Program 264 : Replace the Substring with the given String

Program 264 : Replace the Substring with the given String

#include<stdio.h>
#include<string.h>
main()
{
int i,j,flag=0,len1,len2,replaceLen,start,end;
char str[100],substr[100],replace[20],temp[120];
printf("Enter a string\n");
gets(str);
printf("Enter a substring to be replaced with\n");
gets(substr);
printf("Enter String to replace\n");
gets(replace);
 len1=strlen(str);
len2=strlen(substr);
replaceLen=strlen(replace);
strcpy(temp,str);
for(i=0;i<=len1-len2;i++)
{
 start=i;
    for(j=i;j<i+len2;j++)
    {
        flag=1;
        if(str[j]!=substr[j-i])
        {
            flag=0;
            break;
        }
        else{
         end=j;
        }
    }
    if(flag==1)
    break;
}
if(flag==1)
{
    int k=0;
    if(len2==replaceLen){
       for(i=0;i<len1;i++)
       {
     if(i>=start && i<=end)
     {
      str[i]=replace[k];
      k++;
     }
       }
    }
    if(replaceLen-len2>0)
    {
     for(i=0;i<len1;i++)
       {
     if(i>=start && i<=end)
     {
      str[i]=replace[k];
      k++;
     }
       }
     k=0;
     int x;
     //Storing for continuing copy o remaining string in the original one
     x=end+1;
     
     //For adding the remaining string left in replace
     for(i=0;i<len1+(replaceLen-len2);i++)
     {
      if(i<=end)
      {
       temp[i]=str[i];
      }
      //from end of the substring to length of extra string in replace
      if(i>end && i<=end+(replaceLen-len2))
      {
        temp[i]=replace[len2+k];
     k++;
      }
      if(i>end+(replaceLen-len2))
      {
       temp[i]=str[x++];
      }
     }
     temp[i]='\0';
     strcpy(str,temp);
    }
    
    if(replaceLen-len2<0)
    {
        int rem=end+(replaceLen-len2);
      for(i=0;i<len1;i++)
       {
     if(i>=start && i<=rem)
     {
      str[i]=replace[k];
      k++;
     }
       }
     k=0;
     int x;
     //Storing for continuing copy o remaining string in the original one
     x=end+1;
     
     //For adding the remaining string left in replace
     for(i=0;i<len1+(replaceLen-len2);i++)
     {
      if(i<=end)
      {
       temp[i]=str[i];
      }
      
      if(i>end+(replaceLen-len2))
      {
       temp[i]=str[x++];
      }
     }
     temp[i]='\0';
     strcpy(str,temp);
    }
    printf("String After replacing is\n%s\n",str);
}
else
{
        printf("Entered Substring not Found\n");
}
}

Explanation:

//Coming Soon

Output: