I keep stumbling on the format specifiers for the printf() family of functions. What I want is to be able to print a double (or float) with a maximum given number of digits
This can't be done with the normal printf
format specifiers. The closest you could get would be:
printf("%.6g", 359.013); // 359.013
printf("%.6g", 359.01); // 359.01
but the ".6" is the total numeric width so
printf("%.6g", 3.01357); // 3.01357
breaks it.
What you can do is to sprintf("%.20g")
the number to a string buffer then manipulate the string to only have N characters past the decimal point.
Assuming your number is in the variable num, the following function will remove all but the first N
decimals, then strip off the trailing zeros (and decimal point if they were all zeros).
char str[50];
sprintf (str,"%.20g",num); // Make the number.
morphNumericString (str, 3);
: :
void morphNumericString (char *s, int n) {
char *p;
int count;
p = strchr (s,'.'); // Find decimal point, if any.
if (p != NULL) {
count = n; // Adjust for more or less decimals.
while (count >= 0) { // Maximum decimals allowed.
count--;
if (*p == '\0') // If there's less than desired.
break;
p++; // Next character.
}
*p-- = '\0'; // Truncate string.
while (*p == '0') // Remove trailing zeros.
*p-- = '\0';
if (*p == '.') { // If all decimals were zeros, remove ".".
*p = '\0';
}
}
}
If you're not happy with the truncation aspect (which would turn 0.12399
into 0.123
rather than rounding it to 0.124
), you can actually use the rounding facilities already provided by printf
. You just need to analyse the number before-hand to dynamically create the widths, then use those to turn the number into a string:
#include <stdio.h>
void nDecimals (char *s, double d, int n) {
int sz; double d2;
// Allow for negative.
d2 = (d >= 0) ? d : -d;
sz = (d >= 0) ? 0 : 1;
// Add one for each whole digit (0.xx special case).
if (d2 < 1) sz++;
while (d2 >= 1) { d2 /= 10.0; sz++; }
// Adjust for decimal point and fractionals.
sz += 1 + n;
// Create format string then use it.
sprintf (s, "%*.*f", sz, n, d);
}
int main (void) {
char str[50];
double num[] = { 40, 359.01335, -359.00999,
359.01, 3.01357, 0.111111111, 1.1223344 };
for (int i = 0; i < sizeof(num)/sizeof(*num); i++) {
nDecimals (str, num[i], 3);
printf ("%30.20f -> %s\n", num[i], str);
}
return 0;
}
The whole point of nDecimals()
in this case is to correctly work out the field widths, then format the number using a format string based on that. The test harness main()
shows this in action:
40.00000000000000000000 -> 40.000
359.01335000000000263753 -> 359.013
-359.00999000000001615263 -> -359.010
359.00999999999999090505 -> 359.010
3.01357000000000008200 -> 3.014
0.11111111099999999852 -> 0.111
1.12233439999999995429 -> 1.122
Once you have the correctly rounded value, you can once again pass that to morphNumericString()
to remove trailing zeros by simply changing:
nDecimals (str, num[i], 3);
into:
nDecimals (str, num[i], 3);
morphNumericString (str, 3);
(or calling morphNumericString
at the end of nDecimals
but, in that case, I'd probably just combine the two into one function), and you end up with:
40.00000000000000000000 -> 40
359.01335000000000263753 -> 359.013
-359.00999000000001615263 -> -359.01
359.00999999999999090505 -> 359.01
3.01357000000000008200 -> 3.014
0.11111111099999999852 -> 0.111
1.12233439999999995429 -> 1.122
Your code rounds to three decimal places due to the ".3" before the f
printf("%1.3f", 359.01335);
printf("%1.3f", 359.00999);
Thus if you the second line rounded to two decimal places, you should change it to this:
printf("%1.3f", 359.01335);
printf("%1.2f", 359.00999);
That code will output your desired results:
359.013
359.01
*Note this is assuming you already have it printing on separate lines, if not then the following will prevent it from printing on the same line:
printf("%1.3f\n", 359.01335);
printf("%1.2f\n", 359.00999);
The Following program source code was my test for this answer
#include <cstdio>
int main()
{
printf("%1.3f\n", 359.01335);
printf("%1.2f\n", 359.00999);
while (true){}
return 0;
}
I search the string (starting rightmost) for the first character in the range 1
to 9
(ASCII value 49
-57
) then null
(set to 0
) each char right of it - see below:
void stripTrailingZeros(void) {
//This finds the index of the rightmost ASCII char[1-9] in array
//All elements to the left of this are nulled (=0)
int i = 20;
unsigned char char1 = 0; //initialised to ensure entry to condition below
while ((char1 > 57) || (char1 < 49)) {
i--;
char1 = sprintfBuffer[i];
}
//null chars left of i
for (int j = i; j < 20; j++) {
sprintfBuffer[i] = 0;
}
}
Some of the highly voted solutions suggest the %g
conversion specifier of printf
. This is wrong because there are cases where %g
will produce scientific notation. Other solutions use math to print the desired number of decimal digits.
I think the easiest solution is to use sprintf
with the %f
conversion specifier and to manually remove trailing zeros and possibly a decimal point from the result. Here's a C99 solution:
#include <stdio.h>
#include <stdlib.h>
char*
format_double(double d) {
int size = snprintf(NULL, 0, "%.3f", d);
char *str = malloc(size + 1);
snprintf(str, size + 1, "%.3f", d);
for (int i = size - 1, end = size; i >= 0; i--) {
if (str[i] == '0') {
if (end == i + 1) {
end = i;
}
}
else if (str[i] == '.') {
if (end == i + 1) {
end = i;
}
str[end] = '\0';
break;
}
}
return str;
}
Note that the characters used for digits and the decimal separator depend on the current locale. The code above assumes a C or US English locale.
What about something like this (might have rounding errors and negative-value issues that need debugging, left as an exercise for the reader):
printf("%.0d%.4g\n", (int)f/10, f-((int)f-(int)f%10));
It's slightly programmatic but at least it doesn't make you do any string manipulation.
Why not just do this?
double f = 359.01335;
printf("%g", round(f * 1000.0) / 1000.0);