I get an "undefined reference to ..." for two of my functions - c++

Question

I have created three separate files. The first is the main.cpp the 2nd is a header file called "statistics.h" which has the declarations for the two functions i am getting the error and the 3nd is a file called statistics.cpp which holds the implementation of the two functions.
Here is my main file:
#include <iostream>
#include "statistics.h"
using namespace std;
int main()
{
cout<<"This program provides the average and the standard deviation of 1,2,3 or 4 numbers."<< endl;
while(true){
start:
unsigned int howmany;
cout<<endl;
cout<<"How many numbers do u wish to receive as input?? ";
cin >> howmany;
if (howmany>4){
cout<<"You should pick at most 4 numbers u idiot!!!"<< endl<<endl;
goto start;
}
if(howmany==0){
return 0;
}
cout<< endl;
double nums[howmany];
for (int i=0;i<howmany;i++){
cout<<"Give me the number "<<i+1<<":";
cin>>nums[i];
}
double avg=average(nums,howmany); /////////////////////////////
double stdev=standard_deviation(nums,howmany,avg); ////////////////////
cout<< endl<<"Average: "<<avg<<". Standard Deviation: "<< stdev<< endl;
}
}
My header file is:
#ifndef STATISTICS_H_INCLUDED
#define STATISTICS_H_INCLUDED
double average(double ar[],int hm);
double standard_deviation(double ar[],int hm,double avrg);
#endif // STATISTICS_H_INCLUDED
And my implementation file statistics.cpp is:
#include<iostream>
#include"statistics.h"
#include <math.h>
using namespace std;
double average(double ar[],int hm){
double sum=0.0;
double average;
for(int i=0;i<hm;i++){
sum+=ar[i];
}
average=sum/hm;
return average;
}
double standard_deviation(double ar[],int hm,double avrg){
double std_dev;
double sum=0;
double ans;
for(int i=0;i<hm;i++){
sum+= ((ar[i]-avrg)*(ar[i]-avrg));
}
ans=sqrt(sum/hm);
return ans;
}
I am getting the errors in my main file (i have marked the corresponding lines with consecutive ////////). What could be wrong?? I am sure its something stupid from my part.
As i am using codeblocks i finally found the solution. I just needed to add #include "statistics.cpp" to my main.cpp file.

Answer 1

You forgot to link statistics.o together with main.o, so your hypothetical executable will not contain the function definitions.
(The hypothetical executable can thus not exist, so you're getting a linker error.)
Variation 1
Where you're writing something like:
g++ main.cpp -o myExecutable
instead write:
g++ main.cpp statistics.cpp -o myExecutable
Variation 2
Where you're writing something like:
g++ -c main.cpp
g++ main.o -o myExecutable
instead write:
g++ -c main.cpp
g++ -c statistics.cpp
g++ main.o statistics.o -o myExecutable
If you're still having trouble, try reversing the order in which you provide the filenames to g++ (though the above should be correct).
Incidentally, you are not allowed to define an array with variable dimensions. Use a std::vector instead.

Answer 2

As i am using codeblocks i finally found the solution. I just needed to add #include "statistics.cpp" to my main.cpp file.
!!! This is NOT a solution. Do not include implementation files. You will fall into a pit of despair, decay and disorganization.
Everything was fine with your use of header files. You just need to tell your IDE to compile both files as a single project. Consult the documentation.