CODEWITHSUNDEEP
c++arraysmath
anarchy99
anarchy99

Reputation: 1023

Why do I Get Different Results Using Array?

I've made a simple program that counts matrices, here's the code:

#include <iostream>
#include <math.h>
using namespace std;

int main()
{
int result[3] = {0,0,0};
int matrixc[3][6] = {
        {0,0,0,0,0,1},
        {0,0,0,1,1,1},
        {1,0,1,0,0,1}
        };
for(int x=0;x <3;x++)
{
    for(int y=0;y < 6;y++)
    {           
        result[x] += (matrixc[x][y] * pow(2,6-y));
    }
    cout << result[x] << endl;
}
}

The output is what I wanted, it is: 2,14,and 82. But, when I delete the initialization in the integer array of result:

#include <iostream>
#include <math.h>
using namespace std;

int main()
{
int result[3]; //deleted initialization
int matrixc[3][6] = {
        {0,0,0,0,0,1},
        {0,0,0,1,1,1},
        {1,0,1,0,0,1}
        };
for(int x=0;x <3;x++)
{
    for(int y=0;y < 6;y++)
    {           
        result[x] += (matrixc[x][y] * pow(2,6-y));
    }
    cout << result[x] << endl;
}
}

I got odd outputs: 1335484418,32618, and 65617.

Would you like to explain me why would the output be different between an array with and without an initialization?

Actually, I don't want to initialize all result array, because I have a huge data of matrices.

Is it possible if I use std::vector without initializing all of the result array?

Upvotes: 0

Views: 158

Answers (5)

Praetorian
Praetorian

Reputation: 109289

When you delete the initialization of the result array, those locations are initially set to unspecified, arbitrary values. You then add to this arbitrary value within the for loop, and end up those unexpected results. The results may be completely different the next time you run your program.

Also, since result[x] += ... reads from an uninitialized variable, deleting the initialization results in your code having undefined behavior.

If you switch over to using a vector you can zero initialize it as

std::vector<int> result(count);  // zero initialized with `count` elements

or

std::vector<int> result; // contains zero elements
result.resize(count);    // now it contains `count` elements, all set to 0

Upvotes: 1

Jonathan Wakely
Jonathan Wakely

Reputation: 171433

Would you like to explain me why would the output be different between an array with and without an initialization?

Seriously? If you don't initialize the array elements they are not initialized!

That means they contains junk data, so when you do result[x] += xxx; you are adding to junk, so you get more junk.

Actually, I don't want to initialize all "result" array, because I have a huge data of matrices.

Then you shouldn't rely on their initial value being zero.

You can do this:

int result[3] = { };  // initialize all elements to zero

Is it possible if I use std::vector without initializing all of the "result" array?

std::vector always initializes its members.

Upvotes: 1

Shafik Yaghmour
Shafik Yaghmour

Reputation: 158599

As I mentioned in my comment if you do not initialize result it will have undetermined values. Later on your are then adding a value to an unknown value which will still be an unknown value. In this situation you need to initialize your data, can zero initialize like so:

int result[3] = {} ;

Upvotes: 1

Mike Seymour
Mike Seymour

Reputation: 254731

Would you like to explain me why would the output be different between an array with and without an initialization?

Without initialisation, automatic variables aren't initialised. They will have an indeterminate value, depending on what happened to be in the memory they occupy.

Actually, I don't want to initialize all "result" array, because I have a huge data of matrices.

You can zero-initialise the whole array, even if it's huge, like this:

int result[huge] = {};

although, if it is huge, then it shouldn't be an automatic variable. These are typically kept on the stack, which is typically not huge and liable to overflow if you put too much stuff on it.

Is it possible if I use std::vector without initializing all of the "result" array?

Yes, a vector will zero-initialise its elements by default.

Upvotes: 6

juanchopanza
juanchopanza

Reputation: 227558

Without the initialization, the result array contains undetermined values, i.e. values that could be anything that fits into an int.

Upvotes: 1

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