Digit grouping in C's printf

Question

I wish to output large numbers with thousand-separators (commas or spaces) — basically the same as in How to display numeric in 3 digit grouping but using printf in C (GNU, 99).

If printf does not support digit grouping natively, how can I achieve this with something like printf("%s", group_digits(number))?

It must support negative integers and preferably floats, too.

Answer 1

A secure way to format thousand separators, with support for negative numbers:

Because VS < 2015 doesn't implement snprintf, you need to do this

#if defined(_WIN32)
    #define snprintf(buf,len, format,...) _snprintf_s(buf, len,len, format, __VA_ARGS__)
#endif

And then

char* format_commas(int n, char *out)
{
    int c;
    char buf[100];
    char *p;
    char* q = out; // Backup pointer for return...

    if (n < 0)
    {
        *out++ = '-';
        n = abs(n);
    }


    snprintf(buf, 100, "%d", n);
    c = 2 - strlen(buf) % 3;

    for (p = buf; *p != 0; p++) {
        *out++ = *p;
        if (c == 1) {
            *out++ = '\'';
        }
        c = (c + 1) % 3;
    }
    *--out = 0;

    return q;
}

Example usage:

size_t currentSize = getCurrentRSS();
size_t peakSize = getPeakRSS();


printf("Current size: %d\n", currentSize);
printf("Peak size: %d\n\n\n", peakSize);

char* szcurrentSize = (char*)malloc(100 * sizeof(char));
char* szpeakSize = (char*)malloc(100 * sizeof(char));

printf("Current size (f): %s\n", format_commas((int)currentSize, szcurrentSize));
printf("Peak size (f): %s\n", format_commas((int)currentSize, szpeakSize));

free(szcurrentSize);
free(szpeakSize);

Answer 2

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

char *commify(char *numstr){
    char *wk, *wks, *p, *ret=numstr;
    int i;

    wks=wk=strrev(strdup(numstr));
    p = strchr(wk, '.');
    if(p){//include '.' 
        while(wk != p)//skip until '.'
            *numstr++ = *wk++;
        *numstr++=*wk++;
    }
    for(i=1;*wk;++i){
        if(isdigit(*wk)){
            *numstr++=*wk++;
            if(isdigit(*wk) && i % 3 == 0)
                *numstr++ = ',';
        } else {
            break;
        }
    }
    while(*numstr++=*wk++);

    free(wks); 
    return strrev(ret);
}


int main(){
    char buff[64];//To provide a sufficient size after conversion.
    sprintf(buff, "%d", 100);
    printf("%s\n", commify(buff));
    sprintf(buff, "%d", 123456);
    printf("%s\n", commify(buff));
    sprintf(buff, "%.2f", 1234.56f);
    printf("%s\n", commify(buff));
    sprintf(buff, "%d", -123456);
    printf("%s\n", commify(buff));
    sprintf(buff, "%.2lf", -12345678.99);
    printf("%s\n", commify(buff));
    return 0;
}

ADD:

/*
char *strrev(char *str){
    char c,*front,*back;

    for(front=str,back=str+strlen(str)-1;front < back;front++,back--){
        c=*front;*front=*back;*back=c;
    }
    return(str);
}
*/

Answer 3

#include <stdio.h>

void punt(int n){
    char s[28];
    int i = 27;
    if(n<0){n=-n; putchar('-');} 
    do{
        s[i--] = n%10 + '0';
        if(!(i%4) && n>9)s[i--]=' ';
        n /= 10;
    }while(n);
    puts(&s[++i]);
}


int main(){

    int a;
    scanf("%d",&a);
    punt(a);

}

Answer 4

#include <stdio.h>

int main() {
    char str[50];
    int len = 0;   
    scanf("%48[^\n]%n", str, &len);

    int start = len % 3;

    for(int i = 0; i < len; i++) {        
        if(i == start && i != 0) {
            printf(" ");
        } else if((i - start) % 3 == 0 && i != 0) {
            printf(" ");
        }    
        printf("%c", str[i]);
    }   

   return 0;
}

Answer 5

Here is a compact way of doing it:

// format 1234567.89 -> 1 234 567.89
extern char *strmoney(double value){
    static char result[64];
    char *result_p = result;
    char separator = ' ';
    size_t tail;

    snprintf(result, sizeof(result), "%.2f", value);

    while(*result_p != 0 && *result_p != '.')
        result_p++;

    tail = result + sizeof(result) - result_p;

    while(result_p - result > 3){
        result_p -= 3;
        memmove(result_p + 1, result_p, tail);
        *result_p = separator;
        tail += 4;
    }

    return result;
}

For example, a call to strmoney(1234567891.4568) returns the string "1 234 567 891.46". You can easily replace the space with another separator (such as a comma) by changing the separator variable at the top of the function.

Answer 6

If you can use POSIX printf, try

#include <locale.h>
setlocale(LC_ALL, ""); /* use user selected locale */
printf("%'d", 1000000);