Multiplying and displaying the values of an array in C++

Question

I am trying to complete a C++ exercise in which an array is displayed and the user is prompted to input a multiplier, which will result in the initial numbers that were displayed being multiplied by the user's input. Here is the code that I have so far:

#include <iostream>
using namespace std;

int main()
{

    int array[5] = { 1, 2, 3, 4, 5 };
    for (const auto& a : array)
    {
        std::cout << a << std::endl;

    }
    double multiplier;
    cout << "Input a multiplier: ";
    cin >> multiplier;
    for (int array = 1; array <= 5; ++array)
    {
        array == multiplier * array;
        std::cout << array << std::endl;
    }

}

When it runs, it prints the correct array, with a newline being created after each array value, and prompts the user for the multiplier. However, when the multiplier is inputted, the values do not change. Here is an example output:

1
2
3
4
5
Input a multiplier: 2                                                                  
1                                                                                      
2                                                                                      
3                                                                                      
4                                                                                      
5  

The goal is to get this output:

    1
    2
    3
    4
    5
    Input a multiplier: 2                                                                  
    2                                                                                      
    4                                                                                     
    6                                                                                      
    8                                                                                      
    10  

Any help or code improvement would be appreciated, as figuring out how to multiply and display the multiplied values is the only thing needed to be done in order to complete the exercise. Thank you in advance!

Answer 1

You have quite a few things going on that are incorrect.

First is your over scoping (maybe not the right term) the array value.

You have an array value declared outside of your for loop, and you're using the value array again inside of the for loop for the counter. So you're not actually doing anything to your array. You're actually trying to do something to the initialization variable.

Second you're using == instead of =

You're using a comparison operator (==) instead of an assignment operator (=), but there's other big no no's going on.

array is an int[5] not just an int

To actually modify each element of your array, you need to reference the index by saying array[index] where index is a value of 0 to length of array - 1, so 4 in your case.

You're using a double multiplier and trying to apply it to an int *

If you try to multiply by a double value like 2.5, your array values are going to be int's and not a number with a decimal value. You should make your int array [] into a double array []

Not accessing your array, starting from index 0

When looping through an array, the first index is always 0. You're using array = 1 and array <= 5. This would skip your first index, and give you an out of bound index at the end. You should use int index = 0; index < 5; index++; instead.

You're trying to print array

Since array is an int [] printing array in the for loop like that will just give you the address of where the array is. You'll have to either print out each index or use the enhanced for loop method after you've applied your multiplier.

If you're wanting to use your above implementation, do this

for (int index = 0; index < 5; index++)
{
    array[index] *= multiplier; // Or you can use array[index] = multiplier * array[index]
    std::cout << array[index] << std::endl;
}

Answer 2

A simpler approach would be to use another range-based for loop instead of the indexed version:

for (auto& a : array)
{
    a *= multiplier;
    std::cout << a << std::endl;
}

If you don't need to update the array itself (after all, your code doesn't actually read from the array again), then you a very similar, though more straightforward version might be applicable:

for (const auto& a : array)
{
    std::cout << a * multiplier << std::endl;
}

Answer 3

There are three big issues with your code:

1. Naming conventions. Do not name your array and your temporary for loop variable the same thing. This will cause an issue further down the line, which I'll illustrate.

2. Incorrect operator As minterm has mentioned, you are using a comparison operator instead of the equal operator. But that alone will not fix your issue.

3. Not accessing array values You are not actually multiplying the array values with the multiplier. You have to access the elements, which means you cannot start the index at 1.

for (int i = 0; i < 5; i++){
    array[i] *= multiplier
    cout << array[i] << endl;
}

Answer 4

Use = instead of ==. Currently, it just evaluates a logical statement instead of setting it to a new value. Also, you need to change "int array" in the for loop to a different name so as to not get it confused with the array called array. Call the variable in the for loop something else, like "int i". So then the line in question would not be "array == multiplier*array", but instead something like "int j = multiplier * array[i]", and then have it print out j instead of array.