C program to compare two strings without using string function(strcmp)

This C program is to compare two strings without using string function(strcmp).For example, str1=”code” and str2=”code” then on comparing we find that the two strings are equal.

Logic

We first check if both their lengths are equal.If strings are not equal, we need not go further with computation, thus increasing the efficiency of the program.If they are equal then we compare each string character by character.

Dry Run of the program

Take input str1 and str2.Let us take str1=code and str2=code

flag is set to 1 i.e. flag=1;

len1=strlen(str1) hence len1=4

len2=strlen(str2) hence len2=4

if(len1==len2)  i.e if(4==4) true

1st iteration for(i=0;i<len1;i++)   i .e.   for(i=0;0<4;i++)

if(str1[i]!=str2[i]) i.e if(str1[0]!=str2[0])  i.e if(c!=c) false

2nd iteration for(i=1;i<len1;i++)   i .e.   for(i=1;1<4;i++)

if(str1[i]!=str2[i]) i.e if(str1[1]!=str2[1])  i.e if(o!=o) false

3rd iteration for(i=2;i<len1;i++)   i .e.   for(i=2;2<4;i++)

if(str1[i]!=str2[i]) i.e if(str1[2]!=str2[2])  i.e if(d!=d) false

4th iteration for(i=3;i<len1;i++)   i .e.   for(i=3;3<4;i++)

if(str1[i]!=str2[i]) i.e if(str1[3]!=str2[3])  i.e if(e!=e) false

For loop ends here as i=4 which is not less than len1(4)

if(flag==1)  true

Hence the two strings are equal.

Program

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

void main()
{
    char str1[100],str2[100];
    int i,flag=1,len1,len2;

    printf("Enter string str1\n");
    gets(str1);
    
    printf("Enter string str2\n");
    gets(str2);
    
    len1=strlen(str1);
    len2=strlen(str2);
	
    if(len1==len2)
    {	
    	for(i=0;i<len1;i++)
    	{
        	if(str1[i]!=str2[i])
        	{
            		flag=0;
	                break;
		}
        }
    }
 
    else
    {
    	flag=0; 
    }	

    if(flag==1)
    {
        printf("The two string are EQUAL!!!\n");
        printf("%s(str1) = %s(str2)",str1,str2);
    }

    else
    {
        printf("The two string are NOT EQUAL!!!\n");
        printf("%s(str1) != %s(str2)",str1,str2);
    }
}

Output