Not declared in Scope - c++

Question

I have wrote a simple average calculation program trying to calculate a semester average. When I compile the code I get an error telling me my 'inputExam' function was not declared in this scope. I've researched the error message and I can't figure out what to do to fix it.
I also get this error for the other functions, but once I understand my error I think I can fix the others.
#include <iostream>
using namespace std;
int main()
{
double finalExam=0.0;
double midterm = 0.0;
double quizzes = 0.0;
double labs = 0.0;
double semGrade=0.0;
midterm=inputExam("Midterm");
finalExam=inputExam("Final");
quizzes=inputAndAvgQuizzes();
labs=inputAndAvgLabs();
semGrade=(midterm*.2)+(finalExam*.2)+(labs*.5)+(quizzes*.1);
cout<<"Your End of Semester Grade is: " semGrade;
return 0;
}
double inputExam(string examType)
{
double grade;
cout<< "Enter the " examType " Score: ";
cin>>grade;
return (grade);
}
double inputAndAvgLabs()
{
double num [4];
double sum;
double avg;
if (int a=0, a<3,a++)
{
cout<<"What is the grade?"<<endl;
cin>>num[a]>>endl;
}
if (int a=0, a<3, a++)
{
sum=sum+num[a];
}
avg=sum/4;
return avg;
}
double inputAndAvgQuizzes()
{
double num[3];
double sum;
double avg;
double lowest = num[0];
if (int a=0, a<2,a++)
{
cout<<"What is the grade?"<<endl;
cin>>num[a]>>endl;
}
if (lowest>num[1])
{
lowest=num[1];
}
if (lowest>num[2])
{
lowest=num[2];
}
sum=num[1]+num[2]+num[3]-lowest;
avg=sum/2;
return avg;
}

Answer 1

You need to let it be known that those functions exist so add prototypes for your functions above main or define your functions there. Like so:
...
double inputExam(string examType);
double inputAndAvgLabs();
double inputAndAvgQuizzes();
int main() { ... }
//definitions after main
..or copy paste all those definitions above the call to main like so:
...
// Function defs here
// Prototypes no longer needed
...
int main() {...}
// Defs no longer needed here
Alternatively you can put all the definitions in an external file and compile it into the project via a make file or better yet, and as you progress, create classes in header and implementation files and include them in your file the same way(sort of) that you do #include <iostream>.
Another small nugget of advice would be to avoid using namespace std;. If not only in theory it's bad practice and can lead to namespace clashing in larger projects. If you, like me, hate typing std::string ... then add using std::string; to your code for the same ease of use.

Answer 2

In C/C++ you need to declare the function before you use it. In this case, it simply means declaring function prototypes before your main function and then implementing them after the main function.
Example:
// declare a prototype
double Function(int variable);
int main()
{
Function(5);
return 0;
}
// Implement the function
double Function(int variable)
{
/* Do Something */
}
Alternatively, you could change your code to the form:
// Implement the function first
double Function(int variable)
{
/* Do Something */
}
int main()
{
Function("Testing");
return 0;
}