string format for printf floating point values

Question

I have a question about using printf.

char str[8];
float val = 2.334563;
sprintf(str, format, val);
printf("val = %s.\n", str);

val = -23.34563;
sprintf(str, format, val);
printf("val = %s.\n", str);

val = -0.02334563;
sprintf(str, format, val);
printf("val = %s.\n", str);

val = 233;
sprintf(str, format, val);
printf("val = %s.\n", str);

The expected output follows:

val = +2.3345
val = -23.345
val = -0.0233
val = +233.00

What format string do I need for that? Thank you for your attention.

Alok Save · Accepted Answer · 2011-10-30 15:34:01Z

3

what happened to the good old %f

answered Oct 30, 2011 at 15:34

Alok Save

207k56 gold badges448 silver badges539 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Joe McGrath Over a year ago

Does not get desired output. His question should of asked more directly.

Alok Save Over a year ago

@JoeMcGrath: This Q clearly smells of Homework & I do not appreciate writing any form of code for Homework Questions other than giving pointers or ideas. Reason: I do not want an programmer sitting right beside me(Albeit a few yrs from now) who cannot write a source code given a hint.

Joe McGrath Over a year ago

Ok thanks. I am new to Stack Overflow and did not realize. Just trying to answer questions. I am a hobbyist programmer who hasn't been to school. Did not think of homework assignments. Will remember to do this in future.

Alok Save Over a year ago

@JoeMcGrath: Yes,please.I understand your enthusiasm and willingness to help but We do not really want to turn SO in to a Do My Homework Website.Usually,for Q's which smell of homework We would like to see what the OP has tried,their efforts and their ideas to solve the problem.Then provide subtle hints/clues which will give them a direction and unblock them but also leaves them enough sizeable homework to do.As a hobbyist programmer you would sure know,there is no alternative to hard work.Welcome to SO and See you around.All the Best :)

Shamim Hafiz - MSFT · Accepted Answer · 2011-10-30 15:36:37Z

1

"%f"

example

printf("%f\n", floatVal);

answered Oct 30, 2011 at 15:36

Shamim Hafiz - MSFT

22.2k44 gold badges122 silver badges183 bronze badges

2 Comments

user1020803 Over a year ago

I need fixed size output string.

Bill Lynch Over a year ago

nor will "%f" show the + sign in front of positive numbers.

Praetorian · Accepted Answer · 2011-10-30 15:55:39Z

1

The following (almost) does what you want. Note that I changed the number of characters in the str array from 7 to 8; since all of your output strings contain 7 characters the NULL termination performed by sprintf will cause buffer overflow otherwise.

The only difference between my results and yours is the rounding performed by sprintf. AFAIK, the only way to get around this is to pre-truncate the number you want to print using floor; for example, to print 2 digits without rounding float f = floor( 1.8888 * 100 ) / 100;

#include <stdio.h>

int main(void)
{
  char str[8];

  {
    float val = 2.334563f;
    sprintf(str, "%+6.*f", 4, val);
    printf("val = %s.\n", str);
  }

  {
    float val = -23.34563f;
    sprintf(str, "%+6.*f", 3, val);
    printf("val = %s.\n", str);
  }

  {
    float val = -0.02334563f;
    sprintf(str, "%+6.*f", 4, val);
    printf("val = %s.\n", str);
  }

  {
    float val = 233.0f;
    sprintf(str, "%+6.*f", 2, val);
    printf("val = %s.\n", str);
  }

  return 0;
}

Output:

val = +2.3346.
val = -23.346.
val = -0.0233.
val = +233.00.

answered Oct 30, 2011 at 15:55

Praetorian

110k20 gold badges248 silver badges340 bronze badges

1 Comment

Joe McGrath Over a year ago

This requires calculating needed number of decimal places manually in code. what is the way to do this with code for any case?

Joe McGrath · Accepted Answer · 2011-10-30 16:48:44Z

1

use snprintf() to truncate string at exactly 8 characters including \0

Format string:

"%#+.5f"

%   modifier
#   force decimal place
+   show + or -
.5  precision of 5
f   use precision as places after decimal

Code:

char str[8];
float val = 2.334563;
snprintf(str,8, "%#+.5f", val);
printf("val = %s\n", str);

val = -23.34563;
snprintf(str,8, "%#+.5f", val);
printf("val = %s\n", str);

val = -0.02334563;
snprintf(str,8, "%#+.5f", val);
printf("val = %s\n", str);

val = 233.001;
snprintf(str,8, "%#+.5f", val);
printf("val = %s\n", str);

Output:

val = +2.3345
val = -23.345
val = -0.0233
val = +233.00
val = +0.0003

edited Oct 30, 2011 at 16:48

answered Oct 30, 2011 at 15:36

Joe McGrath

1,4991 gold badge10 silver badges26 bronze badges

2 Comments

user1020803 Over a year ago

I think snprintf(str, 8, "%#+.5f", 0.0003); best of all

Joe McGrath Over a year ago

@user1020803 That does work in all cases. Got lost in all my changes not sure why i was still using sig figs after i switched to snprintf(). Thanks. edited to reflect.

Bill Lynch · Accepted Answer · 2011-10-30 15:46:28Z

0

The only thing that I can come up with is as follows:

const char *format = "val = %+6.*f\n";
int places_past_decimal = ???;
printf(format, places_past_decimal, 2.334563f);

In this case, you will need to pass an additional argument (places_past_decimal) which is the number of digits remaining in the number that aren't used by the left side of the number.

answered Oct 30, 2011 at 15:46

Bill Lynch

82.3k16 gold badges132 silver badges181 bronze badges

1 Comment

user1020803 Over a year ago

I thought about it, but hoped for the best solution.

Collectives™ on Stack Overflow

string format for printf floating point values

5 Answers 5

4 Comments

2 Comments

1 Comment

2 Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

4 Comments

2 Comments

1 Comment

2 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related