How can I make a function to remove double letters in C?

Question

I am trying to make a function that removes double letters from a string. The function is only supposed to remove double letters next to each other, not in the whole string. e.g 'aabbaa' would become 'aba' (not 'ab'). Im a fairly new to c programming and dont fully understand pointers etc. and need some help. Below is what I have so far. It does not work at all, and I have no idea what to return since when I try and return string[] it has an error:

char doubleletter( char *string[] ) {
char surname[25];
int i;
for((i = 1) ; string[i] != '\0' ; i++) {

    if (string[i] == string[(i-1)]) {   //Supposed to compare the ith letter in array with one before
        string[i] = '\0' ;              //Supposed to swap duplicate chars with null
    }


}
surname[25] = string;

return surname ;

Answer 1

Your idea behind the code is right, but you are making two fundamental mistakes:

1. You return a char [] from a function that has char as return type. char [], char * and char are three different types, even though in this case char [] and char * would behave identically. However you would have to return char * from your function to be able to return a string.

2. You return automatically allocated memory. In other languages where memory is reference counted this is OK. In C this causes undefined behavior. You cannot use automatic memory from within a function outside this very function. The memory is considered empty after the function exits and will be reused, i.e. your value will be overwritten. You have to either pass a buffer in, to hold the result, or do a dynamic allocation within the function with malloc(). Which one you do is a matter of style. You could also reuse the input buffer, but writing the function like that is undesirable in any case where you need to preserve the input, and it will make it impossible for you to pass const char* into the function i.e. you would not be able to do do something like this:

const char *str = "abbc"; ... doubleletter(str,...);

If I had to write the function I would probably call it something like this:

int doubleletter (const char *in, size_t inlen, char *out, size_t outlen){
    int i;
    int j = 0;
    if (!inlen) return 0;
    if (!outlen) return -1;
    out [j++] = in[0];
    for (i = 1; i < inlen; ++i){
        if (in[i - 1] != in[i]){
            if (j > outlen - 1) return -1;
            out[j++] = in[i];
        }
    }
    out[j] = '\0';
    return j - 1;
}

int main(void) {
    const char *str1 = "aabbaa";
    char out[25];
    int ret = doubleletter(str1, strlen(str1), out, sizeof(out)/sizeof(out[0]));
    printf("Result: %s", out);
    return 0;
}

Answer 2

Try the following. It is a clear simple and professionally-looked code.:)

#include <stdio.h>

char * unique( char *s ) 
{
    for ( char *p = s, *q = s; *q++; )
    {
        if ( *p != *q ) *++p = *q;
    }

    return s;
}


int main(void) 
{
    char s[] = "aabbaa";

    puts( unique( s ) );

    return 0;
}

The output is

aba

Also the function can be rewritten the following way that to escape unnecassary copying.

char * unique( char *s ) 
{
    for ( char *p = s, *q = s; *q++; )
    {
        if ( *p != *q )
        {
            ( void )( ( ++p != q ) && ( *p = *q ) );
        }
    }

    return s;
}

Or

char * unique( char *s ) 
{
    for ( char *p = s, *q = s; *q++; )
    {
        if ( *p != *q && ++p != q ) *p = *q;
    }

    return s;
}

It seems that the last realization is the best.:)

Answer 3

One example, It does not modify input string and returns a new dynamically allocated string. Pretty self explanatory I think:

char *new_string_without_dups(const char *input_str, size_t len)
{
    int i = 1;
    int j = 0;
    char tmpstr[len+1] = {0};

    for (; i < len; i++) {
        if (input_str[i] == input_str[i-1]) {
            continue;
        }
        tmpstr[j] = input_str[i];
        j++;
     }

     return strdup(tmpstr);
}

Don't forget to free the returned string after usage.

Note that there are several ways to adapt/improve this. One thing now is that it requires C99 std due to array size not being known at compile time. Other things like you can get rid of the len argument if you guarantee a \0 terminated string as input. I'll leave that as excercises.

Answer 4

I would recommend using 2 indices to modify the string in-place:

void remove_doubles(char *str)
{

    // if string is 1 or 0 length do nothing.
    if(strlen(str)<=1)return; 

    int i=0;   //index (new string)
    int j=1;   //index (original string)


    // loop until end of string
    while(str[j]!=0)
    {
        // as soon as we find a different letter,
        // copy it to our new string and increase the index.
        if(str[i]!=str[j])
        {
            i++;
            str[i]=str[j];
        }

        // increase index on original/old string
        j++;
    }

    // mark new end of string
    str[i+1]='\0';

}

Answer 5

First of all delete those parenthenses aroung i = 1 in for loop (why you put them there in the first place ?
Secondly if you put \0 in the middle of the string, the string will just get shorter.
\0 terminates array (string) in C so if you have:
ababaabababa
and you replace second 'a' in pair with \0:
ababa\0baba
effectively for compiler it will be like you just cut this string to:
ababa

Third error here is probably that you are passing two-dimensional array to function here: char *string[] This is equivalent to passing char **string and essentialy you are passing array of strings while you wanna only to pass a string (which means a pointer, which means an array: char *string or ofc char string[])

Next thing: you are making internal assumption that passed string will have less than 24 chars (+ \0) but you don't check it anywhere.

I guess easiest way (though maybe not the most clever) to remove duplicated chars is to copy in this for loop passed string to another one, omitting repeated characters.