CODEWITHSUNDEEP
cgcc
kay dee
kay dee

Reputation: 53

Out of bound array elements being printed using gcc in Win8

I'm compiling the following code in my m/c using codeblocks and mingw32-gcc.exe v.4.8.1(tdm-2). 

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

int main()
{
    int a[8] = {3,1,10,-1,7,2,-8,1};
    int b[8] = {3,1,10,-1,7,2,-8,1};
    int i=0,alength = (int)sizeof(a)/sizeof(a[0]);

    for(i = 0; i < alength; i++){
            if(a[i] > a[i+1]){
                a[i+1] = a[i] + a[i+1];
                a[i] = a[i+1] - a[i];
                a[i+1] = a[i+1] - a[i];
            }
    }

    printf("Original array size is %d \n", alength);
    printf("Original array elements are given below: \n");
    printf("Sizeof(a) is %d \n", sizeof(a));
    for(i = 0; i < alength; i++){
        printf("\n b[%d] = %d", i, b[i]);
    }

    for(i = 0; i < alength; i++){
        printf("\na[%d] = %d", i, a[i]);
    }
}

It's giving me the following output. Here for each array, two extra elements are being printed. 

Original array size is 10 
Original array elements are given below: 
Sizeof(a) is 32 

 b[0] = 3
 b[1] = 1
 b[2] = 10
 b[3] = -1
 b[4] = 7
 b[5] = 2
 b[6] = -8
 b[7] = 1
 b[8] = 1
 b[9] = 3
a[0] = 1
a[1] = 3
a[2] = -1
a[3] = 7
a[4] = 2
a[5] = -8
a[6] = 1
a[7] = 8
a[8] = 10
a[9] = 9

But compiling the same code in online compiler gives the correct output. OS is Win8 64bit. Any help in explaining and resolving the difference would be appreciated..

Upvotes: 1

Views: 78

Answers (2)

haccks
haccks

Reputation: 106092

Accessing out of bound array element invokes undefined behavior and this will result in any expected or unexpected result.

Change 

 for(i = 0; i < alength; i++){
        if(a[i] > a[i+1]){ ...
        ...
  }

to 

 for(i = 0; i < alength-1; i++){
        if(a[i] > a[i+1]){ ...
        ...
 }

Upvotes: 5

Blindy
Blindy

Reputation: 67439

What you have is the very definition of undefined behaviour, specifically the one pass bubble sort you're trying to write. i goes from 0 to alength-1, covering the entire array, then you index a[i+1], which is going to be past the end.

If I had to guess specifically what's causing the difference, one machine is building in debug mode introducing a sentinel between a and b, so in one instance you're comparing a[7] with either b[0] (going to swap) or with a[8] (the sentinel, which is usually a large number so not going to swap).

TL;DR: Undefined behaviour.

Upvotes: 4

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