Segmentation fault when passing a pointer to an array into a function (C++)

Question

I'm trying to get a large array of random numbers, then pass that array into a function that will create a new array with 10 elements whose values correspond with the values in the large array. I pass the parameter start into the function to indicate the index of the large array from which I will start copying the values into the smaller array.

In case my explanation isn't clear, I have an array A of 20 randomly generated numbers:
A = {1, 2, 3, .... 20}

I want to make an array B that holds the first 10 numbers:
B = {1, 2, 3, .... 10}

and an array C that holds the second 10:
C = {11, 12, 13, .... 20}

I'm able to generate the random numbers and display them with a for loop, but once I start trying to make the new arrays I get a segmentation fault in terminal:
Segmentation fault (core dumped)

Below is my code.

int main() {

    int size = 20;
    int *arrA = getRandomScores(size);

    int *arrB = applyScores(arrA, 0);
    int *arrC = applyScores(arrA, 10);

    for(int i = 0; i < 10; i++) {
        cout << arrB[i] << endl;
    }
    cout << endl << endl;
    for(int i = 0; i < 10; i++) {
        cout << arrC[i] << endl;
    }

    return 0;
}

int *applyScores(int *arr, int start) {
    int *newArr;

    for(int i = 0; i < 10; i++) {
        newArr[i] = arr[start];
        start++;
    }    

    return newArr;
}

int *getRandomScores(int size) {
    int *arr;

    //return null if size is zero or negative
    if (size <= 0)
        return NULL;

    //dynamically allocate the array
    arr = new int[size];

    //seed the random number generator
    srand(time(0));

    //populate the array with random numbers
    for(int i = 0; i < size; i++)
        arr[i] = rand() % 100;

    return arr;
}

I'm on linux, so I was told to use valgrind to see a more detailed error message. Below is what valgrind gave me. This is my first encounter with a segmentation fault, so forgive me if the answer is trivial with valgrind's help.

valgrind error log screenshot

Karsten Koop · Accepted Answer · 2016-11-21 09:17:50Z

3

You declare newArr as a pointer to int, but it points nowhere. You could create a new array like this:

int *newArr = new int[10];

but remember to delete the array when you are done with it:

delete[] arrB;
delete[] arrC;

Or you could simply use std::vector<int> for your arrays A,B and C (or at least B and C).

answered Nov 21, 2016 at 9:17

Karsten Koop

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Comments

Domso · Accepted Answer · 2016-11-21 09:17:05Z

1

you never allocate the memory for B and C

int *applyScores(int *arr, int start) {
    int *newArr = new int[10];

    for(int i = 0; i < 10; i++) {
        newArr[i] = arr[start];
        start++;
    }    

    return newArr;
}

answered Nov 21, 2016 at 9:17

Domso

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Comments

stefaanv · Accepted Answer · 2016-11-21 10:02:11Z

1

As valgrind states (use of uninitialised value, which means the pointer and invalid write, which means write to where the pointer happens to refer to): you don't allocate memory to copy the random scores to arrB and arrC. arrNew in applyScores() is just a pointer, so it can't store an array. The easiest way to avoid this kind of problems is to use standard library containers like std::vector or std::array, which do what you expect and avoid manual memory management.

edited Nov 21, 2016 at 10:02

answered Nov 21, 2016 at 9:19

stefaanv

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