How to parameterize precision of float on printf format

Question

I have a C program that output values to stdout from measure. I use printf to display them and the format allows me to change the precision

 printf("Scan=%d S_0=%.5f S_1=%.5f S_2=%.5f S_3=%.5f", scanId, S0, S1, S2, S3);

My issue is that I have a lot of printing like this one and if I want to change the precision quickly I have to get on each one and change it. I was wondering if it was possible to use something like a #define to have it on a precise location and it will apply on each printf at compilation. I know it is not possible easily without recompile but I'm ok with that. I know something similar as I ask here in python is:

f"{s_0:.{prec}f}"

No quick or easy or global-defaulty way, no. If you want to you can say int prec = 5; printf("Scan=%d S_0=%.*f S_1=%.*f S_2=%.*f S_3=%.*f", scanId, prec, S0, prec, S1, prec, S2, prec, S3);, but that may be more trouble than it's worth. — Steve Summit
– Steve Summit, Commented Feb 9, 2022 at 23:30

dbush · Accepted Answer · 2022-02-09 23:55:09Z

7

You can use a * symbol in place of the precision, in which case the precision is given as a parameter. Then you can define a constant to pass to printf.

#define P_FLOAT 5

printf("Scan=%d S_0=%.*f S_1=%.*f S_2=%.*f S_3=%.*f", 
       scanId, P_FLOAT, S0, P_FLOAT, S1, P_FLOAT, S2, P_FLOAT, S3);

answered Feb 9, 2022 at 23:55

dbush

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SGeorgiades · Accepted Answer · 2022-02-09 23:36:15Z

6

One way would be to do a #define (as you guessed) like this:

#define PFLT "%.5f"

printf("Scan=%d S_0=" PFLT " S_1=" PFLT " S_2=" PFLT " S_3=" PFLT, scanId, S0, S1, S2, S3);

Then to change the precision you just need to alter the #define and recompile.

answered Feb 9, 2022 at 23:36

SGeorgiades

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