printf double with up to n decimals but not more as needed

Question

I want to trail the output of a double to 2 decimals or less. Consider following example:

#include <stdio.h>

int main() {
    printf( "%.2f\n", 2.    );
    printf( "%.2f\n", 2.5   );
    printf( "%.2f\n", 2.25  );
    printf( "%.2f\n", 2.125 );
}

The output is

2.00
2.50
2.25
2.12

Is there any way to print the values with less than 2 decimals, in case less are needed? I mean the desired output is

Answers with iomanip are allowed. I'm focusing on C++, not C but the example is c-only.

I know, that it is often not possible because double values are not precise, but in my examples they are because all fractional parts are powers of 2

Live example

You can have separate setprecision's if you know the outputs? But if you are trying to check if the next value is 0 then exclude it then it is not possible. — Omid CompSCI, Nov 08 '16 at 06:46
The example is constructed to be minimal. Please assume, that value is unknown but I don't want to print useless zeros at the end — meddle0106, Nov 08 '16 at 06:48
Not fantastic but sprintf result in a string and then right trim it, manually... — Djee, Nov 08 '16 at 06:50
@OmidCompSCI that's what one often expected. Pocket calculators never output trailing zeros — phuclv, Nov 08 '16 at 09:28

chqrlie · Answer 1 · 2016-11-08T14:29:26.140

2

If the absolute value is known to be between 0.006 and 999.995, you could use format %.3g:

#include <stdio.h>

int main() {
    printf( "%.3g\n", 2.    );
    printf( "%.3g\n", 2.5   );
    printf( "%.3g\n", 2.25  );
    printf( "%.3g\n", 2.125 );
}

Output is

But for the general case, trimming the string converted by snprintf() seems required:

void print_double_with_at_most_2_decimal_figures(double x) {
    char buf[400];
    int i = snprintf(buf, sizeof buf, "%.2f", x);
    while (i > 0 && buf[i - 1] == '0')
        buf[--i] = '\0';
    if (i > 0 && buf[i - 1] == '.')
        buf[--i] = '\0';
    fputs(buf, stdout);
}

edited Nov 08 '16 at 14:29

answered Nov 08 '16 at 07:17

chqrlie

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I usually use your 'general' case as well – not with such a counterintuitive function name, though :P – Jongware Nov 08 '16 at 09:29
1

@RadLexus: of course! I made the name a bit more specific ;-) – chqrlie Nov 08 '16 at 14:30

score -3 · Answer 2 · answered Nov 08 '16 at 06:53

-3

One way to do it is to take the value that is 2.00 and then assign it to a variable. This should take care of the extra 0's and give you the correct answer.

answered Nov 08 '16 at 06:53

Max Ho

17

1

No it won't!!!! – barak manos Nov 08 '16 at 06:58

printf double with up to n decimals but not more as needed

2 Answers2