Bytebook Compiler

Getting Started

Variables, Data Types & Constants

Store values in variables, choose the right data type, print them with the right format specifier, and lock values with constants.

4 min read5 runnable examples4-question quiz

A variable is a named box in memory that holds a value. Every variable in C has a type, which decides how many bytes the box uses and what kind of value it can hold.

C
#include <stdio.h>

int main(void) {
    int age = 19;          // whole number
    float cgpa = 8.75f;    // decimal number
    char grade = 'A';      // single character

    printf("Age: %d\n", age);
    printf("CGPA: %.2f\n", cgpa);
    printf("Grade: %c\n", grade);

    age = age + 1;         // variables can change
    printf("Next year: %d\n", age);
    return 0;
}
Output
Age: 19
CGPA: 8.75
Grade: A
Next year: 20
Try it Yourself

Declaring and naming variables

C
int count;            // declaration (value is garbage until you assign one)
count = 10;           // assignment
int total = 0;        // declaration + initialisation
int a = 1, b = 2;     // several at once

Naming rules (these are called identifiers):

  • Use letters, digits and underscores only: total_marks, num1.
  • Must not start with a digit: 1num is invalid.
  • Cannot be a keyword such as int, return, while.
  • Case matters: Sum and sum are different variables.

Data types

Type Typical size Holds Example
char 1 byte one character or small integer 'A'
short 2 bytes small integers 300
int 4 bytes integers, about ±2.1 billion -42
long long 8 bytes very large integers 9000000000LL
float 4 bytes decimals, ~7 digits of precision 3.14f
double 8 bytes decimals, ~15 digits of precision 3.14159265

Add unsigned to an integer type to store only non-negative values with double the positive range. Sizes can vary between machines, so C gives you the sizeof operator to check:

C
#include <stdio.h>

int main(void) {
    printf("char      : %zu byte\n", sizeof(char));
    printf("short     : %zu bytes\n", sizeof(short));
    printf("int       : %zu bytes\n", sizeof(int));
    printf("long long : %zu bytes\n", sizeof(long long));
    printf("float     : %zu bytes\n", sizeof(float));
    printf("double    : %zu bytes\n", sizeof(double));
    return 0;
}
Output
char      : 1 byte
short     : 2 bytes
int       : 4 bytes
long long : 8 bytes
float     : 4 bytes
double    : 8 bytes
Try it Yourself

Format specifiers

The specifier must match the variable's type, or printf prints nonsense.

Specifier Type Specifier Type
%d or %i int %c char
%u unsigned int %s string
%ld long %x / %o hex / octal
%lld long long %e scientific notation
%f float / double %p pointer address
%.2f 2 decimal places %zu sizeof result
C
#include <stdio.h>

int main(void) {
    int n = 255;
    double pi = 3.14159265;
    printf("%d in hex is %x and in octal is %o\n", n, n, n);
    printf("pi = %f\n", pi);
    printf("pi = %.3f\n", pi);
    printf("pi = %e\n", pi);
    printf("[%5d] [%-5d]\n", 42, 42);   // width 5, right and left aligned
    return 0;
}
Output
255 in hex is ff and in octal is 377
pi = 3.141593
pi = 3.142
pi = 3.141593e+00
[   42] [42   ]
Try it Yourself

Characters are numbers

A char stores a small integer — its ASCII code. 'A' is 65, 'a' is 97 and '0' is 48. That is why you can do arithmetic on characters:

C
#include <stdio.h>

int main(void) {
    char c = 'A';
    printf("%c has ASCII code %d\n", c, c);
    printf("Next letter: %c\n", c + 1);
    printf("Lowercase: %c\n", c + 32);
    printf("Digit '7' as a number: %d\n", '7' - '0');
    return 0;
}
Output
A has ASCII code 65
Next letter: B
Lowercase: a
Digit '7' as a number: 7
Try it Yourself

Constants

Some values should never change. C gives you two ways to make them:

C
#include <stdio.h>
#define GRAVITY 9.8          // preprocessor constant: text replaced before compiling

int main(void) {
    const int DAYS_IN_WEEK = 7;   // typed constant: the compiler refuses changes
    printf("Weeks in 28 days: %d\n", 28 / DAYS_IN_WEEK);
    printf("Weight of 50 kg: %.1f N\n", 50 * GRAVITY);
    return 0;
}
Output
Weeks in 28 days: 4
Weight of 50 kg: 490.0 N
Try it Yourself

Trying DAYS_IN_WEEK = 8; would be a compile error: assignment of read-only variable.

Test yourself

4 questions · pick an answer to check it instantly

  1. 1

    Which is a valid variable name in C?

  2. 2

    Which format specifier prints a double with exactly two decimal places?

  3. 3

    What does this print?

    C
    char c = 'B';
printf("%d", c);
  4. 4

    What is the main difference between const int X = 5; and #define X 5?

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