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Is there a way to allow the user to populate an Array with "cin"?

Jan 30, 2017 at 12:48am
willmannix123 (11)
I have an excercise to do for college and one of the questions is as follows

a. Write a prototype for a function LargestInputValue that will allow the user to
enter a list of positive integers, followed by a terminator value –1. The
function should return the largest value entered.
b. Write a definition for the function LargestInputValue
c. Write a test application that will use the function LargestInputValue


The problem I'm having is how do I allow the user to enter there own numbers using cin, I know it involves a for loop.

lets say I declared an array in main int Array[20];

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#include iostream;
using namespace std;

int main()
{
for (int i = 0; i < 20; i++)
	{

		cin >> First[i];


	

	}


	cout << First;

}
Edit & run on cpp.sh


But when I try output my array, It gives me the a hexdecimal memory address. How would I go about outputing All the elements in the array on the screen that the user just populated? Also can you explain Why the memory address is showing up, I know its completley wrong and the cout << First makes no sense but Im just giving you an idea what I'am trying to achieve.
Last edited on Jan 30, 2017 at 12:53am
Jan 30, 2017 at 1:21am
integralfx (897)
You would need to loop through the array like you did with cin, except with cout.
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// named constant; avoid magic numbers
const int array_size = 20;

for(int i = 0; i < array_size; i++)
{
    cout << First[i] << ' ';
}
Jan 30, 2017 at 1:36am
mbozzi (3925)
But when I try to output my array, I get a hexadecimal memory address.

I'm assuming you have an array int a[10];, and you wrote std::cout << a << "\n"; to try and print it.

The simple answer is that a unqualified identifier of array type (i.e., a) "decays" to a pointer to its' first element in many (but not all) cases. This is one such case.
A pointer is a memory address; std::ostream::operator<<(), AKA (cout <<) happens to stream pointers in hexadecimal format by default.

The exact rules are quite complex and will assume a lot of knowledge about the language. If you're really interested, you can find them here:
http://en.cppreference.com/w/cpp/language/array
That site usually nicely paraphrases the horse's mouth, found starting here
http://eel.is/c++draft/conv#array-1

Here's an example of how to print the contents of an array:
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# include <iostream>

int main() {
    constexpr int sz = 10;
    int vals[sz], i = 0;
    
    for (i = 0; i < sz; ++i) {
        std::cin >> vals[i];
        if (! std::cin) { // error!
            std::cerr << "error populating array\n";
            return 1; // bail out.
        }
    }
    
    for (int i = 0; i < sz - 1; ++i) 
        std::cout << vals[i] << ", ";
    std::cout << vals[sz - 1] << "\n";
}


http://coliru.stacked-crooked.com/a/f60420ed6d2b2836
Jan 30, 2017 at 2:16am
closed account (E0p9LyTq)
allow the user to enter a list of positive integers, followed by a terminator value –1.

Don't try using a static-sized array, use a dynamic array such as a vector.

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#include <iostream>
#include <vector>

int main()
{
   int input_value = 0;
   std::vector<int> list;

   std::cout << "input a list of values (-1 to stop):\n";

   while (true)
   {
      std::cin >> input_value;

      if (input_value == -1)
      {
         break;
      }

      // add a new value to the vector
      list.push_back(input_value);
   }

   // define a variable to check for largest value entered
   int largest = 0;
   
   std::cout << "\nThe contents of the vector:\n";
   
   // use a range based for loop to print out the contents of the vector
   for (auto x: list)
   {
      std::cout << x << '\n';
      
      if (largest < x)
      {
         largest = x;
      }
   }
   
   std::cout << "\nThe largest value entered was: " << largest << '\n';
}
Edit & run on cpp.sh

input a list of values (-1 to stop):
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175
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-1

The contents of the vector:
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175
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The largest value entered was: 175

Is this the most efficient way to fulfill that objective? Doubtful, this was a quick and dirty knock-up.

Most instruction, classroom or online tutorial, avoid using the language's better features that end up making it harder to write good programs.
Last edited on Jan 30, 2017 at 2:33am
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