Computer Programming I

Computer Programming I UP ITTC AY 2009-2010

Control Structures Lesson 4

Control Structures
Kinds  

Sequence Control Structures Selection Control Structures






of Control Structures

if if-else switch

Repetition Control Structures




while do-while for

Sequential Program int main() { statement1; statement2; …. statementN; }

statement1

statement2

statementN

if Statement


used to conditionally execute a statement or block of statement.

if (expression) statement;

True or False





In C, every expression has a numeric value An expression is ‘true’ when its value is nonzero it is false it evaluates to zero. Therefore, in the following – if (expression) statement statement is executed if expression evaluates to non-zero.

if-else Statement if (expression) statement1; else statement2;








if expression is false, statement2 is executed both statements can be (and very often are) replaced by blocks of statements (“compound statements”) The else portion is optional.

Multiple if-else constructs


Multiple if-else constructs can also be used to choose between three of more options. TRUE

if (expression1) statement1; else if (expression2) statement2; else statement3;

expression1

statement1

FALSE expression2 FALSE statement3

rest of program

TRUE

statement2

The conditional or ternary operator: ‘?: ‘


The ?: operator is a more efficient form for expressing simple if statements.




It has the following form: <expression1>? <expression2>: <expression3>




It simply states: if (<expression1>) <expression2>; else <expression3>;

The conditional or ternary operator: ‘?: ‘


Example: 

Suppose we want to assign the maximum of a and b to z. Using the ? operator, we have the following statement: z = (a > b) ? a : b;




which is the same as: if (a > b)

z = a; else

z = b;

Exercise 6





Write two version of a program using if-else and the conditional operator to determine whether a year is a leap year or not. Hint: Any year exactly divisible by 4 is a leap year. However, if that same year is exactly divisible by 100 it is not, unless it is also exactly divisible by 400.

switch Statement


A multi-way conditional statement  similar to the if … else if … else …  allows the selection of an arbitrary number of choices based on an integer value switch (<expression>) { case : <statements> case : <statements> …. …. case : <statements> default : <statements> }

switch Statement


expression must have an integer value (char, int)




when the switch statement is executed:  the expression is evaluated  if a case matches the value of the expression, the program jumps to the first statement after that case label  otherwise, the default case is selected  the default is optional

while loop
The while loop has the following while ()

syntax:

<statement> 

where,





The boolean_expression in the syntax can be any expression that evaluates to TRUE or FALSE only. The statement can be either a simple statement or a compound statement.

while loop FALSE expression 



The statement is executed as long as condition is true. The loop terminates when the condition is no longer true.

TRUE statement/s

rest of program

while loop
Here

is a simple example of the use of the while loop: i=1 while (i<=5) { printf("%d",i); i++; }

while loop
There

are some things you have to remember when using the while loop. First

and foremost is that the while loop must terminate eventually. This means that you have to make sure that the boolean expression must evaluate to FALSE at some foreseeable point. Loops that do not terminate are called infinite loops.

do-while loop statement/s TRUE

do { statement(s); } while ( condition );

condition FALSE rest of program

Similar to while loops Except the condition is evaluated after the loop body The loop body is always executed at least once, even if the expression is never true 

do-while loop
Example:

i=1; do { printf("%d", i); i++; } while (i<=5);

for loop initialization

for ( ; ; ) { statement(s); }

condition

statement(s)

increment

rest of program

break and continue
C

provides two commands to control how we loop:  

break – to exit from a loop or a switch continue – skip an iteration of the loop

Exercise 7





Write a program that prints a 10 x 10 multiplication table. Write a program that ask the user to enter a number from 1 to 12 and prints the corresponding month. Example: 1 January, 2 February

Subprograms Lesson 5

Introduction
C

provided subprograms
Procedures
Scope of identifiers
Parameters
Functions

Subprograms



A subprogram is a sequence of program statements that have common goals. There are two types of subprograms:  




Procedures Functions

The difference between a procedure and a function is that a function returns a value (of a specific type) after it finishes executing.

C-provided Subprograms
C

provides us with a lot of procedures and functions that we can use for our different purposes.
All we need to do is to “include” the header where the procedure/function belongs to by using the #include precompiler directive.

C-provided Subprograms
Here

are some of the most popular C procedures and functions: printf – prints formatted text on screen (stdio.h)  scanf – read formatted text from keyboard(stdio.h) 

C-provided Subprograms
Here

are some of the most popular C procedures and functions: 





strcpy – copies one string to another (string.h) strcmp – compares two strings (string.h) strlen – gets the length of a string (string.h)

Procedures
The syntax for definition of a procedure: void <procedure_name> () { <statements> }

Procedures
A

procedure should be called by a running function before it will be executed. This is also true for functions.
To call: <procedure_name>();

Procedures
Example: void p(int n) {

} TO CALL:/*placed in function that will run*/ p(1); p(i); /* assume: int i = 1; */

Scope of identifiers






In C, there is such a thing as the scope of an identifier. The scope of an identifier is confined in the block where it is declared. All the variables/constants that are declared at the start of a block is said to be local with respect to everything outside the block, but is said to be global with respect to everything inside the block.

Parameter Passing
Parameter

passing

A way of passing information from one subprogram to another.



Parameter Passing
There  

are two types of parameters:

Formal parameters Actual parameters

Functions
As

was said before, functions are different from procedures only in the sense that, when called, it returns a value of a specific type to whatever procedure or function that called it. Otherwise, they are very much the same.

Functions
Here

is the syntax for the definition of functions: () { <statements> }

Procedure/Function Declaration


The following are the syntax for procedure and function declaration, respectively. void<procedure_name>(); ();




We do not really need to use subprogram declarations. That is if all functions called are placed before the calling function.

