Back to Chapter 1

Sub Programs Activities

Constructs

  1.  global choice
 global cm
 global inches

 procedure main()
     while True // iteration/loop
         menu()
 endprocedure

 procedure menu()
     print(“1. cm to inches”)
     print(“2. inches to cm”)
     print(“3. Quit”)
     choice = 0
     while choice < 1 or choice > 3
         choice = input(“Enter choice”)
         if choice == "1" then // branching/selection
             cm_to_in()
         elseif choice == "2" then
             in_to_cm()
         else
             quit()
 endprocedure

 procedure cm_to_in()
     cm = input("Enter cm: ") // sequence
     inches = cm * 0.393700787
     print (inches&" inches")
 endprocedure

 procedure in_to_cm()
     inches = input("Enter inches: ")
     cm = inches * 2.54
     print (cm&" cm")
 endprocedure

  2. [above]

  3. for i in {0..25}

  4. while i != 3.14

  5. When the number of iterations needed is known

  6. When the setup for the loop is the same as the body of the loop

Global and Local variables

  1.  #include <stdbool.h>
 #include <stdio.h>

 int main() {
     while (true) {
         printf("1. cm -> in\n2. in -> cm\n3. Quit\n>>> ");
         int input = getchar() - '0';
         switch (input) {
         case 1:
             double user_length;
             printf("\nEnter length in cm\n>>> ");
             scanf("%d", &user_length);
             user_length *= 0.393700787;
             printf("\n%.2d inches\n");
             break;
         case 2:
             double user_length;
             printf("\nEnter length in inches\n>>> ");
             scanf("%d", &user_length);
             user_length *= 2.54;
             printf("\n%.2d cm\n");
             break;
         case 3:
             return 0;
         default:
             printf("\nInvalid selection.\n");
         }
     }
 }

  2. ​

    • Global variables are variables that are accessible from any part of the program.
    • Local variables are variables that are only accessible from the scope they are defined in (or any sub-scope)
    • Scopes are blocks of code with access to a specific set of variables. In Python, only functions and classes create their own scopes. In JavaScript, any pair of curly braces ({}) can create its own scope - but only variables created with let obey this.
    • Scope and local variables can be used to minimise the memory access dangers within your program - preventing unexpected changes to a value due to a subroutine overwriting it - however globals can be used to reduce nesting call parameters - if a function at the bottom of the call stack requires a parameter in order to localise the parameter it must be passed to or generated by the functions leading to it

Parameter passing by value and by reference

Passing by reference

#include <stdio.h>

int substract_deductions(int pay, int percent) {
    return (pay * percent) / 100;
}

void calculate_pay(int *pay) {
    *pay = *pay - subtract_deductions(pay, 22);
}

int main() {
    int pay = 2000;
    calculate_pay(&pay);
    printf("%i\n", pay);
    return 0;
}

#include <stdio.h>

int substract_deductions(int pay, int percent) {
    return (pay * percent) / 100;
}

void calculate_pay(int pay) {
    pay = pay - subtract_deductions(pay, 22);
}

int main() {
    int pay = 2000;
    calculate_pay(&pay);
    printf("%i\n", pay);
    return 0;
}