C Programming

Strings

Strings

Array of characters – most common type of array in C Let’s make them easier for use

Denote the end of array using a special character We won’t need to indicate the length each

and every time Easier initialization syntax

String Initialization

Use a string instead of the usual braces and comma notation

char str[] = "Text";char s[] = "Hello, World!";

String Termination

Strings terminate with the special character '\0' (ascii code 0)

To hold a string of N characters we need an array of length (at least) N + 1

's' '#' ' ' 'f' 'd' 'y' '4' '7' '$' '_' 'e' 'g' 'd' '.' 'p''H' 'e' 'l' 'l' 'o' ' ' 'w' 'o' 'r' 'l' 'd' 'g' 'd' '.' 'p''H' 'e' 'l' 'l' 'o' ' ' 'w' 'o' 'r' 'l' 'd' 'd' '.' 'p''\0'

Terminator

The array

The string

String Initialization

Use a string instead of the usual braces and comma notation

char str[] = "Text";

Shorthand for the longer but equivalent

char str[] = {'T', 'e', 'x', 't', '\0'};

String Length

int string_length(char str[]){ int i = 0; while (str[i] != '\0') ++i;

return i;}

Printing Strings

We can use printf to print strings Use %s to print from the string until '\0'

Printing Stringsint main(void){ char str[] = "Hello, World!";

printf("%s\n", str);

str[5] = '\0'; printf("%s\n", str);

str[10] = '#'; printf("%s\n", str);

str[5] = ';'; printf("%s\n", str);

return 0;}

Hello, World!

Hello

Hello

Hello; Wor#d!

Reading Strings

getchar read character by character

scanf multiple character in a single read will only read until the first space / newline scanf("%s", str);

NO ‘&’

#define MAXLINE 100

int main(void){ char str[MAXLINE + 1]; /* one more place for the '\0' */ char c; int i = 0;

c = getchar(); while (c != '\n' && i < MAXLINE) { str[i] = c; ++i; c = getchar(); } str[i] = '\0'; /* Terminate the string */

printf("The string you entered is: %s\n", str);

return 0;}

Example – Using getchar()

Reading strings - scanf

scanf reads in letters until a space or newline ('\n') is encountered

The maximum length can be stated in the parentheses:scanf("%10s", str); read 10 characters and terminate with'\0'

(str should be of size at least 11)

Example – using scanf

#define MAXLINE 100

int main(void) {

char str[MAXLINE + 1];

printf("Please enter a string:\n");

scanf("%100s", str);

printf("The string you entered is: %s\n", str);

return 0;}

Input: fun and games with scanf

output: The string you entered is: fun

scanf problem

After using scanf the next character that will be read is the space or newline.

For example:scanf("%s", str);scanf("%c", &letter);

Here letter has the value ‘ ’ or newline (‘\n’).

Solving the problem

We need to read and discard the unwanted newline.

Either use getchar orinform scanf to expect spaces (also newline) before the next character.

scanf("%s", str);scanf(" %c", &tav);

Exercise

Implement the function:

void replace(char str[], char from, char to);

The function replaces every occurrence of the first char with the second one.

Write a program to test the above function read a string from the user (no spaces) and two characters pass them as arguments to your function print the result

Example input: “papa” ‘p’ ‘m’ output: “mama”

Solution

void replace(char str[], char from, char to){ int i;

for (i = 0; str[i] != '\0'; ++i) { if (str[i] == from) str[i] = to; }}

Solution#define MAX_STR_LEN 100

int main(void){ char str[MAX_STR_LEN + 1]; char from, to;

printf("Please enter a string (no spaces)\n"); scanf("%100s", str);

printf(“Character to replace: "); scanf(" %c", &from);

printf(“Character to replace with: "); scanf(" %c", &to);

replace(str, from, to);

printf("The result: %s\n", str);

return 0;}

String library

Like in the case of stdio.h and math.h, we have a special library for handling strings

We should #include <string.h>

String library

All functions assume that a string ends with ‘\0’. Useful functions:

int strlen(char s[]) returns the length of s

int strcmp(char cs[], char ct[]) compares cs with ct

strcpy(char s[], char ct[])copies the contents of ct to s

strcat(char s[], char ct[])Concatenate ct to s

and more… see string.h library

Exercise (1)

Implement the functionvoid string_swap(char str1[], char str2[]);

The function accepts two strings and swaps them.

Solution – string_swapvoid string_swap(char str1[], char str2[]){ int i = 0, temp, str1_len = strlen(str1), str2_len = strlen(str2); int max;

if (str1_len > str2_len) max = str1_len; else max = str2_len;

for (i = 0; i <= max; ++i) { temp = str1[i]; str1[i] = str2[i]; str2[i] = temp; }}

Exercise (2)

Write a program that reads 10 words from the user and sort them.Use a two-dimensional array to store the

wordsUse strcmp to compare two words

A word is a sequence of characters without spaces of length 20 or less

Sort Reminder

set n to number of words to be sortedrepeat

for counter = 1 to n - 1 do   if key[counter] > key[counter+1] then     swap the words; end if end do  n = n - 1;until n = 1

Solution - main

int main(void){ char words[WORDS_NUM][WORD_SIZE + 1]; int i = 0, j = 0; /* read words from user */

for (i = WORDS_NUM; i >= 1; --i) { for (j = 0; j < i - 1; ++j) { if (strcmp(words[j], words[j + 1]) > 0) string_swap(words[j], words[j + 1]); } }

/* print sorted words */ return 0;}