My run-time code isn't outputting anything?

**Thisgurl** · 12-05-2013

This program is supposed to sort an array of random numbers and print out the time it took for the sort to take place. I'm currently not getting any output. Help would be appreciated.

Code:

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

int swap,temp,i,j,s,a[];
float function_time;
clock_t start;
clock_t end;

int main()
{
 printf("Enter total number of elements: ");
 scanf("%d",&s);

 a[i] = rand()%10000;
 for(i=0;i<s;i++)
 {
  scanf("%d", &a[i]);
 }
 start = clock();
 for(i=1;i<s;i++)
 {
  temp=a[i];
  j=i;
  for(j=i-1;j>=0 && a[j]>temp; j--)
  {
   a[j+1]=a[j];
  }
  a[j+1]=temp;
 }
 
 end = clock();
 function_time = (float)(end-start)/(CLOCKS_PER_SEC); // Time in seconds
 printf("\nTime for Insertion Sort is %f seconds\n", function_time);
}

**Elkvis** · 12-05-2013

your code has lots of problems:

1. you never specify a size for your array, and it is assumed to have 1 element.
2. all of your variables are uninitialized, making line 15 particularly troublesome.
3. you use global variables for everything. why? you have one function, and no external code needs to access them. make them local to main.

**anduril462** · 12-05-2013

Originally Posted by Elkvis

your code has lots of problems:

1. you never specify a size for your array, and it is assumed to have 1 element.

GCC assumes 1 element in such cases. Do you know if that is guaranteed by the standard, and if so, in what section?

Originally Posted by Elkvis

2. all of your variables are uninitialized, making line 15 particularly troublesome.

Not quite. Since they're all global, and have static storage duration, thus are all initialized. They're just initialized to zero (except for the clock_t, whose initialization depends on how it's implemented)

Originally Posted by Elkvis

3. you use global variables for everything. why? you have one function, and no external code needs to access them. make them local to main.

Yes. Global variables are bad.

**Elkvis** · 12-05-2013

Originally Posted by anduril462

GCC assumes 1 element in such cases. Do you know if that is guaranteed by the standard, and if so, in what section?

I don't know what the standard says. I used GCC to compile it, and that's what it said. Just a guess, but it's probably implementation-defined or unspecified.

Not quite. Since they're all global, and have static storage duration, thus are all initialized. They're just initialized to zero (except for the clock_t, whose initialization depends on how it's implemented)

fair enough, but line 15 still has a problem, because depending on how the standard defines arrays like this, even accessing index zero may be undefined.

**laserlight** · 12-05-2013

Originally Posted by anduril462

GCC assumes 1 element in such cases. Do you know if that is guaranteed by the standard, and if so, in what section?

The type is an incomplete type:

Originally Posted by C99 Clause 6.2.5 Paragraph 22a

An array type of unknown size is an incomplete type. It is completed, for an identifier of that type, by specifying the size in a later declaration (with internal or external linkage).

Originally Posted by C99 Clause 6.7.5.2 Paragraph 4a

If the size is not present, the array type is an incomplete type.

Unless C11 changed something here, I believe that we are in the realm of undefined behaviour since the type is never completed yet the object is used. I reason that since the array has static storage duration, gcc automatically does something equivalent to:

Code:

int a[] = {0};

hence the corresponding warning about assuming that the array has one element.

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