Strings

1. Strings

2. Strings • Sentences in English are implemented as strings in the C language. • Computations involving strings are very common. • E.g. • Is string_1 the same as string_2 ? • Append string_1 at the end of string_2 • Sort a collection of strings in ascending order • Find occurrence of a character in a given sentence (string)

3. Strings • The string in C programming language is actually a one-dimensional array of characters which is terminated by a null character '\0‘. • The following declaration and initialization create a string consisting of the word "Hello“: • char greeting[6] = {'H', 'e', 'l', 'l', 'o', '\0'};

4. Strings • This is how this char array looks in memory contains the above string • Be careful about the length of string’s array length • No bounds checking by the compiler • May result in unpredictable results

5. Strings Example – Placement of NULL • Write a program to declare a char array, initialize it to the string “Hello” and then move around the NULL character to see its effects

6. Strings Manipulation • Because dealing with strings is cumbersome, a library of functions is available for programmers to manipulate strings • Some of the more frequently used string manipulation functions are: • strlen(str); //returns length of a str • strcpy(dest_str, src_str);//copies source str in deststr • strcat(dest_str, src_str); //append source str at the end of deststr • strcmp(Str1, Str2);// compares two strings lexicographically //returns 0 if both are equal, negative number if LHS is < RHS

7. Strings Example – strlen() function • Write a program to declare a char array, initialize it to a string and then traverse the string in forward and backward direction

8. Strings Example – strcpy(), strcat() functions • Write a program to declare two char arrays and initialize them to some strings. Declare a third char array and copy a concatenation of the first two strings into the third array.

9. String I/O • Another convenient way for string I/O is: • gets(str); //read a string from user and store in str • puts(str); //print a string contained in str

10. Strings Example – strcmp() function • Write a program to declare two char arrays and initialize them to some strings. Compare the two strings using strcmp() function

11. Strings Example – strcmp() function • Write a program to input a list of names. Sort the list of names in ascending (lexicographical / dictionary) and descending orders.

12. Algorithm – Sorting an array of names (strings) • You have a 2-Dimensional array containing 4 names char names[4][50]; The array has 4 rows and 50 columns each row can hold one name of length 50 characters (including the NULL that comes at the end of a string

13. Algorithm – Sorting an array of names (strings) • To input names, you only need to specify the row where the name is to be stored for(i=0;i<10;++i) gets(str[i]); Here, i is the index of the row where every name will be stored The function gets() takes care of where every character of a name goes and also the string terminating NULL character

14. Algorithm – Sorting an array of names (strings) for(i=0;i<10;++i) gets(str[i]); After the for loop finishes execution the array of names will look like this: names =

15. Algorithm – Sorting an array of names (strings) Loops to compare strings and swap places for(i=0;i<3;++i) for(j=i+1;j<4 ;++j){ if(strcmp(str[i],str[j])> 0) { strcpy(temp,str[i]); strcpy(str[i],str[j]); strcpy(str[j],temp); } }

16. Algorithm – Sorting an array of names (strings) • start from row # 1 (variable i) • Compare row 1 with rows 2, 3 and 4 (up to i-1), one row at a time • Smaller of the two names being compared must occupy the upper row Col 1, col 2, col 3, …. Col 50 Row 1 Row 2 Row 3 Row 4

17. Algorithm – Sorting an array of names (strings) Compare and swap if(strcmp(str[i],str[j])> 0) i j

18. Algorithm – Sorting an array of names (strings) The array now looks like this i a d a m \0 … t y l e r \0 … s o p h i a \0 … r a c h e l \0 … j

19. Algorithm – Sorting an array of names (strings) Compare and swap if(strcmp(str[i],str[j])> 0) i a d a m \0 … t y l e r \0 … s o p h i a \0 … r a c h e l \0 … j

20. Algorithm – Sorting an array of names (strings) Compare and swap if(strcmp(str[i],str[j])> 0) i a d a m \0 … t y l e r \0 … s o p h i a \0 … r a c h e l \0 … j

21. Algorithm – Sorting an array of names (strings) i is incremented (i++), j is re-initialized to j=i+1 Compare and swap if(strcmp(str[i],str[j])> 0) i a d a m \0 … t y l e r \0 … s o p h i a \0 … r a c h e l \0 … j

22. Algorithm – Sorting an array of names (strings) Array now looks like this i a d a m \0 … s o p h i a \0 … t y l e r \0 … r a c h e l \0 … j

23. Algorithm – Sorting an array of names (strings) Compare and swap if(strcmp(str[i],str[j])> 0) i a d a m \0 … s o p h i a \0 … t y l e r \0 … r a c h e l \0 … j

24. Algorithm – Sorting an array of names (strings) Array now looks like this i a d a m \0 … r a c h e l \0 … t y l e r \0 … s o p h i a \0 … j

25. Algorithm – Sorting an array of names (strings) i is incremented (i++), j is re-initialized to j=i+1 Compare and swap if(strcmp(str[i],str[j])> 0) i a d a m \0 … r a c h e l \0 … t y l e r \0 … s o p h i a \0 … j

26. Algorithm – Sorting an array of names (strings) Compare and swap if(strcmp(str[i],str[j])> 0) i a d a m \0 … r a c h e l \0 … s o p h i a \0 … t y l e r \0 … j

27. Algorithm – Sorting an array of names (strings) Finally, the sorted array looks like this a d a m \0 … r a c h e l \0 … s o p h i a \0 … t y l e r \0 …

28. Strings Example – counting words • Write a program to input a sentence (string) and count the number of words in it.

29. Strings Example – frequency of a character • Write a program to input a sentence (string) and find the frequency of a given character.

30. Strings Example – Palindrome or Not • Write a program to input a word and find out if it is a palindrome. • Civic, mom, dad, kayak, radar, madam, racecar • Function strrev(str) reverses a string