C++ vector practice undeclared variable

Question

Its probably an easy solution, but I am still new to programming, it says that my ivec is undeclared. Here were the instructions and my code:

1. Query the user for a size..
2. Create a vector of the size specified by the user.
3. Using a loop, fill this vector with random numbers.  Each random number should be in the range 0 to 99 (inclusive).
4. Write a function that finds the mean of a vector.  The function should have the following prototype:  double mean(vector & data);
5. Use this function to find the mean of the vector created earlier and print the result.
6. Using a loop, subtract the mean from every element in the vector.  (By the way, this vector should now have positive and negative numbers.)

This is my code

#include <iostream>
#include <vector>
#include <ctime>
#include <cstdlib>

using namespace std;

//prototype
double mean(vector<int> & data);

int main()
{
    const int SIZE = 10;
    vector<int> ivec(SIZE);
    int srand(time(0));
    double m;

    for(int i = 0; i < ivec.size(); i++)
        {
            ivec[i] = rand() % 99 + 1;

        }

    m = mean(ivec);

    for(int i = 0; i < ivec.size(); i ++)
        {
            ivec[i] = ivec[i] - mean;
        }

    return 0;
}

double mean(vector<int> & data)
       {
           double dSum = 0;
           for(int i = 0; i < ivec.size; i++)
            {
                dSum += data[i];
            }
           return dSum/ivec.size;

       }

It says that my ivec is undeclared so how would I declare. I would appreciate the help

Answer 1

The parameter of your mean function is called data, so that is what you need to use:

   double mean(const vector<int>& data)
   {
       double dSum = 0;
       for(int i = 0; i < data.size(); i++)
        {
            dSum += data[i];
        }
       return dSum/data().size;

   }

Also note that size is a member function, so you need to call is as size(). Also, the vector should be passed by const reference, because it is not being modified.

Note: An idiomatic way to calculate the mean would be to use std::accumulate:

if (!ivec.empty())
{
  double mean = std::accumulate(ivec.begin(), ivec.end(), 0.0)/ivec.size();
}

Answer 2

You have several errors in function mean. First of all std::vector has no data member with name size . It has a member function with name size. So instead of for example expression

ivec.size

there must be

ivec.size()

And the second error is that instead of nane ivec you have to use name data because name ivec is not declared in this function.

So the function should look as

double mean( const vector<int> &data )
{
   double dSum = 0.0;

   for ( std::vector<int>::size_type i = 0; i < data.size(); i++ )
   {
      dSum += data[i];
   }

   return ( data.size() == 0 ? 0.0 : dSum / data.size() );
}

I changed the type of the parameter to const vector<int> & because the vector itself is not changed in the function.

It would be even better to substitute the ordinary for statement for the range-based for statement. in this case the function will look as

double mean( const vector<int> &data )
{
   double dSum = 0.0;

   for ( int x : data ) dSum += x;

   return ( data.size() == 0 ? 0.0 : dSum / data.size() );
}

These two statements

   double dSum = 0.0;

   for ( int x : data ) dSum += x;

in fact correspond to the body of standard algorithm std::accumulate

So you could write (provided that you included header <numeric>)

#include <numeric>

//...

double mean( const vector<int> &data )
{
   double dSum = std::accumulate( data.begin(), data.end(), 0.0 );

   return ( data.size() == 0 ? 0.0 : dSum / data.size() );
}

Also take into account that your program does not fully satisfies the assignment. In the assignment there is written

Query the user for a size.
Create a vector of the size specified by the user.

Also after this loop

for(int i = 0; i < ivec.size(); i ++)
    {
        ivec[i] = ivec[i] - mean;
    }

I think you should output the vector that to show new values of its elements.