how to handle pointer in for loop

Question

This throws an error id returned 1 error status. The loop is working but I cant print the final concatenated string. When i try to print the final string it does nothing.

char str1[50], str2[50], str[100];
char *p1, *p2, *p3;
int i = 0, j = 0;

p1 = &str1[0];
p2 = &str2[0];
p3 = &str[0];

printf("enter a string:");
gets(str1);

printf("enter a string:");
gets(str2);

while (i <= strlen(str1) + strlen(str2)) {
    if (i != strlen(str1)) {
        *(p3 + i) = *(p1 + i);
        i++;
    } else {
        *(p3 + i) = *(p2 + j);
        j++;
    }

Welcome to Stack Overflow! Why is the gets function so dangerous that it should not be used? — Sourav Ghosh
– Sourav Ghosh, Commented Apr 25, 2019 at 12:35
"but i cant print the final concatenated string" - why not show the code which does the printing? Also, are you adding a NUL terminator correctly? — Angew is no longer proud of SO
– Angew is no longer proud of SO, Commented Apr 25, 2019 at 12:36
Welcome to Stack Overflow! Instead of describing your code which did not work, can you show it? — Sourav Ghosh
– Sourav Ghosh, Commented Apr 25, 2019 at 12:36
Welcome to Stack Overflow! We need to see a minimal reproducible example for this problem. Also: idownvotedbecau.se/nomcve — Sourav Ghosh
– Sourav Ghosh, Commented Apr 25, 2019 at 12:37

Blaze · Accepted Answer · 2019-04-25 12:55:44Z

1

This check here is wrong:

if(i!=strlen(str1))

When i is bigger than strlen(str1) it will get false again, but it is supposed to stay true. Change it to this:

if (i < strlen(str1))

Furthermore, you're not increasing i in the else block. This is going to cause an endless loop with ever-growing j, at least until the undefined behavior will cause your program to exit, possibly through an access violation. Try this instead:

else {
    *(p3 + i) = *(p2 + j);
    i++; // increase i in both cases
    j++;
}

Or better yet, make it unconditional. For instance, you could take the i++ out of the body change the loop to this:

for (i = 0; i <= strlen(str1) + strlen(str2); i++)

Also, may I interest you in the array subscript notation? Instead of this:

if (i < strlen(str1)) {
    *(p3 + i) = *(p1 + i);
}

Do this:

if (i < strlen(str1)) {
    p3[i] = p1[i];
}

It's much cleaner this way and people who will work with your code will appreciate it.

edited Apr 25, 2019 at 12:55

answered Apr 25, 2019 at 12:42

Blaze

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4 Comments

Sourav Ghosh Over a year ago

// increase i in both cases..rather, make it unconditional.

machine_1 Over a year ago

And please do suggest the array subscript notation: p3[i] & p2[j].

Blaze Over a year ago

@SouravGhosh good idea. I edited the answer to include suggesting a for loop with an i++, I guess that would be a clean approach.

Blaze Over a year ago

@machine_1 Great suggestion, added it to the answer. I have to admit, looking at the *(p3 + i) = *(p1 + i) construct takes me some time to figure out what's going on.

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