How to Compare Strings in C

String Comparison in C

In strings strcmp() function is used to compare two strings under a common header file called string.h .This function returns a negative,zero or a positive integer depending on the string pointed to,by str1 to string pointed to by str2.

Syntax strcmp

int strcmp(const char *str1,const char *str2);

Arguments

• str1 is first array to compare.
• str2 is the second array to compare.

Return values

• If str1=str2 function returns 0.
• If str1<str2 function returns a negative integer.
• If str1>str2 function returns a positive integer.
• If string doen’t exists in str2 then it returns NULL.

How return value of strcmp is calculated

strcmp() function cannot compare strings by length. It compares strings by its ASCII values. If s1=”apple” and s2=”banana”, when we compare both the strings like strcmp(s1,s2), then this function returns a negative integer because ASCII value of ‘a’ is less than the ASCII value of ‘b’. If both the first characters are same then it goes for second characters.

Examples

Example  – Compare Strings using strcmp() function

If two strings are equal, strcmp(s1, s2) should return zero.

C Program

#include <stdio.h>
#include <string.h>
int main()
{
	char s1[]="apple";
	char s2[]="banana";
	if(strcmp(s1,s2)==0)
		printf("equal");
	if (strcmp(s1,s2)<0)
		printf("s1 less than s2");
	if(strcmp(s1,s2)>0    )
		printf("s1 greater than s2");

	return 0;
}

Output

s1 less than s2

Example Compare Strings using Recursion

In string comparison using recursion we are using three conditions. If one of the pointer is NULL pointer then it returns -2 in order to stop the process. If two strings are identical it returns 0 after comparing every character in one string with another string. We recursively call CompareStrings() function in order to compare both the strings.

C Program

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

int main() {
	char str[100],s[100];
	printf("enter strings to compare");
	gets(str);
	gets(s);
	printf("the result of comparison is %d\n",CompareStrings(s, str));
	return 0;
}

int CompareStrings(char *s, char *s1) {
	// if one of the pointer is a NULL pointer return directly -2
	// in order to stop the process
	if(s==NULL || s1==NULL)
			return -2

	if(strcmp(s,s1)==0) // the two strings are identical
		return 0;

	if((s[0])==(s1[0]) && (s[0])==((s1+1)[0]))
		CompareStrings(s, ++s1);
	else if((s[0])==(s1[0]) && (s1[0])==((s+1)[0]))
		CompareStrings(++s, s1);
	else if((s[0])==(s1[0]))
		CompareStrings(++s, ++s1);
	else
		return -1;
}

Example Compare Strings using Pointers

In this example, we will use string pointers and write a function to compare the two strings.

C Program

#include<stdio.h>
int compstring(char* s1, char* s2);
int main() {
	char s1[100], s2[100];
	int result;
	
	printf("Input a string1\n");
	gets(s1);
	
	printf("Input a string2\n");
	gets(s2);
	
	result = compstring(s1,s2);
	
	if (result == 0)
		printf("The strings are same.\n");
	else
		printf("The strings are different.\n");

	return 0;
}
 
int compstring(char *s1, char *s2) {
	while (*s1== *s2) {
		if (*s1 == '\0' || *s2 == '\0')
			break;
		s1++;
		s2++;
	}

	if (*s1 == '\0' && *s2== '\0')
		return 0;
	else
		return -1;
}

Conclusion

In this C Tutorial – Compare Strings, we have gone through the syntax of strcmp() inbuilt function and two other ways using recursion and pointers respectively with Example C Programs.