Remove extra white spaces in C++

Question

I tried to write a script that removes extra white spaces but I didn't manage to finish it.

Basically I want to transform abc sssd g g sdg gg gf into abc sssd g g sdg gg gf.

In languages like PHP or C#, it would be very easy, but not in C++, I see. This is my code:

#include <iostream>
#include <stdio.h>
#include <stdlib.h>
#include <cstring>
#include <unistd.h>
#include <string.h>

char* trim3(char* s) {
    int l = strlen(s);

    while(isspace(s[l - 1])) --l;
    while(* s && isspace(* s)) ++s, --l;

    return strndup(s, l);
}

char *str_replace(char * t1, char * t2, char * t6)
{
    char*t4;
    char*t5=(char *)malloc(10);
    memset(t5, 0, 10);
    while(strstr(t6,t1))
    {
        t4=strstr(t6,t1);
        strncpy(t5+strlen(t5),t6,t4-t6);
        strcat(t5,t2);
        t4+=strlen(t1);
        t6=t4;
    }

    return strcat(t5,t4);
}

void remove_extra_whitespaces(char* input,char* output)
{
    char* inputPtr = input; // init inputPtr always at the last moment.
    int spacecount = 0;
    while(*inputPtr != '\0')
    {
        char* substr;
        strncpy(substr, inputPtr+0, 1);

        if(substr == " ")
        {
            spacecount++;
        }
        else
        {
            spacecount = 0;
        }

        printf("[%p] -> %d\n",*substr,spacecount);

        // Assume the string last with \0
        // some code
        inputPtr++; // After "some code" (instead of what you wrote).
    }   
}

int main(int argc, char **argv)
{
    printf("testing 2 ..\n");

    char input[0x255] = "asfa sas    f f dgdgd  dg   ggg";
    char output[0x255] = "NO_OUTPUT_YET";
    remove_extra_whitespaces(input,output);

    return 1;
}

It doesn't work. I tried several methods. What I am trying to do is to iterate the string letter by letter and dump it in another string as long as there is only one space in a row; if there are two spaces, don't write the second character to the new string.

How can I solve this?

Answer 1

There are already plenty of nice solutions. I propose you an alternative based on a dedicated <algorithm> meant to avoid consecutive duplicates: unique_copy():

void remove_extra_whitespaces(const string &input, string &output)
{
    output.clear();  // unless you want to add at the end of existing sring...
    unique_copy (input.begin(), input.end(), back_insert_iterator<string>(output),
                                     [](char a,char b){ return isspace(a) && isspace(b);});  
    cout << output<<endl; 
}

Here is a live demo. Note that I changed from c style strings to the safer and more powerful C++ strings.

Edit: if keeping c-style strings is required in your code, you could use almost the same code but with pointers instead of iterators. That's the magic of C++. Here is another live demo.

Answer 2

Here's a simple, non-C++11 solution, using the same remove_extra_whitespace() signature as in the question:

#include <cstdio>

void remove_extra_whitespaces(char* input, char* output)
{
    int inputIndex = 0;
    int outputIndex = 0;
    while(input[inputIndex] != '\0')
    {
        output[outputIndex] = input[inputIndex];

        if(input[inputIndex] == ' ')
        {
            while(input[inputIndex + 1] == ' ')
            {
                // skip over any extra spaces
                inputIndex++;
            }
        }

        outputIndex++;
        inputIndex++;
    }

    // null-terminate output
    output[outputIndex] = '\0';
}

int main(int argc, char **argv)
{
    char input[0x255] = "asfa sas    f f dgdgd  dg   ggg";
    char output[0x255] = "NO_OUTPUT_YET";
    remove_extra_whitespaces(input,output);

    printf("input: %s\noutput: %s\n", input, output);

    return 1;
}

Output:

input: asfa sas    f f dgdgd  dg   ggg
output: asfa sas f f dgdgd dg ggg