Printing Value From Array in C-CodePudding

I want to write a method in C which creates an array of monotonically increasing values. It has the inputs of int begin, int end, int interval.

In this example; method should return the array of [0,1,2,3,4,5,6,7,8,9,10]. When I print the results it should print out the first two indexes and get 0 and 1. However, when I print it, it gives 0 for the first one and 9829656 for the second one.

When I only print one index it is always correct, but when I print more than one index, every value except for the first printed one gives a different result. I think the other results are related to memory address since I used pointers.

#include <iostream>
using namespace std;

int* getIntervalArray(int begin, int end, int interval){

    int len = (end - begin) / interval   1;

    int result[11] = {};

    for (int i = 0; i <= len - 1; i  ) {
        result[i] = begin   interval * i;
    }

    return result;
}

int main(){

    int begin = 0;
    int end = 10;
    int interval = 1;

    int* newResult = getIntervalArray(begin, end, interval);

    cout << newResult[0] << endl;
    cout << newResult[1] << endl;

    return 0;

}

CodePudding user response：

You are returning a pointer to a local variable. You can instead return a std::vector by value as shown below:

#include <iostream>
#include <vector>
//return a vector by value
std::vector<int> getIntervalArray(int begin, int end, int interval){

    int len = (end - begin) / interval   1;

    std::vector<int> result(len); //create a vector of size len

    for (int i = 0; i <= len - 1; i  ) {
        result.at(i) = begin   interval * i;
    }

    return result;
}

int main(){

    int begin = 0;
    int end = 10;
    int interval = 1;

    std::vector<int> newResult = getIntervalArray(begin, end, interval);

    //print out elements of returned vector
    for(int i = 0; i < newResult.size();   i)
    {
        std::cout << newResult.at(i) << std::endl;
 
    }
    
    return 0;
}

The output of the above program can be seen here.

CodePudding user response：

A possible solution dynamically allocating the local array, and returning it via a smart pointer:

#include <array>
#include <iostream>
#include <memory>  // make_unique

auto getIntervalArray(int begin, int end, int interval)
{
    int len = (end - begin) / interval   1;

    auto result{ std::make_unique<std::array<int, 11>>() };

    for (int i = 0; i <= len - 1; i  ) {
        (*result)[i] = begin   interval * i;
    }

    return result;
}

int main()
{
    int begin = 0;
    int end = 10;
    int interval = 1;

    auto newResult{ getIntervalArray(begin, end, interval) };

    std::cout << (*newResult)[0] << std::endl;
    std::cout << (*newResult)[1] << std::endl;
    std::cout << (*newResult)[2] << std::endl;

    return 0;
}

Demo

CodePudding user response：

Set the array variable in your function as static. This is because C does not support returning the address of a local variable.

static int result[11];

CodePudding user response：

try this. also add deletion of the newResult

#include <iostream>
using namespace std;

int* getIntervalArray(int begin, int end, int interval){

    int len = (end - begin) / interval   1;

    int* result = new int[len];
    int lastValue = begin;
    for (int i = 0; i <= len - 1; i  ) {
        result[i] = lastValue;
        lastValue  = interval;
    }

    return result;
}

int main(){

    int begin = 0;
    int end = 10;
    int interval = 1;

    int* newResult = getIntervalArray(begin, end, interval);

    cout << newResult[0] << endl;
    cout << newResult[1] << endl;
    
    // add delete here. 
 
    return 0;

}