Strip whitespace from a string in-place?

Question

I saw this in a \"list of interview questions\". Got me wondering.

Not limited to whitespace necessarily, of course, easily generalized to \"removing some specific char

Answer 1

First of all, i<strlen(str) is always an inefficient idiom for looping over a string. The correct loop condition is simply str[i], i.e. loop until str[i] is the null terminator.

With that said, here's the simplest/most concise algorithm I know:

for (size_t i=0, j=0; s[j]=s[i]; j+=!isspace(s[i++]));

Note: My solution is for the question as written in the subject (whitespace) as opposed to the body (particular character). You can easily adapt it if needed.

Answer 2

C doesn't have default arguments, and if you're programming in C++ you should use std::string and remove_if from <algorithm>.

You definitely can make this more efficient, by eliminating the calls to strlen, which are turning an O(N) algorithm into an O(N²) algorithm, and are totally unnecessary -- you're scanning the string anyway, so just look for the NUL yourself.

You can also make this more C-idiomatic, by using two pointers instead of array indexing. I'd do it like this:

void strip_char(char *str, char strip)
{
    char *p, *q;
    for (q = p = str; *p; p++)
        if (*p != strip)
            *q++ = *p;
    *q = '\0';
}

Answer 3

void prepend(char* s,char ch){
    int len = strlen(s);
    memmove(s, s + 1, len - 1);
    s[len - 1] = '\x0';
}


void RemoveWhitespace(char* InStr, char ch){
     int n(0);

if (InStr == NULL){
    return;
}
else if ((*InStr) == '\x0'){
    return;
}   
else if ((*InStr) != ch){
    RemoveWhitespace(InStr + 1,ch);
}
else{
    while ((*InStr) == ch){
        prepend(InStr,InStr[0]);
        n++;
    }
    RemoveWhitespace(InStr + n,ch);
}
}