Lesson Review






A subprogram is a sequence of program statements that have common goals. There are two types of subprograms: Procedures and Functions. The difference between a procedure and a function is that a function returns a value (of a specific type) after it finishes executing. C provides us with a lot of procedures and functions that we can use for our different purposes.

Lesson Review








In C, there is such a thing as the scope of an identifier. The scope of an identifier is confined in the block where it is declared. Parameter passing is a way of passing information from one subprogram to another. There are two types of parameters: Formal parameters are the variables found in the heading of the declaration of a procedure. Formal parameters are considered as local variables. Actual parameters are the variables or expressions used when calling the procedure.

Exercise 9
Write

a function named min that takes two integers as its parameters and returns the value of the smaller of the two parameters.
Write a program that uses the function in the previous problem to determine the smallest of five integers inputted by the user.

Arrays, Strings and Pointers Lesson 6

Introduction




Arrays Strings Pointers

Arrays


C provides us with a way to declare a group of data items and give them a common name. This is called array.




Arrays are declared in the following manner: <array_name> [ ];  where




data_type can be any type discussed so far array_name is an identifier num_elements is a constant integer value greater than 0

Arrays


Examples: int number[100]; float grade[10]; char str[50];

Accessing Array Data


Each element in an array is considered as a single variable and is accessed via its index.




The syntax for accessing a single element in an array is as follows: <array_name> [] where

index is an expression that evaluates to an integer. In using the index, it is important to remember that the ith element is accessed using an index of i-1

Initializing Arrays






You can optionally initialize an array in the declaration itself. The syntax for an array declaration with initialization is: <array_name> [] = { , , ...}; Example: float grade[10] = {83.5, 97.5, 71.0, 88.0};

Strings








When declaring an array of characters, add one to num_elements to accommodate the ‘\0’ (null terminator character) which marks the end of the string. For example, if you want to store a name that can be at most 30 characters, the declaration should be: char name[31]; when you use scanf to let the user enter a string of characters. scanf("%s", name);  scanf

appends ‘\0’ to the end of the string inputted.

Strings


You may also initialize an array of characters with a string constant. So instead of doing it like this, char name[31] = {'V', 'a', 'n'};




you can do it like this, char name[31] = "Van";




but you can’t assign a string constant to it with an assignment statement.

name = "Van";

/* Error: ... */

Strings


C provides you with the functions to manipulate strings. Whenever you want to use these functions, don’t forget to add the preprocessor directive #include <string.h>.

Strings


To assign a string to a string variable, you must use strcpy. char name[31]; strcpy (name, "van");




To compare two strings, use strcmp. if strcmp(name, "van") == 0){ // strcmp returns 0 if they are the same printf("Your name contains 3 characters."); }

Strings


To find the length of a string, use strlen. if (strlen(name) == 3) printf("Your name contains 3 characters.");




To append to the end of a string, use strcat. strcat (name, "essa"); printf("Now your name is vanessa.");

Pointers


A pointer, in its simplest term, is a variable which points at certain memory location.




In much the same way as an integer variable contains a number and a character variable contains a single character, a pointer, a pointer variable contains the address of a memory location.

Uses of Pointers


C uses pointers to create dynamic data structures – data structures built up from blocks of memory allocated from the heap at run-time.




C uses pointers to handle variable parameters passed to functions.




Pointers in C provide an alternative way to access information stored in arrays. Pointer techniques are especially valuable when you work with strings. There is an intimate link between arrays and pointers in C.

Pointers






Pointers that point to a specific data types hold the addresses of either characters, integers, floats, or the like. The syntax for declaring pointers is as follows: *; Examples: int *i_ptr; char *c_ptr; float *f_ptr;

Pointers


In C, we get the address of any variable by using the following syntax: &




Let’s say that we have three other variables: char ch; int num; float dec;

Pointers


And so, we can now perform the following assignment operations: c_ptr = &ch; i_ptr = # f_ptr = &dec;

Pointers





How do we reference the value inside the memory location pointed to by a pointer variable? The syntax for doing this is as follows: *<pointer_variable_name>

Pointers


As an example, we have here a simple C program which writes the letter ‘R’ on screen: #include <stdio.h> main() { char *c_ptr, ch; ch = 'R'; c_ptr = &ch; printf("%c", *c_ptr); }




The method of accessing the value pointed at by a pointer variable is called dereferencing or indirection.

Pointers and Arrays: Pointer Arithmetic


Pointers and arrays are so closely related to each other that they have a correspondence.




If p points to an object in array of the same type:  (p + 1) points to the next object in the array  (p + i) points to the ith object in the array  *(p + 1) refers to the value of the next object in the array  *(p + i) refers to the value of the ith object p points to

Lesson Review









C provides us with a way to declare a group of data items and give them a common name. This is called array. Each element in an array is considered as a single variable and is accessed via its index. C provides you with the functions to manipulate strings. Whenever you want to use these functions, don’t forget to add the preprocessor directive #include <string.h>. A pointer, in its simplest term, is a variable which points at certain memory location.

Exercise 10
Write

a function int getMaximum( int anData[], int nNum ) which returns the highest number among the nNum data items stored in the array anData.
Write a function int getMaximum( int *pnData, int nNum ) which returns the highest number among the nNum data items stored in the array pnData.

Exercise 11
Write

a function void reverseString(char* str) which reverses a string parameter. For example, if the input is “string”, the output should be “gnirts” .
Write a function void toUpperCase(char* str) which converts all lower case letters contained in a string to upper case letters. For example, if the input is “2 String(s)”, the output should be “2 STRING(S)”.

End