从字符串C中删除字符

Question

So i start learning C from a book, and one of the exercises was to create a function that will take 2 string and delete from the first string the characters in the second string.

We stile didn't learn about pointer, so i guess this is possible without them, but when i try to run my code its crush.

The code:

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

char squis(char string[], char sub[])
{
    int i, c;
    char ret_string[strlen(string)];
    int map[strlen(string)];
    for(i=0; i<= strlen(string);i++)
        map[i] = -1;
    for(i=0; i<= strlen(sub);i++)
    {
        while(string[c]!='\0')
        {
            if (string[c]==sub[i])
                map[c] = c;
            c++;
        }
        c=0;
    }
    for(i=0; i<= strlen(string);i++)
    {
        if (map[i]==-1)
            ret_string[c++] = string[i];
    }
    ret_string[c] ='\0';
    return ret_string[0];
}

int main()
{
    char string[] = "string";
    char remove[] = "sasas";
    printf("%s",squis(string,remove));
    return 0;
}

I stile newbie in C, so I think the problem lay on my lack of understanding in the way that C work.

Thanks a lot for help :-)

Update: its seem the problem laying in the return in the end of the function. The function seems to work well when when i print ret_string inside the function(except one bug that make the function ignore the first char in the sub string, but i will deal with it later), but when i try to return the array to print it in the main function its fail.

There is specific rules for returning array in C?

Answer 1

Here is an obvious problem:

int map[strlen(string)];
for(i=0; i<= strlen(string);i++)
    map[i] = -1;

You create an array of strlen(string) characters, and then you initialize strlen(string) + 1 characters in the array. Writing out of bounds of an array leads to undefined behavior , where anything could happen.

Change the loop condition to less-than < . You should probably do it in all your loops.

You have a similar problem with ret_string , which will be the string you return. It's going to be at most strlen(string) characters, but then you need to add one character for the string terminator so the array needs to be strlen(string) + 1 long.

Then you have the problem that the squis function only return a single character but in your printf call you treat this single character as a string. This should make your compiler scream a warning at you. If you fix this by simply returning ret_string you will have another case of undefined behavior , as you then return a pointer to a local variable, which goes out of scope when the function exits, so the returned pointer is no longer valid. And if you decide to allocate ret_string on the heap, with the current call you have a memory leak as then the pointer is not saved so you can free the allocated memory.