Reinterpreting int as float

Question

I'm trying to create a float by setting its bits. I read here that probably the best way is using memcpy. That's the code I'm using

#include <stdio.h>
#include <string.h>

float i2f(int i) {
  float result;

  memcpy(&result, &i, sizeof(int));

  return result;
}

int main() {
  printf("Size of int is %d.\n", sizeof(int));
  printf("Size of float is %d.\n", sizeof(float));

  int i = 0x7fffff;
  float f = i2f(i);

  printf("int value is %d.\n", i);
  printf("float value is %f.\n", f);

  return 0;  
}

The output I get is

Size of int is 4.
Size of float is 4.
int value is 8388607.
float value is 0.000000.

Can someone explain why float value is zero ?

But float is a simple data type, and when it's value is returned, it will be copied. — Stephen Docy, May 10 '18 at 08:43
Because it is so close to zero. Values close to 1.0f are as big as 0x3f800000 as integers. — Aki Suihkonen, May 10 '18 at 08:44
to print the result of `sizeof` you [must use `%zu`](https://stackoverflow.com/q/940087/995714) — phuclv, May 10 '18 at 09:53
Stupid question probably, but what are you actually trying to achieve? — Mr Lister, May 10 '18 at 11:57
@Mr Lister you can see the float value for a given representation. — zzz_zzz, May 13 '18 at 11:26

score 3 · Answer 1 · answered May 10 '18 at 08:46

3

The bits of integer 0x7fffff encode a float value that is very small, less than 2⁻¹²⁶. The %f conversion specifier does not produce enough digits to show the value. Use %g or %e or specify many more digits, such as %.99f.

answered May 10 '18 at 08:46

Eric Postpischil

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Thanks, it worked, could you explain how did you know that it's so close to zero ? – zzz_zzz May 10 '18 at 08:59
@zzz_zzz http://www.binaryconvert.com/convert_float.html , https://en.wikipedia.org/wiki/IEEE_754 – Angew is no longer proud of SO May 10 '18 at 09:06
@zzz_zzz: [https://stackoverflow.com/a/6313398/298225](https://stackoverflow.com/a/6313398/298225). – Eric Postpischil May 10 '18 at 09:11

Reinterpreting int as float

1 Answers1