Weird bug while working with arrays in c++

Question

I was writing a program in which i used a 5x5 array, and i actually came up with a bug.
In order to find it, I tried simplifying the program, and writing another one instead, in which i just wanted to simply show the numbers 1 to 25 using arrays.

#include <iostream>
#include <conio.h>
using namespace std;
main()
{
long int a[4][4];
int m=1;
for(int i=0;i<5;i++)
{for(int j=0;j<5;j++)
{a[i][j]=m;

m=m+1;
}
}

for(int i=0;i<5;i++)
{for(int j=0;j<5;j++)
{
cout<<a[i][j]<<" ";
}
cout<<endl;
}
getch();
}

And what i actually got was this:

1  2  3  4  6
6  7  8  9  11
11 12 13 14 16
16 17 18 19 21
21 22 23 24 25

However, when i tried a different thing and put a cout<<a[i][j]; after a[i][j]=m; and deleted the second part, i got it correct.
Am i missing something here?

Format your code properly. People are more likely to help when the code is readable. — digital_revenant
– digital_revenant, Commented Aug 24, 2013 at 13:43
"Am I missing something here?" -- Indentation. It's not clear what you're asking, what was your expected vs actual output before and after introducing the "deleted second part" change? — George Mitchell
– George Mitchell, Commented Aug 24, 2013 at 13:44

Benjamin Lindley · Accepted Answer · 2013-08-24 13:43:42Z

4

Your array is 4 by 4, but you are treating it as though it is 5 by 5. Your code has undefined behavior. Your loop should only count until i < 4 and j < 4, or you need to declare your array as long int a[5][5];.

By the way, main should have a return type, and it should be int. Anything else is non-standard.

answered Aug 24, 2013 at 13:43

Benjamin Lindley

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2 Comments

CODE Over a year ago

Well, when i say long int a[4][4]; doesn't it create an array with 25 cells? actually 0,1,2,3,4 are counted as five numbers. Isn't a[4][4] defined in this situation?

Benjamin Lindley Over a year ago

@CODE: No, it creates an array with 16 cells. When you create an array of size N, the last valid index is N-1. In your array, a[4][4] does not exist, and the last element is a[3][3].

Udit Bhardwaj · Accepted Answer · 2013-08-24 14:56:21Z

1

when you are declaring a[4][4] it is actually creating a 4X4 matrix that is a matrix that can contain maximum of 16 values, the numbers in the square brackets are to mention size of array. Size is given in as humans count like 1,2,3,4... and the index value from where the program starts storing the input starts from 0. so from here you can conclude that

a[ 1 ] (what we see or think) For computer is a[0]

a[2] (what we see or think) For computer is a[ 1 ]

a[3] (what we see or think) For computer is a[2]

and so on...

in your code change

    a[4][4]

to

    a[5][5]

answered Aug 24, 2013 at 14:56

Udit Bhardwaj

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Comments

Map X · Accepted Answer · 2013-08-24 13:45:39Z

0

long int a[4][4];

It's a 4x4 array.

answered Aug 24, 2013 at 13:45

Map X

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3 Comments

CODE Over a year ago

You mean if i use long int a[5][5]; i will have 25 five cell(as expected) and in this one i have 16?

Lee Taylor Over a year ago

Yes, indexes in C/C++ (and lots of other languages) begin at zero.

CODE Over a year ago

I knew they begin at 0, i actually didn't know they end at N-1.

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