Why is printf not using scientific notation?

Question

I understand that this is a common problem. However I can't find a solid straight answer.

16 ^ 54 = 1.0531229167e+65 (this is the result I want)

When I use pow(16,54), I get:

105312291668557186697918027683670432318895095400549111254310977536.0

Code is as follows:

#include <stdio.h>
#include <math.h>
#include <stdlib.h>

void main(){

   double public;
   double a = 16;
   double b = 54;
   public = (pow(a,b));

   printf("%.21f\n", public);
}

Code executed with:

gcc main.c -lm

What I'm doing wrong?

Your code is exhibits UB (*`main()`* returns an *`int`* - not a *`void`* or anything else no matter what appears to 'work' no matter how many times or how many systems it's run on). ...which I see the answer now points out. Good. Anyway UB is bad news but fortunately this one is easy to fix. — Pryftan, Oct 15 '19 at 12:39

score 33 · Accepted Answer · answered Jan 22 '14 at 16:10

33

What am I doing wrong?

Several things:

Use %.10e format for scientific notation with printf for a printout with ten digits after the dot,
Return an int from your main,
Consider not using public to name a variable, on the chance that your program would need to be ported to C++, where public is a keyword.

Here is how you can fix your program:

#include <stdio.h>
#include <math.h>
#include <stdlib.h>

int main(){

   double p;
   double a = 16;
   double b = 54;
   p = (pow(a,b));

   printf("%.10e\n", p);
   return 0;
}

Demo on ideone.

answered Jan 22 '14 at 16:10

Sergey Kalinichenko

714,442
84
1,110
1,523

1

`public` is not a keyword in C. :) Although its use will be problematic if this code ever moves to C++ – abelenky Jan 22 '14 at 16:11
@abelenky You're right, I missed the tag. Thanks for the correction! – Sergey Kalinichenko Jan 22 '14 at 16:15

score 13 · Answer 2 · answered Jan 22 '14 at 16:09

13

Have you tried:

printf("%e\n", public);

The %e specifier is for scientific notation, as described in the documentation

answered Jan 22 '14 at 16:09

abelenky

63,815
23
109
159

Shafik Yaghmour · Answer 3 · 2014-01-22T16:17:23.870

4

If you need scientific notation you need to use the %e format specifier:

printf("%e\n", public);
        ^^

Also, public is a keyword in C++ and so it would be a good idea to avoid that and any other keywords in case this code needs to be portable.

edited Jan 22 '14 at 16:17

answered Jan 22 '14 at 16:11

Shafik Yaghmour

154,301
39
440
740

1

Amusing puns: *public* [is a] *keyword* (and *public* is in the ghastly C++), *keywords* in *case* (also a keyword in both C and C++ and others too). It's a great idea though to write code with C++ names just to prevent the C++ conversions! But you're right that if there is any chance there will be a desire to port it then better not do that. – Pryftan Oct 15 '19 at 12:48

Why is printf not using scientific notation?

3 Answers3

Linked