C++ function and I/O file - c++

Question

I'm trying to get the numbers of lines of input file to be size of array, but it keeps giving error that expression of the array must be constant. Here is my code:
int main() {
int count = 0;
string line;
ifstream infile("students.dat");
while (getline(infile, line))
count++;
cout << "Numbers of input is: " << count << endl;
const int size = count;
double max, min, average;
int freshman, sophomore, junior, senior;
string firstname[size];
string lastname[size];
string year[size];
double grade[size];
cout << "name\n";
for (int i = 0; i < size; i++) {
infile >> firstname[i] >> lastname[i]
>> year[i] >> grade[i];
}
max = FindMax(size, grade);
cout << "\nHighest grade: " << max;
min = FindMin(size, grade);
cout << "\nLowest grade: " << min;
average = FindAvg(size, grade);
cout << "\nClass average: " << average;
FindYear(size, year);
infile.close();
return 0;
}
Sample input file:
John Omi freshman 66
Katy Hill sophomore 55
Jeff Ozco freshman 90

Answer 1

constant means compile-time constant, which means the compiler would know its exact value at compile time. For example:
10 // constant
#define VAL 10
VAL // constant
const int val = 10 + 1;
val // constant
The size in your code is a variable that its value is not known until the program runs and reads your file, counts its line. It is called a run-time variable.

Answer 2

This here is wrong:
size_t size = count; // obtain the number of elements
string firstname[size]; //< Fails with a compiler error
The declaration something[<size>] tells the compiler to generate space for <size> elements something. Since you know the number of elements (count) only at runtime, the compiler does not know what to do. The size parameter must be a (fixed) number, when the compiler translates your source into binary code.
That said: Please learn the std library. std is part of the C++ language!
Don't ever use [] for vectors/arrays anymore (unless in very specific instances).
The std library has a whole lot of containers that are very easy to use. The most prominent container is std::vector.
Rewrite your code, for example like this:
std::vector<std::string> firstname;
firstname.resize(count);
for (...)
infile >> firstname.at(i);
Or better still:
struct student
{
std::string firstname;
std::string lastname;
...
};
...
std::vector<student> students;
while (!infile.eof()) {
std::string first, last, this, that;
infile >> first >> last >> this >> that;
students.emplace_back(first, last, this, that);
// or alternatively
students.push_back(student(first, last, this, that));
Like I said: The std library is part of the C++ standard.