[C program] convert gigabyte to byte

noir911 · 11-24-2010, 01:13 AM

I'm learning C and trying to write a program to convert gigabyte into byte. It will take 50% of the value given and then convert that value into byte. But the problem is the output goes into minus / negative value. Like, if I enter 22, I get output: -2147483648.

Code:

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

main (int argc, char* argv[])
{
   int gig;
   int byte;
   printf ("Enter gigabyte: ");
   scanf ("%d", &gig);
   byte = (gig*0.5)*1024*1024*1024;
   printf ("Byte size is: %d\n", byte);
   return 0;
}

Thanks.

H_TeXMeX_H · 11-24-2010, 02:41 AM

Use 'unsigned int' instead of 'int'. You may need to use 'unsigned long int' if numbers get really large. Make sure to printf "%u" for unsigned and "%l" for u long int.

http://www.cplusplus.com/reference/c...cstdio/printf/

Also, why 50% ? That doesn't really convert GB to bytes does it ?

noir911 · 11-24-2010, 03:12 AM

Thanks!

I'm now getting positive value. But the output I'm getting is still not right.

eg. 50% of 22GB converted in bytes is 11811160064. But my program is giving me 3221225472.

Code:

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

main (int argc, char* argv[])
{
   unsigned int gig;
   unsigned int byte;
   printf ("Enter gigabyte: ");
   scanf ("%u", &gig);
   byte = (gig*0.5)*1024*1024*1024;
   printf ("Byte size is: %u\n", byte);
   return 0;
}

Thanks.

Also, I'm taking 50% off as a part of the learning process.

H_TeXMeX_H · 11-24-2010, 03:37 AM

Yes, it is caused by integer overflow, do this:

Code:

#include <stdio.h>

int main (void)
{
   unsigned int gig;
   printf ("Enter gigabyte: ");
   scanf ("%u", &gig);
   printf ("Byte size is: %lu\n", (long)1024*1024*1024*gig/2);
   return 0;
}

johnsfine · 11-24-2010, 07:06 AM

Quote:

Originally Posted by H_TeXMeX_H

Use 'unsigned int' instead of 'int'. You may need to use 'unsigned long int' if numbers get really large.

On a 32 bit x86 architecture, you would need 'unsigned long long'.

On a 32 bit x86 architecture, 'unsigned long int' is still the same size as 'unsigned int'

Quote:

Originally Posted by noir911

convert gigabyte into byte.

You should be aware that "gigabyte" is often used to mean 10 to the ninth power, rather than the meaning you seem to intend: 2 to the thirtieth power. Many people observe the convention that GiB unambiguously means 2 to the thirtieth power bytes.

Quote:

It will take 50% of the value given and then convert that value into byte.

Was it luck or understanding that made you multiply by .5 rather than divide by 2?

Dividing by 2 would leave an integer result, so 23/2 would be 11 just like 22/2. Also an integer result would have just 32 bits when multiplied by 1024*1024*1024, so even if that result were stored in an unsigned long long, it would have already been truncated.

Multiplying an int by .5 creates a temporary of type double, which can be multiplied by 1024 three times with no truncation. Then all the truncation occurred when you stored that double into a 32 bit integer. That truncation would be avoided if you had stored into a long long.

Quote:

printf ("Byte size is: %d\n", byte);

But then printing a long long via printf is tricky. In fact I don't remember how you can do that.

You could use some ugly kludge such as

Code:

unsigned long long byte;
...
printf("Byte size is: %u%09u\n",
   (unsigned)( byte/1000000000 ),
   (unsigned)( byte%1000000000 ) );

Edit: GCC (at least version 3.4.6 that I tried) seems to support %lld for printing long long. Previous C compilers I have used didn't. So all you needed was:

Code:

unsigned long long byte;
...
printf("Byte size is: %lld\n", byte);

H_TeXMeX_H · 11-24-2010, 07:37 AM

I would use '%llu', because it is unsigned.

I did forget that int does differ between 32-bit and 64-bit, which is unfortunate. I was going to recommend using inttypes.h to just request a nice large type, but it may just complicate things.

Either way, my example does work, I have tested it (on 64-bit, but it should work on 32-bit too).