CODEWITHSUNDEEP
c++arraysdynamicresize
a_large_farva
a_large_farva

Reputation: 29

How does one dynamically resize arrays?

I'm trying to read an unknown number of elements into the array, when my size reaches the current capacity, I call a function to double the size and copy the contents of the old array into a new array. I got my '&' but it seems it's still passing the array by value.

#include <iostream>
#include <fstream>
using namespace std;
void resize(int*&, int&);

int main() {
    ifstream in("numbers.input");
    int cap = 10;
    double avg;
    int total = 0;
    int size = 0;
    int *arr = new int [cap];
    int temp;

    for(int i =0; i <cap; i++){
        in >> temp;
        if(size >= cap) {
            resize(arr,cap);
        }
        arr[i]=temp;
        total += arr[i];
        size++;
    }
    avg = (double) total/cap;
    cout << cap <<endl;
    cout << size <<endl;
    cout << total <<endl;
    cout << avg;
    return 0;

}
void resize(int *&arr,int &cap) {
    cap*=2;
    int* newArr = new int[cap];
    for(int i = 0; i < cap; i++){
        newArr[i] = arr[i];
    }
    delete [] arr;
    arr = newArr;
}

Upvotes: 1

Views: 900

Answers (2)

The Dark
The Dark

Reputation: 8514

Your resize function is taking the pointer by reference and will modify the value of the variable it is called with. However you have a number of bugs:

  1. You copy cap items out of the old array, but you have already doubled cap, leading to an out of bound access and a possible crash.
  2. Your resize function never gets called, due to a bug in your input loop. You should step through in a debugger (or at least add some trace cout calls) to work out what is going on. Try to figure this one out, if you can't let me know.
  3. Your average is using cap as the divisor, that is not correct.

Note: You should add in your question that you can't use vector, because that would be the normal way to do this.

Note 2: In your question, you should also say exactly what is going wrong with your program - "seems to be passing the array by value" is a bit vague - why do you think it isn't passing by value?

Upvotes: 0

Gerhard Wesp
Gerhard Wesp

Reputation: 329

Everything you try to implement 'by hand' is already in the standard library. Use std::vector<> which implements a doubling/reallocation strategy very similar to what you're proposing.

#include <fstream>
#include <vector>

int main() {
    std::ifstream in("numbers.input");
    std::vector<int> arr;
    int temp;
    while (in >> temp) { arr.push_back(temp); }

    // process your data ...
}

See http://www.cplusplus.com/reference/vector/vector/

To answer the question more literally: Arrays are always passed by reference, typically by passing a pointer to the first element.

Upvotes: 1

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