Variables, types and operators

A supermarket till has to remember how many items you bought, their prices and the total. Programs also keep information in variables. But C is strict: for every variable you must say in advance what its type is — whether it holds a whole number, a fractional number or a character. In return, C uses memory very economically and runs very fast.

Variables and basic types

A variable is declared like this: first the type, then the name, optionally a starting value after =, and a ; at the end. Names may contain letters, digits and _, but cannot start with a digit. age and Age are two different variables. After the declaration you can assign a new value — without writing the type again.

#include <stdio.h>

int main(void) {
    int apples = 7;
    double price = 0.85;
    char size = 'L';
    printf("Apples: %d\n", apples);
    printf("Price: %.2f AZN\n", price);
    printf("Total: %.2f AZN\n", apples * price);
    printf("Size: %c\n", size);
    apples = apples + 3;
    printf("Now: %d apples\n", apples);
    return 0;
}

Apples: 7
Price: 0.85 AZN
Total: 5.95 AZN
Size: L
Now: 10 apples

For fractional numbers you usually choose double: it takes twice as much space as float but is much more precise. The sizeof operator tells you how many bytes a type takes. Its result has the special type size_t and is printed with %zu.

#include <stdio.h>

int main(void) {
    printf("char:   %zu byte\n", sizeof(char));
    printf("int:    %zu bytes\n", sizeof(int));
    printf("float:  %zu bytes\n", sizeof(float));
    printf("double: %zu bytes\n", sizeof(double));
    return 0;
}

char:   1 byte
int:    4 bytes
float:  4 bytes
double: 8 bytes

A char is really a small integer: the computer stores every character as a code. In the ASCII table the code of 'A' is 65 and the code of 'B' is 66. So if you print a character with %d you see its code, and if you add 1 to it you get the next letter.

#include <stdio.h>

int main(void) {
    char letter = 'A';
    printf("%c has code %d\n", letter, letter);
    printf("Next letter: %c\n", letter + 1);
    return 0;
}

A has code 65
Next letter: B

Constants

Some values must never change while the program runs: the number π, the maximum score, a tax rate. There are two ways to make them constant. The const keyword makes a variable read-only: if you try to give it a new value, the compiler reports an error. #define is a preprocessor directive: before compiling, it simply replaces the name with the value in the text. Constant names are usually written in capital letters.

#include <stdio.h>
#define MAX_SCORE 100

int main(void) {
    const double PI = 3.14159;
    double r = 2.0;
    int score = 87;
    printf("Circle area: %.2f\n", PI * r * r);
    printf("Missing points: %d\n", MAX_SCORE - score);
    return 0;
}

Circle area: 12.57
Missing points: 13

Arithmetic operators and integer division

C has the operators +, -, *, / and %. The most important detail: when you divide two integers, the result is an integer too — the fractional part is simply dropped, not rounded. The % operator gives the remainder of the division and works only with integers. If at least one operand is a fractional number, the division is done the normal way.

#include <stdio.h>

int main(void) {
    int a = 17, b = 5;
    printf("%d + %d = %d\n", a, b, a + b);
    printf("%d - %d = %d\n", a, b, a - b);
    printf("%d * %d = %d\n", a, b, a * b);
    printf("%d / %d = %d\n", a, b, a / b);
    printf("%d %% %d = %d\n", a, b, a % b);
    printf("17.0 / 5 = %f\n", 17.0 / 5);
    return 0;
}

17 + 5 = 22
17 - 5 = 12
17 * 5 = 85
17 / 5 = 3
17 % 5 = 2
17.0 / 5 = 3.400000

Casts and shorthand operators

Suppose Murad wants to share 7 manats between 2 friends. 7 / 2 gives 3. To get the correct answer, you must turn one of the values into a double with a cast: (double) 7 / 2. Careful: if you write (double) (7 / 2), the integer division happens first and you get 3.0. In the other direction, (int) 9.99 drops the fractional part and gives 9.

#include <stdio.h>

int main(void) {
    int total = 7, people = 2;
    printf("%d\n", total / people);
    printf("%.1f\n", (double) total / people);
    printf("%.1f\n", (double) (total / people));
    printf("%d\n", (int) 9.99);
    int count = 10;
    count += 5;
    count++;
    printf("%d\n", count);
    return 0;
}

3
3.5
3.0
9
16