13 Es 26 - Strings

Strings 13

Introduction to Programming

Strings • In C, strings like “hello” are just arrays of characters • A string is terminated by the sentinel string character ' \0 ', or null character which may NOT be the last element in the array • Null character: backslash-zero • This is the only character whose bits are all zeroes

Introduction to Programming

Implicit Strings • Created by enclosing some text within double quotation marks. • Compiler creates a hidden char array to represent the characters and ensures that a '\0' is placed after the last character in the string. • ex.: printf(“Welcome\n”); W

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Introduction to Programming

\n \0

Implicit Strings • Why call it hidden array? It has no name to which the program can refer to. • Why implicit? The program does not have any control over the size or number of elements in the array, which was implicitly sized by the compiler.

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Variable/Explicit Strings • Created by defining an explicit array of char and then initializing it with other text. ex.: char name[] = “Cao”; • It is not necessary to state the storage requirements or number of elements, since this will be calculated by the compiler. C

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Examples char first_name[5] = {‘J’,’u’,’a’,’n’,’\0’}; – previous rules on arrays still apply char last_name[3] = “Tamad”; – gcc issues a warning but compiles anyway – “Tam” is stored but not properly terminated – No way of knowing when the actual string really ends. Introduction to Programming

Examples char other[100] = “Maria Makiling”; – specifying a larger size leaves space in the array for other characters to be appended – compiler still appends '\0' after the implicit string char characters[7] = “No null”; – still valid though string is not properly terminated. – No way of knowing when to end. Introduction to Programming

Printing Strings • Strings can be printed per element as they are just arrays: int i; char cool[] = “Programming”; while(cool[i] != ‘\0’) printf(“%c”, cool[i++]);

• Alternately, printf supports “%s” printf(“%s \n”,cool); Introduction to Programming

Printing Strings • Also, we can use the function puts declared in stdio.h puts( cool );

• The use of puts() ALWAYS results in a new line character '\n' being issued at the end of each line of output.

Introduction to Programming

Accepting/Reading Strings • Using scanf() scanf( “%s”, cool );

• When using “%s”, the required address is the address of the first element in the array • The address operator '&' is not required because the name of an array in C represents the address of the first element in the array. Introduction to Programming

Accepting/Reading Strings • Same as scanf( “%s”, &cool[0] );

• Note that the scanf function uses white space characters to separate data into different fields. • We can use gets() declared in stdio.h to permit a whole line to be read and does not use white space characters to separate various items of data entry. Introduction to Programming

Example – scanf() … printf(“Anong pangalan mo? ”); scanf(“%s”, string); printf(“Ikaw si %s.”, string); ...

Anong pangalan mo? Wilmarc Lopez Ikaw si Wilmarc.

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Example – gets() ... puts(“Anong pangalan mo? ”); gets( string ); printf(“Ikaw si ”); puts( string); ...

Anong pangalan mo? Wilmarc Lopez Ikaw si Wilmarc Lopez.

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Assigning to String • Strings may be initialized with “=“ but not assigned to with “=“ • Valid (initialization) – char who[] = “Ser Wilmarc”; • Invalid – who = “Mom Wilmarc”;

Introduction to Programming

Assigning to String #include <stdio.h> #include <string.h> main(void) { char who[20]; strcpy(who, “Manong Fishball”); strcpy(who, “Tuition Fee Increase”); printf(“%s”,who); return 0; } Introduction to Programming

String Handling Functions in string.h char s1[50] = “UP”; char s2[50] = “UP System”; String function

Example

Output/value

String length

strlen(s1);

2

String concatenate

strcat(s1,” Diliman”);

s1 has the value “UP Diliman”

String compare strcmp(s1,s2) Introduction to Programming

String Handling Functions in string.h strlen(s1) – returns the number of characters before '\0' of s1 (either implicit or explicit) char s1[50] = “UP”; char s2[50] = “UP System”; int length; printf(“%d”, strlen(s1)); length = strlen(s2);

• What happens if s1 does not have a '\0'? Introduction to Programming

String Handling Functions in string.h strcpy(destination,source) – copies all of the characters from source (either implicit or explicit), including the '\0' terminator to destination (explicit) • Whatever exists in destination is overwritten • It is assumed that destination has enough space to hold the result • The destination array is returned source = strcpy(destination, “Hello”); Introduction to Programming

String Handling Functions in string.h strcat(s1,s2) – takes two strings (either implicit or explicit) as arguments, concatenates them and puts the result in s1 strcpy(s1,“Hello”); strcat(s1, “ world!”); printf(“%s”,s1);

Introduction to Programming

String Handling Functions in string.h strcmp(s1,s2) – an integer is returned that is less than, equal to or greater than zero, depending on whether s1 lexicographically less than, equal or greater than s2, respectively. – Both can either be implicit or explicit strings. strcmp(“Hello”,”Hell”); strcmp(“Hell”, “Hello”); strcmp(“Sat”,”Cat”); Introduction to Programming

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Lab Exercises 1) Write your own strcmp function. Name it my_strcmp. 2) Write your own strcpy dunction. Name it my_strcpy.

Introduction to Programming