Here is a sample program in C++ to calculate the area and perimeter of a number of rectangles using a for loop:
```
#include <iostream>
using namespace std;
int main() {
int num_rectangles;
cout << "Enter the number of rectangles: ";
cin >> num_rectangles;
for (int i = 1; i <= num_rectangles; i++) {
float length, width, area, perimeter;
cout << "Enter the length and width of rectangle " << i << ": ";
cin >> length >> width;
area = length * width;
perimeter = 2 * (length + width);
cout << "The area of rectangle " << i << " is: " << area << endl;
cout << "The perimeter of rectangle " << i << " is: " << perimeter << endl;
}
return 0;
}
```
Step-by-step explanation:
- The program first asks the user to enter the number of rectangles they want to calculate the area and perimeter for.
- It then uses a for loop to iterate through each rectangle, from 1 to the user-specified number of rectangles.
- For each rectangle, the program asks the user to enter the length and width, and calculates the area and perimeter using the formulas `area = length * width` and `perimeter = 2 * (length + width)`.
- Finally, the program displays the calculated area and perimeter for each rectangle.