function and variable not declared in this scope (c++)

Question

I'm a bit stuck on my homework and could use a little help. I'm definitely not looking for someone to do my homework for me, but I sure could use some suggestions.

I am really sorry for posting on a topic that has already been posted on dozens of times. I've read through what I could find and I'm still stuck.

The program has 3 files associated with it - app.cpp has main, student.h has function prototypes and class definition. student.cpp has all of the functions.

We just started learning about classes, and this assignment is just an exercise using a class. It compares two student IDs to find which is less, and then checks to see if the gpa associated with the ID is high enough for honors.

When I try to compile it, I get a bunch of errors about not being declared in scope. My understanding is that this error happens when something is not declared ahead of time. But I can't figure out what that might be as everything is declared. I had the tutor at school take a quick look at it and she didn't see what the problem was. I'm going to post my 3 files and my error messages - I hope it's not too much!

I really, really appreciate any help!

app.cpp

#include "student.h"

int main()
{
        Student s1("G10", 3.9);
        Student s2("G20", 3.5);

        isLessThanByID(s2);
        qualifyForHonor(minGpaForHonor);

        s1.print();
        s2.print();

        return 0;
}

student.h

#ifndef STUDENT_H
#define STUDENT_H

#include <iostream>
using namespace std;

class Student
{
public:
        Student(const char initId[], double gpa);
        bool isLessThanByID(const Student& aStudent) const;
        bool qualifyForHonor(double minGpaForHonor) const;
        void print()const;

private:
        const static int MAX_CHAR = 100;
        char    id[MAX_CHAR];
        double  gpa;
};      double minGpaForHonor = 3.75;

#endif

student.cpp

void Student::Student(const char initId[], double gpa)
{
        id = initId[];
}
bool Student::isLessThanByID(const Student& aStudent) const

        if(s1.isLessThanByID (s2) == > 0)
                cout << "s1 is greater than s2" << endl;
}
bool Student::qualifyForHonor(double minGpaForHonor, double gpa) const
{
        if (gpa >= minGpaForHonor)
        {
                return true;
        }
}

void Student::print() const
{
        cout << id << '\t' << gpa << endl;
}

errors

g++  -Wall -g -std=c++11  -c -o app.o app.cpp
app.cpp: In function ‘int main()’:
app.cpp:8:2: error: ‘isLessThanByID’ was not declared in this scope
  isLessThanByID(s2);
  ^~~~~~~~~~~~~~
app.cpp:9:2: error: ‘qualifyForHonor’ was not declared in this scope
  qualifyForHonor(minGpaForHonor);
  ^~~~~~~~~~~~~~~
app.cpp:9:2: note: suggested alternative: ‘minGpaForHonor’
  qualifyForHonor(minGpaForHonor);
  ^~~~~~~~~~~~~~~
  minGpaForHonor
<builtin>: recipe for target 'app.o' failed
make: *** [app.o] Error 1

Answer 1

These functions

    isLessThanByID(s2);
    qualifyForHonor(minGpaForHonor);

are non-static member functions of the class Student. So they shall be called with an object of the class the same way as the member function print is called

    s1.print();
    s2.print();

Moreover these member functions have undefined behavior because they either return nothing or in some path of execution return nothing.

bool Student::isLessThanByID(const Student& aStudent) const
{  // <=== you forgot the open brace.:)
        if(s1.isLessThanByID (s2) == > 0)
                cout << "s1 is greater than s2" << endl;
}
bool Student::qualifyForHonor(double minGpaForHonor, double gpa) const
{
        if (gpa >= minGpaForHonor)
        {
                return true;
        }
}

And the first function recursively calls itself in this statement

        if(s1.isLessThanByID (s2) == > 0)

using the undeclared identifier s2.

The functions can be defined for example the following way

bool Student::isLessThanByID(const Student& aStudent) const
{
    return strcmp( id, aStudent.id ) < 0;       
}

bool Student::qualifyForHonor(double minGpaForHonor ) const
{
    return minGpaForHonor <= gpa;
}

Also pay attention to that this variable

double minGpaForHonor = 3.75;

is defined twice because its definition is included in the header.

You could declare the variable either like

const double minGpaForHonor = 3.75;

or

inline const double minGpaForHonor = 3.75;

Answer 2

It seems that you are confusing the scopes. For example, consider this class, and how is the proper way of invoking its methods.

class Foo {
    void Bar1();
    static void Bar2();
};

Foo foo;
foo.Bar1();
foo::Bar2();

This is also valid for variables. Therefore you should use s1.isLessThanByID(s2); and s1.qualifyForHonor(minGpaForHonor); instead.

Your function qualifyForHonor should be static or left out of the class, since it does not use any member, just the parameters passed to it.

Also, this line is a bit strange for me: if(s1.isLessThanByID (s2) == > 0), what were you trying to achieve in here?

Answer 3

isLessThanByID and qualifyForHonor are member functions in the class Student, therefore should be invoked on a instance of class Student.

        isLessThanByID(s2);
        qualifyForHonor(minGpaForHonor);

should be changed to

        s1.isLessThanByID(s2);
        s1.qualifyForHonor(minGpaForHonor);

Having said this, you apperantly have more errors than this. The function declaration bool qualifyForHonor(double minGpaForHonor) const in Student class doesn't match with its signature in the definition as well.