Replacing one character with two in C

Question

I want to replace a character by two characters in my string.

void strqclean(const char *buffer)
{
  char *p = strchr(buffer,'?');
  if (p != NULL)
    *p = '\n';
}

int main(){
    char **quest;
    quest = malloc(10 * (sizeof(char*)));
    quest[0] = strdup("Hello ?");
    strqclean(quest[0]);
    printf(quest[0]);
    return;
}

This works fine, but in fact I want to replace my "?" by "?\n". strcat doesn't works with pointers is that right ? I can find a solution adding a character in my string and replace it by '\n', but that's not what I really want.

Thanks !

you can't 1) modify a `const` string and 2) add more data than the string could contain. — Jean-François Fabre, Jan 14 '18 at 21:51
Please post the [Minimal, Complete, and Verifiable example](http://stackoverflow.com/help/mcve) which shows what you have tried. The posted code won't compile (even when put in a wrapper). `*p = strchr(buffer,'?')` ==> `char *p = strchr(buffer,'?')` — Weather Vane, Jan 14 '18 at 21:52
Your code shouldn't work, `buffer` points to `const` string, you can change the pointer but not what's behind it. — Pablo, Jan 14 '18 at 21:52
@Pablo not necessarily since it's a parameter. But it's still _wrong_ — Jean-François Fabre, Jan 14 '18 at 21:52
@Jean-FrançoisFabre yes, thank you, I realized that after I pressed ENTER. — Pablo, Jan 14 '18 at 21:53
Note that `'\n'` is typically **one** character in a string, but **two** characters in the source code. — Weather Vane, Jan 14 '18 at 21:55
Don't do this: `printf(quest[0]);`, use `puts(quest[0])` instead — Pablo, Jan 14 '18 at 22:23

Pablo · Accepted Answer · 2018-01-14T23:18:54.727

EDIT

In your initial answer you mentioned that you wanted to append a newline after the ?, but now this reference is gone.

My first answer addressed this, but since it's gone, I'm not sure what you really want.

NEW ANSWER

You have to change your strqclean function

// remove the const from the parameter
void strqclean(char *buffer)
{
  char *p = strchr(buffer,'?');
  if (p != NULL)
    *p = '\n';
}

OLD ANSWER

strcat works with pointers, but strcat expects C-string and expects that the destination buffer has enough memory.

strcat allows you to connate strings. You can use than to append the \n if the ? character is always at the end of the string. If the character that you want to replace is in the middle, the you have to insert the character in the middle. For that you can use use memmove that allows you to move chunks for memory when the destination and source overlap.

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

int main(void)
{
    char line[1024] = "Is this the real life?Is this just fantasy";
    char *pos = strchr(line, '?');
    if(pos)
    {
        puts(pos);
        int len = strlen(pos);
        memmove(pos + 2, pos + 1, len);
        pos[1] = '\n';
    }
    puts(line);
    return 0;
}

Replacing one character with two in C

1 Answers1