Pow function does not work as expected in Code :: Blocks IDE
I have a question from my junior and I cannot fix it. Below is the code it uses in the Code :: Blocks IDE, just downloaded from the official Code :: Blocks site .
This is the hello world console console project, which he only modified slightly using a header file math.h
and using a pow()
function.
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
int main()
{
printf("Hello world! %d\n",pow(2,2));
return 0;
}
Should the output of this code be Hello world! 4
right? But voila, it’s always Hello world! 0
, if I don’t use printf("Hello world! %f\n", pow(2,2));
, which syntactically is, yes, the perfect and correct thing to do. But then that's a completely different story.
The Pow function should return 4 double
, of course. So what's going on? printf()
does not work correctly, or there is a problem with pow()
.
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printf("Hello world! %d\n",pow(2,2));
"The result of this code should be Hello world! 4 correct?"
Not. Since this behavior is undefined, anything can happen. Since @Mureinik posted what is likely to happen in this itinerant situation, you could understand why you saw 0. But in the end, C shouldn't be doing that way.
... If any argument is not the correct type for the corresponding conversion specification, the behavior is undefined .. C11 §7.21.6.1 9
Casting to is (int)
also a problem.
1) the int
range is much less than double
, and for pow(2,100)
that fails.
2) Casting does not int
truncate the fractional part by the result double
, and the accuracy is pow()
not defined. This will give an amazing result when pow()
sort of 7.99999999999999
and 7
printed rather than hoping for 8.000000
.
If your code needs a cardinality integer function, consider unsigned long long int pow or fooobar.com/questions/141070 / ... or search around.
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The return value pow()
is double
, as you can see here:
http://www.tutorialspoint.com/c_standard_library/c_function_pow.htm
So, you have to cast the return value to int like this:
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
int main()
{
printf("Hello world! %d\n",(int)pow(2,2));
return 0;
}
Otherwise, as you've seen, it's output 0!
Another example to show this can be tried:
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
int main()
{
printf("Hello world! %d\n",4.000000);
return 0;
}
As you will see, the output is also 0 because it is a double value!
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As you noted, it pow
returns double. printf
however is unaware of the types passed to it on the stack, and if you pass double
where expected int
, it will just take the first sizeof(int)
bytes from the stack and interpret them as int
. For example, on my machine (gcc (GCC) 4.4.7 20120313 (Red Hat 4.4.7-4) on x86_64), this program printed Hello world! -1954378952
.
If you want to treat the result as int
using %d
, you must explicitly specify it as such:
printf("Hello world! %d\n", (int)pow(2,2));
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pow(2,2)
will always return a double character.
The printf()
% d% and I both used to print int.Esli you use% d or% i to double, it would create problems. To display the double use of% f or% lf you will get the correct answer
printf("Hello world! %f\n",pow(2,2));
or
printf("Hello world! %lf\n",pow(2,2));
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