(), c!= ’z’) A character is read from the user and stored in c and a condition is formed on the value of c. sizeof operator is a compile time operator and which is performed on only one operand to find the number of bytes the operand occupies. The operand may be a variable, constant or a data type. This operator is generally used to find the size of arrays, structures, etc. Eg: x= sizeof(sum); printf(“The size of int is %d“,sizeof(int));

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Operator Precedence and Associativity: In general expressions can involve many operators and when multiple operators are used, the order in which operations are performed should be defined. The precedence and associativity of operators are used to determine how an expression involving multiple operators should be evaluated. There are different levels of precedence and the operation involving highest precedence operator will be evaluated first. The operators of same precedence are evaluated either from left to right or right to left depending on the associativity property of the operator. Most of the operator will have left to right associativity where some of them have right to left associativity. Here is a table:

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Decision Making Statements: Simple if statement: The if statement is a powerful decision making statement and is used to control the flow of execution of statements. The general form of a simple if is: if (test expression) { Statement-block; } Statement-x; The statement block can be a single statement or a group of statements. If the test-expression is true, the statement block is executed and the control continues to statement-x. If the testexpression is false then statement block inside the ‘if’ is skipped and the execution continues with statement-x. Therefore when the test-expression is true both statement block and statementx are executed where as in other case only statement-x is executed. The flow of simple can be represented in the following way:

Eg: Program to check if a number is even or not. #include<stdio.h> void main() { int num; printf(“Enter the number: ”); scanf(“%d”, &num); if(num%2==0) printf(“The entered number %d is a even number”, num);

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if(num%2==1) printf(“The entered number %d is a odd number”, num); } De Morgan’s Rule: when applying logical NOT on complex relational expressions we have to keep in mind that the expressions will be evaluated using De-Morgan’s rule. Eg: !(a||b && !(p>q)) will be equivalent to !x && !b || !p < !q !(a && b) ≈ !a || !b if..else statement: This is the extension of simple if statement and the general form is if(test-expression) { Statement-block-x; //true block } else { Statement-block-y //false block } Statement-block-z; If the test-expression is true, then the Statement-block-x (true block) is executed; if the testexpression is false, then the Statement-block-y is executed. In both the cases the control is passed to Statement-block-z. The flow of the if..else statement can be show in the following way:

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Eg: The above program can be re-written in the following way. #include<stdio.h> void main() { int num; printf(“Enter the number: ”); scanf(“%d”, &num); if(num%2==0) printf(“The entered number %d is a even number”, num); else printf(“The entered number %d is a odd number”, num); } Note:  An else cannot exist without the if block; else should always pair with if.  After a single if block, there can be only one else block following it.  There cannot be any other statements between if block and else block. Nested if – else statements: When a series of decisions are involved, we may have to use more than on if-else statements in the nested form and it can be something like this. if(condition-1) { Statement-block-a; if(condition-2) Statement-block-b; else { Statement-block-c; } if(condition-3) { Statement-block-d; } } else { Statement-block-e; } Statement-block-f;

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The sequence of execution of the statements can be described using the following table: Lets represent condition as c-1,c-2, etc and statement as s-a, s-b, etc. c-1

c-2

c-3

s-a (executed Y/N) s-b s-c s-d s-e s-f

True

True

True

Y

Y

N

Y

N

Y

True

True

False

Y

Y

N

N

N

Y

True

False

False

Y

N

Y

N

N

Y

True

False

True

Y

N

Y

Y

N

Y

N

N

N

N

Y

Y

False True/False True/False

Flow Chart: Entry

C-1 true

Statement-a false

C-2 true false

Statement-b

Statement-c

Statement-e false

C-3

true

Statement-d

Statement-f

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if-else ladder: The if-else ladder is another way of using multiple if. This is used when choice has to be made among multiple blocks of statements based on various test conditions. The general form of ifelse ladder is: if(condition 1) statement-1; else if(condition-2) statement-2; else if(condition-n) statement-n; else statement-default; statement-x; In the above illustration when condition-1 is true only statement-1 is executed and when condition-2 is true only statement-2 is executed and in general when condition-n is true statement-n is executed. Therefore statement 1 through n are executed mutually exclusive (if one is executed rest all are skipped). When all n conditions are false, then the statement-default in the last else is executed. In an ‘if-else’ ladder each else block is paired with the ‘if’ statement immediately above it. The flow chart for the above general form is as follows:

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Switch Statement:

A switch control structure or statement is an alternative to if..else ladder. It is also known as multiple branch selection statement. In order to use switch control statement, four keywords are required. They are: switch, case and break. The general form follows: switch (expression) { case value-1: statement block-1; break; case value-2: statement block-2; break; ……………………… ……………………… default: default-block; break; } statement-x; • •

• • •

•

•

The expression is an integer expression or a character. Cannot be a real or other expression. The value-, value-2, … are the constants or constant expressions and are known as case labels. These labels are the possible values of the expression given in switch. Case labels end with “:” which are followed by statement blocks. There cannot be two case labels with same value. When a switch is executed, the value of expression is compared with case labels and the statement block defined in that case is executed. The break statement at the end of each block causes the exit from the switch statement, there by achieving the exclusivity among the multiple statement blocks. In other words; when expression evaluates to value-1, only statement-block1 is executed; if it is value-2 only statement-block-2 is executed and so on. The default executed when the value of expression matches none of the case labels. The default section is generally defined at the end of switch and it is optional. When it is used as last section, usage of break statement is also optional. When default is not defined and none of the case labels match the expression value, the execution continues to statement-x without executing any block in the switch.

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Absence of break in any of the case block makes the execution to continue with the next case statements as well until break is reached or end of switch is reached. (Eg: Lab Ex-10) Flow Chart illustrating the selection of statements in switch:

Ex:

printf(“Enter day value”); scanf(“%d”,&day); switch(day) { case 1: printf(“Sunday”);break; case 2: printf(“monday”);break; case 3: printf(“tueday”);break; case 4: printf(“wednesday”);break; case 5: printf(“thursday”);break; case 6: printf(“friday”);break; case 7: printf(“Saturday”);break; default: printf(“Illegal choice”); }

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The goto statement:




C supports goto statement to support unconditional branching from one point to another point of program. In other words goto statement makes the flow of control to jump to a location in the program unconditionally. goto requires a label to identify the place where the control has to be jumped. Label can be any valid name followed by a colon “:”. The label is place just above the statements where the control has to be transferred. Depending on place of the label in the program goto will result in either forward jump or backward jump. Below is a illustration:

Eg: main() { int x,y; read: scanf(“%d”, &x); if(x<0) goto read; y=x*x; if(y<25) goto read; }

Note: In a structured programming like C, it not essential to use goto. Since the control is made to jump unconditionally here and there, the best practice is to avoid goto statements until unless it is inevitable (not avoidable). In a nested loop structure when a break statement is used in the inner most loops, it will just break the inner most loop and continue the execution of outer loop(s). If there is requirement where in all loops’ execution has to be stopped we can use goto. Eg: for(i=1:i<4;i++) { for(j=1;j<4;j++) { if(j==3)

for(i=1:i<4;i++) { for(j=1;j<4;j++) { if(j==3)

break; printf(“%d-%d ”,i,j); } } printf(“ --End”); Ouput: 1-1 1-2 2-1 2-2 3-1 3-2 --End

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goto label1; printf(“%d-%d ”,i,j); } } label1: printf(“ --End”); 1-1 1-2 --End

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