C Programming

1. C Programming Strings

2. 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

3. String Initialization • Use a string instead of the usual braces and comma notation char str[] = "Text";char s[] = "Hello, World!";

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

5. 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'};

6. String Length int string_length(char str[]){int i = 0;while (str[i] != '\0') ++i;return i;}

7. Printing Strings • We can use printfto print strings • Use %s to print from the string until '\0'

8. Printing Strings int 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!

9. 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 ‘&’

10. Example – Using getchar() #defineMAXLINE100 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; }

11. 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)

12. Example – using scanf #define MAXLINE100 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

13. 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’).

14. 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);

15. 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”

16. 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; } }

17. 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; }

18. 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>

19. 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

20. Exercise (1) • Implement the function void string_swap(char str1[], char str2[]); • The function accepts two strings and swaps them.

21. Solution – string_swap void 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; } }

22. Exercise (2) • Write a program that reads 10 words from the user and sort them. • Use a two-dimensional array to store the words • Use strcmpto compare two words • A word is a sequence of characters without spaces of length 20 or less

23. 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

24. 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; }