Lecture 6:  String Processing

 

Objectives of this lecture

q       Learn how to declare, initialize, and process strings

q       Learn the basic string functions

q       Be able to apply strings in problem solving

 

Declaration & Initialization

q       A string is a sequence of characters enclosed in double quotes

q       It is represented in C as an array of characters

q       However, one additional cell is required for storing the terminating character, ’\0’,called NUL

q       Example:  Even though the string ”I like C” has 8 characters, we need to declare a character array of size 9.

 

char str[9]=”I like C”;

q       Note that in the example above, the null character is automatically inserted by the compiler

q       All the string functions in C require the presence of the NUL character to work correctly.

q       Note that strings (like arrays) depends on the programmer to give enough size.  If size is not enough, data will be read anyway, replacing memory cells not meant for the string – leading to unpredictable results.

q       Strings can also be declared without specifying the size as in:

char str[]=”I like C”;

In this case, sufficient storage is allocated including that for NUL

q       Because a string is an array of characters, an array of strings is a 2-D array of characters.

q       Example:   To declare an array of strings to store week days.

    Char day[7][10]={”Monday”, ”Tuesday”, ”Wednesday”,

      Thusday”, ”Friday”, ”Saturday”, ”Sunday”}

 

String I/O

q       The easiest way to input string is by using the gets function

Example:       char name[81];

printf(”Input your name”);

gets(name);

 

q       Also the easiest way to output string is by using the puts function as in:

puts(name);

q       gets allows you to read a string that could include blanks.  It terminates reading on encountering the return character.

q       puts advances to the next line after printing

q       You can also use the scanf/printf functions for string I/O using the %s format specifier.

q       However,  scanf terminates reading on encountering the blank character, so first and last names must be read separately:

char first[10],last[10];

printf(”\nEnter your first name: ”);

scanf(”%s”,first);

printf(”\nEnter your last name: ”);

scanf(”%s”,last);

printf(”\nYour full name is : %s %s”,first,last);

 

q       Note that the address of operator, &, is not used in the scanf function – the name of a string (or array) is an address.

 

Built-in string functions:

q       Built-in string processing functions can be accessed through the header file, string.h

q       Some of the common functions are:

strcat(string1, string2) à appends strings2 to string1

strcpy(string1, string2) à replaces string1 with string2

strcmp(string1, string2) à compares string1 and string2

and  returns:

Negative integer if string1 < string2

0 if string1 == string2

positive integer if string1 > string2.

 

strlen(string1) à returns the length of string1 excluding NUL

strchr(string, ch) à searches for the occurrence of ch in string

 

The following program illustrates the use of string functions:

 

#include <string.h>

#include <stdio.h>

 

main()

{    char s1[81], s2[81];

     gets(s1);

     gets(s2);

     printf(”lengths: %d  %d\n”, strlen(s1),  strlen(s2));

     if (!strcmp(s1, s2))

        printf(”The strings are equal\n”);

     strcat(s1,s2);

    printf(”%s\n”,s1);

    return 0;

}

 

Note: It is important to remember that strcmp() returns false if the strings are equal, so be sure to use ! to reverse the condition

 

Built-in character functions:

q       Built-in character functions can be accessed through the header file, ctype.h

q       Some of the common functions are:

isupper(ch) à returns 1 if ch is uppercase, returns 0 otherwise

isalpha(ch) à returns 1 if ch is an alphabet (lower or upper case)

tolower(ch) à converts a character to its lower case if it has one

toupper(ch) à converts a character to its upper case if it has one

Example:

Problem: Write a program that reads a string and print the number of vowels, number of upper case and number of lower case letters .  It should also print the string with the first letter capitalized and the remaining in lower case.

 

Reasoning:  Need to use string & character processing functions such as strlen, tolower, etc.

Input:  the string

Output: number of lower, upper and vowels and the formatted string.

Process: apply string/character functions to count as well as converts characters in the string.

 

Solution:

Step 1: INPUT str

Step 2: find the length of str (n)

Step 3: Initialize counters (vowels, uppers, lowers) to zero

Step 4: LOOP n times

             IF str[i] is a vowel

                Vowels=vowels+1

Step 5: LOOP n times

              IF str[i] is an alphabet

                  IF str[i] is upper

                      Uppers=uppers+1

                  ELSE

                     Lowers=lowers+1

Step 6: coverts first character to upper

Step 7: LOOP n-1 times

               Convert str[i] to lower

Step 8: OUTPUT vowels, lowers, uppers, str

Step 9: STOP

 

/* prints the number of vowels, upercase & lowercase in a

   given string. Also capitalize the first letter of string */

 

#include <stdio.h>

#include <string.h>

#include <ctype.h>

 

int isvowel(char ch);

 

 

main()

{  char str[81];

   int len, i, nvowel=0, nupper=0, nlower=0;

 

   printf("Enter your string >");

   gets(str);

 

   len=strlen(str);

 

   for (i=0; i<=len; i++)

       if (isvowel(str[i]))

             nvowel++;

 

   for (i=0; i<=len; i++)

       if isalpha(str[i])

            if (isupper(str[i]))

               nupper++;

            else

               nlower++;

 

   str[0]=toupper(str[0]);

 

   for (i=1; i<=len; i++)

       str[i]=tolower(str[i]);

 

   printf("Number of vowels     = %d\n", nvowel);

   printf("Number of lower case = %d\n", nlower);

   printf("Number of upper case = %d\n", nupper);

   printf("Capitalised string   = %s\n", str);

 

   return 0;

 }

 

/* returns true if a given character is a vowel */

 

 int isvowel(char ch)

{ int vowel;

      vowel=tolower(ch)=='a' ||

              tolower(ch)=='i' ||

              tolower(ch)=='o' ||

              tolower(ch)=='u' ||

              tolower(ch)=='e';

 

   return vowel;

}