C program to check if a substring exists in a string or not

C program to check if a substring exists in a string :

In this C programming tutorial, we will learn how to check if a substring exists in a string or not. If the substring exists, the position of this substring in the string should be printed. For example, for the string “Hello World!!”, substring World appears on position 7 of the main string. Similarly, if the substring doesn’t exist, print one failed message.The user will enter both of these inputs. Before going through the steps, let’s take a look at the program first. We are using a recursive approach in the below program :

C program: Recursive method to check if a substring exists in a string :

#include<string.h>
#include<stdio.h>
#include<ctype.h>

//12
int findSubString(char* mainString,char* subString,int mainLength,int subLength,int position,int subPosition){
	//13
	if(position >= mainLength || subPosition >= subLength){
		return 1;
	}

	//14
	if(isspace(subString[subPosition])){
		return 1;
	}

	//15
	if(mainString[position] == subString[subPosition]){
		return findSubString(mainString,subString,mainLength,subLength,position + 1,subPosition + 1);
	}

	//16
	return 0;
}



int main()
{
	//1
	char mainString[100];
	char subString[100];

	//2
	int mainLength;
	int subLength;
	int i;

	//3
	int found = 0;

	//4
	printf("Enter main string : ");
	fgets(mainString, 100, stdin); 

	//5
	printf("Enter sub string : ");
	fgets(subString, 100, stdin);

	//6
	mainLength = strlen(mainString);
	subLength = strlen(subString);

	//7
	for(i = 0; i < mainLength - 1 ; i++){
		//8
		if(mainString[i] ==  subString[0]){
			//9
			if(findSubString(mainString,subString,mainLength,subLength,i,0)){
				//10
				printf("Input substring is found on position %d ",i+1);
				found = 1;
				break;
			}
		}
	}

	//11
	if(found == 0){
		printf("Input substring is not found in the string...");
	}
}

Sample Output :

Enter main string : Hello World!!
Enter sub string : World
Input substring is found on position 7

Enter main string : This is a ball
Enter sub string : ball
Input substring is found on position 11

Enter main string : These are balls and bats
Enter sub string : balls
Input substring is found on position 11

Enter main string : This is a cat
Enter sub string : dog
Input substring is not found in the string...

Enter main string : Hello Everyone
Enter sub string : Hello
Input substring is found on position 1

Explanation :

The commented numbers in the above program denote the step number below :

1. Create two char array mainString and subString to store the user input main and substrings.
2. Create three integer variables. mainLength to store the length of the main string, subLength to store the length of the sub-string and  _i _ to use in for-loop.
3. Create one flag found and set the value to 0. 1 means sub-string is found in the main string and 0 means not found.
4. Ask the user to enter the main string, read it and store it in array mainString.
5. Similarly, read and store the sub-string in variable subString.
6. Calculate the lengths of both of these strings and store it in mainLength and subLength variables.
7. Start a for loop to read each character of the main string one by one.
8. For each character, compare it with 0 position character of the sub-string.
9. If both are equal, call findSubString function. This function recursively checks if all characters of the sub-string exist in the main string or not. It returns 1 if true.
10. If the sub-string exists in the main string, print out its position and set the value of flag found as 1.
11. If the substring is not found, print one message.
12. findSubString(char* mainString,char* subString,int mainLength,int subLength,int position,int subPosition) is the main function to find sub-string in a string. It returns 1 if found. It calls itself again and again recursively to compare characters of both strings.
13. Return 1 if the current position of any of the string is more than its length.
14. Check if a character is space or empty by using isSpace() function. If current character of sub-string is empty, return 1.
15. If current character of both of these strings is equal, call the same function recursively. Increment position for both strings by 1.
16. Return 0 if nothing is true above.