Variables: named boxes

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A variable is a box with a name

int marks = 72; makes a box called marks that holds whole numbers (int) and puts 72 in it.

In C you must say the type when you create the variable. After that, you use just the name.

Read this lesson as text

Declare a variable with its type, give it a value, and print it.

A variable is a box with a name

int marks = 72; makes a box called marks that holds whole numbers (int) and puts 72 in it.

In C you must say the type when you create the variable. After that, you use just the name.

Watch memory change

#include <stdio.h>

int main() {
    int marks = 72;
    int bonus = 5;
    int total = marks + bonus;
    printf("Total: %d\n", total);
    return 0;
}

Line 4: A box named marks is created, holding 72.
Line 5: Another box, bonus, holding 5.
Line 6: C reads marks and bonus, adds them, and stores 77 in a new box.
Line 7: %d is replaced by the value in total.

Name or value?

#include <stdio.h>

int main() {
    int apples = 4;
    printf("apples = %d\n", apples * 3);
    return 0;
}

What does this print?

apples = 12
12 = 12
apples = apples * 3

Show the answer

apples = 12 Text in quotes is printed as it is; the %d is filled by the value of apples * 3.

Spot the mistake

#include <stdio.h>

int main() {
    int price = 40;
    printf("%d\n", Price);
    return 0;
}

Which line has the mistake?

Show the answer

Line 5. C cares about capitals: Price and price are different names, and Price was never declared.

Exercise: Area and perimeter

Make two int variables, length = 12 and width = 5.

Print Area: 60 and then Perimeter: 34, worked out from the variables.