Replace a word in string

Question

How do i make below program work properly, The main problem i have seen so far is str1 is not defined properly which may be the real cause for the program not working properly.

#include<stdio.h>
#include<string.h>
int main()
{
    char string[]="We will rock you";
    char s1[10],s2[10];
    printf("Enter string 1 ");
    gets(s1);
    printf("Enter string 2 ");
    gets(s2);
    int start,end,compare;
    for(int i=0;string[i]!='\0';i++)
     if(string[i]==s1[0])
     {
        start=i;
        break;
     }
    //printf("%d",start);
    end=start+strlen(s1);
    //printf("\n%d",end);
    char str1[30],check[10];

//Defining string 1
    for(int i=0;i<start;i++)
     str1[i]=string[i];
    //printf("\n%sd",str1);
    
//Defining check
    for(int i=start;i<end;i++)
     check[i-start]=string[i];
     
    //printf("\n%s\n",check,str1);
    compare=strcmp(check,s1);
    //printf("\n%d",compare);
    
    if(compare==0)
     strcat(str1,s1);
     printf("\n%s",str1);
     
    for(int i=end,j=strlen(str1);i<strlen(string);i++)
    {
        str1[j]=string[i];
    }
    strcpy(string,str1);
    
    printf("\n%s",string);
}

I know this is not the best way to do it, it has so many loopholes as it wont work for words appearing again and it may also change words like (ask, task or asking) if str1 is given ask. But still help me , What am i doing wrong???

Answer 1

What am i doing wrong???

For starters the function gets is unsafe and is not supported by the C Standard. Instead either use scanf or fgets.

If in this for loop

int start,end,compare;
for(int i=0;string[i]!='\0';i++)
 if(string[i]==s1[0])
 {
    start=i;
    break;
 }

the condition string[i]==s1[0] does not evaluate to true then the variable start will have an indeterminate value because it is not initialized and all the subsequent code after the for loop invokes undefined behavior because there is used the uninitialized variable start.

If the condition evaluates to true then the value of end

end=start+strlen(s1);

can be larger than the length of the original string string. That again can invoke undefined behavior in this for loop

for(int i=0;i<start;i++)
 str1[i]=string[i];

After this for loop

for(int i=start;i<end;i++)
 check[i-start]=string[i];
 
//printf("\n%s\n",check,str1);
compare=strcmp(check,s1);

the array check does not contain a string. So calling the function strcmp also invokes undefined behavior.

It seems that in this call there is at least a typo.

if(compare==0)
 strcat(str1,s1)

it seems you mean

strcat( str1, s2 );
              ^^^ 

If s1 was not found in string then this loop

for(int i=end,j=strlen(str1);i<strlen(string);i++)
{
    str1[j]=string[i];
}

just does not make a sense.

Pay attention to that in general the length of s2 can be greater than the length of s1. In this case you may not change s1 to s2 within string declared like

char string[]="We will rock you";

because that results in accessing memory outside the array.