fprintf that uses constant space - c

Question

I have the problem with fprintf. What I want is to use as much of space as possible staying in the same column but that means changing amount of decimal floating numbers. I have a bit problem to describe it so maybe just example:
0.001234
0.123456
1.234567
12.34567
123.4567
I tried to play with g and f options but it didn't work for me (i.e. things like %8.6g or %8.6f).

Answer 1

This is a tricky to survive corner cases.
With different precision specifiers, the number of digits of the left side of the decimal point changes.
printf("%.3f", 9.996) --> "9.996";
printf("%.2f", 9.996) --> "10.00"; // Same print width with different precision.
Given the various rounding modes, +/- numbers, NaN, INF, and details of the typical binary representation of floating point numbers, attempting to predict the width a printf() for a given floating point number is, at best, an approximation. Rather than out-think printf(), simple sprintf() with a best guess precision. Use sprintf() return value and adjust the precision as needed.
Solution:
Estimate width.
Print to a buffer.
Adjust if needed.
.
#include <float.h>
int fprintf_Width(FILE *outf, double a, int width) {
// Subtract 2 for a potential leading "0."
int prec = width - 2;
if (prec < 0 || prec > 100 /* TBD upper bound */)
return 0; // `width` out of range
// By some means, determine the approximate precision needed.
char buf[1 + 1 + DBL_MAX_10_EXP + 1 + prec + 1];
int len = sprintf(buf, "%#.*f", prec, a);
prec -= len - width;
if (prec < 0)
return 0; // `a` too big
// Try it out
len = sprintf(buf, "%#.*f", prec, a);
// Adjust as needed
if (len > width) {
prec--;
if (prec < 0)
return 0; // `a` too big
sprintf(buf, "%#.*f", prec, a);
}
len = fprintf(outf, "%s", buf);
return len;
}

Answer 2

Acknowledgement: This approach fails with certain numbers with certain width-specifications, as pointed out in the comments.
One thing you could do is to calculate the amount of digits before the decimal point, and then use the printf Precision Specification to truncate the digits that would make the number too long.
You could create a custom function, taking the FILE pointer to use with fprintf, float nubmer that you want to print and the desired amount of characters you want to have; 3 arguments.
The next thing you should do is to determine the amount of digits there before the decimal point. Having a new int variable and putting the float you have into it, then dividing the new int by 10s until it is zero should do it. But beware, 0.001234 also has one digit before the decimal point, although its int evaluation would not show one.
Then all you'd have to do is to reduce the width received by the amount of digits before the decimal point, plus 1 for the decimal point itself (or rather minus in this case), and this shall be your Precision Specification for the fprintf you'll use.
So, in the end, your fprintf should look somewhat like this with this method:
fprintf( fp, "%.*f", Width - digitsbeforeDpoint - 1 );
Here is an example that I've done, working as far as I can test. It also has some further precautions, making sure that every input is reasonable and assuring that the float given is qualified to print out the amount of digits you'd desire:
#include <stdio.h>
void fixdLengthfprintf( FILE * _File, float a, int width ) {
if ( !_File ) return;
if ( width < 3 ) return;
int b = a;
do {
width--;
b /= 10;
} while ( b );
width--;
if ( width > 0 ) fprintf( _File, "%.*f\n", width, a );
}
int main( ) {
float numbers[7] = { // prints as:
1.123456, // 1.123456
12.1234567, // 12.12346 (apparently rounded)
123456.0123456, // 123456.0 (heavily rounded)
1234567.0123456, // doesn't get printed
12345678.0123456, // doesn't get printed
123456789.0123456, // doesn't get printed
0.123456789, // 0.123457
};
// I don't allow them to get printed because else they'd look like:
// 1.123456
// 12.12346
// 123456.0
// 1234567
// 12345678.000000
// 123456792.000000
// 0.123457
for ( int i = 0; i < 7; i++ ) fixdLengthfprintf( stdout, numbers[i], 8 );
fflush( stdin );
getchar( );
return 0;
}