In this recipe, you will learn how to create a program that displays the first character to be repeated in a string. For example, if you enter the string racecar, the program should give the output as The first repetitive character in the string racecar is c. The program should display No character is repeated in the string if a string with no repetitive characters is entered.

1. Define two strings called str1 and str2. Your strings can be of any length, but the last position in the string is fixed for the null character \0:

char str1[80],str2[80];

2. Enter characters to be assigned to str1. The characters will be assigned to the respective index locations of the string, beginning with str1[0]:

printf("Enter a string: ");
scanf("%s",str1);                

3. Compute the length of str1 using the strlen library function. Here, the first character of str1 is assigned to str2:

n=strlen(str1);
str2[0]=str1[0];

4. Use a for loop to access all of the characters of str1 one by one and pass them to the ifexists function to check whether that character already exists in str2. If the character is found in str2, this means it is the first repetitive character of the string, and so it is displayed on the screen:

for(i=1;i < n; i++)
{
    if(ifexists(str1[i], str2, x))
    {
          printf("The first repetitive character in %s is %c", str1, 
          str1[i]);
          break;
    }
}

5. If the character of str1 does not exist in str2, then it is simply added to str2:

else
{
    str2[x]=str1[i];
    x++;
}

The repetitive.c program for finding the occurrence of the first repetitive character in a string is as follows::

#include<stdio.h>  
#include<string.h>
int ifexists(char u, char z[],  int v)
{
    int i;
    for (i=0; i<v;i++)
        if (z[i]==u) return (1);
    return (0);
}

void main()
{
    char str1[80],str2[80];
    int n,i,x;
    printf("Enter a string: ");
    scanf("%s",str1);
    n=strlen(str1);
    str2[0]=str1[0];
    x=1;
    for(i=1;i < n; i++)
    {
        if(ifexists(str1[i], str2, x))
        {
            printf("The first repetitive character in %s is %c", str1, 
            str1[i]);
            break;
        }
        else
        {
            str2[x]=str1[i];
            x++;
        }
    }
    if(i==n)
        printf("There is no repetitive character in the string %s", str1);
}

Now, let's go behind the scenes to understand the code better.

Let's assume that we have defined a string, str1, of some length, and have entered the following characters—racecar.

Each of the characters of the string racecar will be assigned to the respective index locations of str1, that is, r will be assigned to str1[0], a will be assigned to str1[1], and so on. Because every string in C is terminated by a null character, \0, the last index location of str1 will have the null character \0, as follows:

Using the library function strlen, the length of str1 is computed and a for loop is used to access all of the characters of str1, one by one, except for the first character. The first character is already assigned to str2, as shown in the following diagram:

Each character that is accessed from str1 is passed through the ifexists function. The ifexists function will check whether the supplied character already exists in str2 and will return a Boolean value accordingly. The function returns 1, that is, true, if the supplied character is found in str2. The function returns 0, that is, false, if the supplied character is not found in str2.

If ifexists returns 1, this means that the character is found in str2, and hence, the first repetitive character of the string is displayed on the screen. If the ifexists function returns 0, this means that the character does not exist in str2, so it is simply added to str2 instead.

Since the first character is already assigned, the second character of str1 is picked up and checked to see if it already exists in str2. Because the second character of str1 does not exist in str2, it is added to the latter string, as follows:

The procedure is repeated until all of the characters of str1 are accessed. If all the characters of str1 are accessed and none of them are found to exist in str2, this means that all of the characters in str1 are unique and none of them are repeated.

The following diagram shows strings str1 and str2 after accessing the first four characters of str1. You can see that the four characters are added to str2, since none of them already exist in str2:

The next character to be accessed from str1 is c. Before adding it to str2, it is compared with all the existing characters of str2 to determine if it already exists there. Because the c character already exists in str2, it is not added to str2 and is declared as the first repeating character in str1, as follows:

Let's use GCC to compile the repetitive.c program, as follows:

D:\CBook>gcc repetitive.c -o repetitive

Let's run the generated executable file, repetitive.exe, to see the output of the program:

D:\CBook>./repetitive
Enter a string: education
There is no repetitive character in the string education

Let's run the program again:

D:\CBook>repetitive
Enter a string: racecar
The first repetitive character in racecar is c

Voilà! We've successfully found the first repeating character in a string. 

Now, let's move on to the next recipe!