What should be the proper declaration of array while finding the largest number in array?

Question

C++ This is my code in C++ for finding the largest number in array. When I was running in my IDE then there was no compilation error but it was not giving me output. I think the problem is in the declaration of array at line 8. I replaced the array declaration from line 8 to line 11 then it is working fine in my IDE. So I didn't get it that why the declaration of array was not working at line 8?

#include <bits/stdc++.h>
using namespace std;
int largest_in_array(int a[], int n);

int main() // main function
{
    int n; // User will enter the size of array 
    int arr[n]; // Line 8
    cout << "Enter the size of array: " << endl;
    cin >> n;
              // Line 11
    cout << "\nEnter the elements of array: " << endl;

    for (int i = 0; i < n; i++) // This loop will run for each element of array that user wants to enter
    {
        cout << "Enter the " << (i + 1) << " element:";
        cin >> arr[i];
        cout << endl;
    }
    cout << "Elements are: [";
    for (int i = 0; i < n; i++) // Prints the elements of array
    {
        // cout << "Enter the " << (i + 1) << " element:";
        cout << arr[i] << " ";
        // cout << endl;
    }
    cout << "]";

    int res = largest_in_array(arr, n); //Function call
    cout << "\nLargest element in array is: " << arr[res] << endl;
    return 0;
}

int largest_in_array(int a[], int n) // function that will return the index of largest element in array
{
    int max = 0;
    for (int i = 1; i < n; i++)
    {
        if (a[max] < a[i])
        {
            max = i;
        }
    }
    return max;
}

You declare `int arr[n];` before the user enters `n` - what value do you expect `n` to have at that point? Also `int arr[n]` is a VLA which is not part of standard C++. Make it `std::vector arr(n);` instead — Ted Lyngmo, Oct 03 '21 at 07:19
In line 8 the input has not been read. How is the program supposed to know information not yet present? Note that regardless of the order of the statements this only works with a non-standard extension to C++. The C++ standard only allows for fixed size array declarations where the number of elements is known at compile time. You should use a data structure that allows you to dynamically allocate the elements, e.g. `std::vector` — fabian, Oct 03 '21 at 07:21
Recommended reading: [Why should I not #include ?](https://stackoverflow.com/q/31816095/2752075), [Why is “using namespace std;” considered bad practice?](https://stackoverflow.com/q/1452721/2752075) — HolyBlackCat, Oct 03 '21 at 08:42

Ted Lyngmo · Accepted Answer · 2021-10-03T08:44:56.077

You declare int arr[n]; before the user has entered a value into n. n has an indeterminate value when you read it and create arr.
You don't check that the user enters a positive value into n. Zero and negative sized arrays are not valid.

Other points:

bits/stdc++.h is not a standard header which makes your program not portable. Use the proper header files, like iostream etc.
arr[n] is a Variable Length Array (VLA) which is not part of standard C++. Make it a std::vector<int> arr(n); instead.
The use of std::endl is unnessesary. There is no need to flush the output streams here. Use \n instead.

Example:

#include <iostream>
#include <limits>
#include <vector>

int largest_in_array(const std::vector<int>& a) {
    int max = 0;
    for(int i = 1; i < a.size(); i++) {
        if(a[max] < a[i]) {
            max = i;
        }
    }
    return max;
}

int main() // main function
{
    int n; // User will enter the size of array
    std::cout << "Enter the size of array:\n";

    // check that input succeeds and that the value is valid
    if(!(std::cin >> n) || n < 1) return 1;

    std::vector<int> arr(n);
    std::cout << "\nEnter the elements of array:\n";

    for(int i = 0; i < n; i++)
    {
        std::cout << "Enter the " << (i + 1) << " element:";
        if(!(std::cin >> arr[i])) {
            std::cout << "invalid input, bye bye\n";
            return 1;
        }
    }
    std::cout << "Elements are: [";
    for(int i = 0; i < n; i++)
    {
        std::cout << arr[i] << " ";
    }
    std::cout << "]";

    int res = largest_in_array(arr); // Function call
    std::cout << "\nLargest element in array is: " << arr[res] << '\n';
}

That said, you could however use the standard algorithm std::max_element instead of writing your own. It returns an iterator to the maximum element. You could also make use of range-based for loop when you don't need to know the index in the array, as in your second loop.

Example:

#include <algorithm>
#include <cstddef>
#include <iostream>
#include <iterator>
#include <limits>
#include <vector>

int main() {
    int n; // User will enter the size of array
    std::cout << "Enter the size of array:\n";
    if(!(std::cin >> n) || n < 1) return 1;

    std::vector<int> arr(n);
    std::cout << "\nEnter the elements of array:\n";

    for(int i = 0; i < n; i++) // This loop will run for each element of
                               // array that user wants to enter
    {
        std::cout << "Enter the " << (i + 1) << " element:";
        if(!(std::cin >> arr[i])) {
            std::cout << "invalid input, bye bye\n";
            return 1;
        }
    }
    std::cout << "Elements are: [";
    for(auto value : arr) {          // a range-based for loop
        std::cout << value << ' ';
    }
    std::cout << "]\n";

    auto res = std::max_element(arr.begin(), arr.end());
    std::cout << "Largest element in array is: " << *res << '\n';

    std::size_t index = std::distance(arr.begin(), res);
    std::cout << "which has index " << index << '\n';
}

score 0 · Answer 2 · answered Oct 03 '21 at 07:22

When you have int n on line 8 it is initialized when you use it to create the array. When n is initialized explicitly, it's value is undefined behavior. You may be creating an array larger than the n you inputted on line 10, resulting in the array having extra random junk, it may be smaller meaning your program read memory it really shouldn't, etc.

What should be the proper declaration of array while finding the largest number in array?

2 Answers2