C++ mag( ) function and prototype

Assignment I am having trouble with:

Complete 'this program', by adding a function called mag( ) to calculate the magnitude of a vector.  You'll need to add a prototype before main( ), and the implementation (header & body) of the function after main( );

reminder: mag = sqrt (x^2 + y^2)

Calling it with something like:

            vec_length = mag(x, y);

program to be completed:

#include<iostream>
#include<cmath>

using namespace std;

//      for part a, put your function prototype for mag here!


//............................................................

int main(void)
{
    double x = 0;
    double y = 0;
    double v=3.0, w=4.0;
    double m;
   
    cout << "Please enter the X coordinate: ";
    cin >> x;
    cout << "Please enter the Y coordinate: ";
    cin >> y;
   
    m = mag(x,y);       // you need to write this function!
    cout << "The magnitude of ( "<< x << " , "<< y <<" ) is: " << m << endl;
   
    m = mag(v,w);       // now get the magnitude of vector (v,w)
    cout << "The magnitude of ( "<< v << " , "<< w <<" ) is: " << m << endl;
   
   
    return 0;
}


//.................put your mag function here...................
// be sure to add comments describing the inputs and outputs so a reader would
// know how to use it.
Alyssa ZukasAsked:
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evilrixSenior Software Engineer (Avast)Commented:
So, what is it you are specifically having trouble with?
murugesandinsShell_script Automation /bin/bash /bin/bash.exe /bin/ksh /bin/mksh.exe AIX C C++ CYGWIN_NT HP-UX Linux MINGW32 MINGW64 SunOS Windows_NT started powershellCommented:
@Alyssa Zukas
Based on your comment, we feel that you need to learn the following:
1. How to write prototype of function.
2. How to handle when user input being invalid
3. define the function where ever required and check static function or available across multiple files,
All above needs to be done before implementation during analysis itself.
Here goes related code doing the same.

#include <limits.h>
#include <stdio.h>
#include <sys/errno.h>
#include <string.h>
#include <iostream>
#include <cmath>
using namespace std;
// Function returning magnitude of first parameter(x) and second parameter (y) points.
double mag( const double &, const double & );
// First  parameter referring constant reference for x point
// Second parameter referring constant reference for y point
int main(void)
{
	double x = 0.0;
	double y = 0.0;
	double v=3.0, w=4.0;
	double m = 0.0;
	cout << "Enter X coordinate: ";
	cin >> x;
	while ( cin.fail() )
	{
		cin.clear();
		cin.ignore( INT_MAX,'\n');
		fflush(stdin);
		cout << "Enter only numbers.";
		cout << "Enter X coordinate: ";
		cin >> x;
	}
	cout << "Enter Y coordinate: ";
	cin >> y;
	while ( cin.fail() )
	{
		cin.clear();
		cin.ignore( INT_MAX,'\n');
		fflush(stdin);
		cout << "Enter only numbers.";
		cout << "Enter Y coordinate: ";
		cin >> y;
	}
	m = mag(x,y);       // you need to write this function!
	cout << "The magnitude of ( "<< x << " , "<< y <<" ) is: " << m << endl;
	m = mag(v,w);       // now get the magnitude of vector (v,w)
	cout << "The magnitude of ( "<< v << " , "<< w <<" ) is: " << m << endl;
	return 0;
}
// Function returning magnitude of first parameter(x) and second parameter (y) points.
double mag( const double &x, const double &y)
// First  parameter referring constant reference for x point
// Second parameter referring constant reference for y point
{
	return sqrt( pow( x, 2) + pow( y, 2));
}

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Example output:
$ /usr/bin/g++ -g -Wall magnitude.c -o a.out
$ ./a.out
Enter X coordinate: Murugesan Dinesh
Enter only numbers.Enter X coordinate: 3
Enter Y coordinate: Alyssa Zukas
Enter only numbers.Enter Y coordinate: 4
The magnitude of ( 3 , 4 ) is: 5
The magnitude of ( 3 , 4 ) is: 5
$

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murugesandinsShell_script Automation /bin/bash /bin/bash.exe /bin/ksh /bin/mksh.exe AIX C C++ CYGWIN_NT HP-UX Linux MINGW32 MINGW64 SunOS Windows_NT started powershellCommented:
Best solution provided.
Closing this, since no comment added from Alyssa Zukas.
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