how to pass array by value in c++

Question

i have the following code

#include <iostream>

using namespace std;

int *sort(int arr[]){
    for(int i =0; i< 1; i++){
        for(int j= 1; j< 5; j++){

            if(arr[i]< arr[j]){
                int temp = arr[i];
                arr[i] =arr[j];
                arr[j] = temp;
            }

        }

    }
    for(int k=0; k<5;k++){
        cout<<arr[k];
    }
return arr;
}

int main(){  
    int arr[5] = {1,2,3,4,5};
    sort(arr);
    for(int m=0; m<5; m++){
        cout<<arr[m];
    }
}

my first problem is that how to pass array by value so that the original array does not change, and secondly the my sort function does not generate correct sorting i want to sort array in decreasing order.

Answer 1

In idiomatic C++, an array arr of 5 ints is declared std::array<int, 5> arr; or std::vector<int> arr(5);. Swapping of values is std::swap. Functions should either alter their parameters and return void, or alter a copy and return that value.

You also don't loop through every element in your outer loop.

#include <array>
#include <iostream>

void sort(std::array<int, 5> & arr){
    for(int i = 0; i < 5; i++){
        for(int j = 1; j < 5; j++){
            if(arr[i] < arr[j]){
                std::swap(arr[i], arr[j]);
            }
        }
    }
}

int main(){  
    std::array<int, 5> arr = {1,2,3,4,5};
    sort(arr);
    for(int m=0; m<5; m++){
        std::cout<<arr[m];
    }
}

However, the easiest way to sort is to use <algorithm>'s std::sort, which takes a range as two Iterator parameters

#include <algorithm>
#include <array>
#include <iostream>

int main(){  
    std::array<int, 5> arr = {1,2,3,4,5};
    std::sort(arr.begin(), arr.end(), std::greater<int>{});
    for(int m=0; m<5; m++){
        std::cout<<arr[m];
    }
}

Answer 2

1) you can use c++ swap function instead using a temp variable

swap(arr[i], arr[j]);

2) your for loops should look like this

for(int i = 0; i < 5; i++) {
    for(int j = i; j < 5; j++) {

The outer loop has to iterate through the whole array (from 0 to 4) and the inner has to iterate through the 'right side' of the array, because the left side is already sorted

so we can't pass array by value

https://stackoverflow.com/a/16137997/6521788 https://stackoverflow.com/a/16137995/6521788

You can just copy the values into a new array

Answer 3

Basic answer: you cannot pass arrays by value in C++.

Solution 1: Make a copy of your C-style array before passing it to your function, and remove your return statement as it won't be needed.

Solution 2: Use std::array, which is a container which provides the same functionality as a basic array, and as a copy constructor and copy assignment operators.

Other remark: The STL does provide sort algorithms (std::sort), so in your future programmer's life do not implement sorting algorithms yourself, unless you do something really specific where performance matters.

Answer 4

your question indicates that there might be quite a bit of confusion as to how one deals with pointers in C++. If my assumption is incorrect then just skip ahead(i will provide "my solution"/correction at the end of this post).

To answer your first question: you can't pass C-style arrays per value in C++, nor by reference for that matter. Your only "option" would be looping through the pointer until '\n' is reached & store the contents in a new array created in your function.

which would be something along the lines of

'{
    int arrx[5] = {};

    for(int i = 0; i < 5; i++) 
    {
        arrx[i] = arr[i];
    }
    for (int j = 0; j < 5; j++) // print
    {
        std::cout << arrx[j];
    }
    return 0;
}`

A couple of other things about pointers:

int *sort(int arr[]){ -> arr[] passes a pointer to arr(same as *arr)

Pointer point to the first element in the memory pointed to & have no inherit size meaning

sizeof(arr) / sizeof(arr[0])

would not work inside your sort function(which could lead to problems if you dont know exactly how long the array you want to search is). a simple solution would be to pass the size of your array by value/reference to your function. Another solution would be to loop through the array itself via something akin to

for(;arr != '\n'; arr++)
{
//your code here

You can get a lot more information about pointers than i could ever give you by visiting the various tutorial sites out there. for example: https://www.tutorialspoint.com/cplusplus/cpp_pointers.htm

http://www.cplusplus.com/doc/tutorial/pointers/

But now back to your original question and hopefully an answer to it.

int *sort(int arr[]) {
    for (int i = 0; i< 1; i++) { 

should be corrected to for(int i=0; i< 5-1; i++)

the sorting function you are trying to implement is called bubblesort. for further information & how exactly bubblesort works please refer to https://de.wikipedia.org/wiki/Bubblesort

        for (int j = 1; j< 5; j++) {

should be corrected to for(int i=0; i< 5-1-i; i++) because the last elements are already in place.

        if (arr[i]< arr[j]) {

should be corrected to if (arr[j]< arr[j+1]) because you want to compare(for example) the first element & the next element to get it in descending/ascending order. this gets done for every number pair in your array.

and finally:

        int temp = arr[i];
        arr[i] = arr[j];
        arr[j] = temp;

should look something along the lines of:

        int temp = arr[j+1];
        arr[j+1] = arr[j];
        arr[j] = temp;

please note that you can also reformat it to:

        int temp = arr[j];
        arr[j] = arr[j+1];
        arr[j+1] = temp; // same output

as an example: if arr[j] = 5 and arr[j+1] = 6 then you store the value j+1(6) in temp, overwrite arr[j+1] with j(5) and lastly store 6 at j to get descending order(hope i got that right now, but you get the gist)

lastly; as others have correctly pointed out already, you can use std::swap instead of using key/temp variables.