Trouble understanding how elements of a certain function work - c++

Question

I am required to fully understand the following code :
#include <iostream>
using namespace std;
double area(double length, double width);
double time(double p_area, double h_area, double mow_rate);
int main() {
double d_plot_length, d_plot_width, d_home_side, d_mow_rate;
double plot_area, home_area, time_taken;
// I've used double for all of these to get the most precise values possible, something I'd only really consider doing on small programmes such as this
cout << "What is the length of the plot? In meters please." << endl;
cin >> d_plot_length;
cout << "What is the width of the plot? In meters please." << endl;
cin >> d_plot_width;
cout<< "What is the size of the side of the house? In meters please." << endl;
cin >> d_home_side;
cout << "What is the rate at which you are going to be mowing? In meters per minute please" << endl;
cin >> d_mow_rate;
// Just getting all the data I need from the user
plot_area = area(d_plot_length, d_plot_width);
home_area = area(d_home_side, d_home_side);
time_taken = time(plot_area, home_area, d_mow_rate);
cout << "It will take " << time_taken << " minutes to mow this lawn. Better get cracking" << endl;
return 0;
}
double area(double length, double width) {
double value;
value = length * width;
return value;
}
double time(double p_area, double h_area, double mow_rate) {
double value;
value = (p_area - h_area) / mow_rate;
return value;
}
I am struggling to understand how the time() function works.
So far I understand that :
time_taken , gets its value from the time() function: time(plot_area, home_area, d_mow_rate).
The time() function gets its values from the function declaration at the bottom.
double time(double p_area, double h_area, double mow_rate) {
double value;
value = (p_area - h_area) / mow_rate;
return value;
}
However, this is where I'm stuck. The user is asked to enter values for d_plot_length, d_plot_width, etc. So I cannot understand how the compiler knows what these values p_area, and h_area actually are.
I realise that somehow the area() function is being used to aid the time() function, but as far as I'm aware the variables P_area etc within the time() function do not have values assigned to them.
Please can someone fill in the gaps in my understanding.
To be more precise, I want to know exactly how time_taken is displayed on the screen, from the start of the process, to the cout. Like I say I am familiar with most areas but not all.

Answer 1

In your program, you had computed the following values:
plot_area = area(d_plot_length, d_plot_width);
home_area = area(d_home_side, d_home_side);
When the method area(double,double) is invoked, the resultant double value gets stored in these variables.
Then you have the function call:
time_taken = time(plot_area, home_area, d_mow_rate);
This is the call by value type of function invocation. A copy of the values in the variables, plot_area, home_area and d_mow_rate are passed to the function. In the time(double, double, double) the computing is done upon the basis of the logic you had defined in this method and the resultant value is returned to the function call in the main() method.
Please note that the function call is of call by value and hence only a copy of the values are passed to the arguments mentioned in the function time(double, double, double) even though the variable names are the same in the main() and in the function call.
For further reading, I will suggest you to have a look at the following links:
Call By
Value
Call By
Reference
Call By
Pointer