Variables and types
Store values in named boxes of memory, and learn why every box in C has a type and a size.
- Declare and initialize variables of the basic types
- Choose between
int,doubleandcharfor a value - Use
sizeofand explain why types have sizes
A variable is a named piece of memory that holds a value. In C you must say what kind of value each variable holds - its type - before you use it. The type tells the compiler how many bytes to set aside and how to interpret them.
1#include <stdio.h>
2
3int main(void) {
4 int age = 30;
5 double height = 1.75;
6 char grade = 'A';
7
8 printf("Age: %d\n", age);
9 printf("Height: %.2f m\n", height);
10 printf("Grade: %c\n", grade);
11 return 0;
12}Age: 30 Height: 1.75 m Grade: A
Declaring and initializing
int age = 30; does two things: it declares a variable named age of type int, and initializes it with the value 30. You can also declare first and assign later:
int score; // declared, but holds garbage until assigned!
score = 100; // assignment
score = score + 5;Variable names can use letters, digits and underscores, cannot start with a digit, and are case-sensitive: total and Total are different variables. Pick names that say what the value means: student_count, not n.
The basic types
| Type | Holds | Typical size | printf format |
| --- | --- | --- | --- |
| char | one character (really a small integer) | 1 byte | %c |
| int | whole numbers | 4 bytes | %d |
| long | bigger whole numbers | 8 bytes on most 64-bit systems | %ld |
| float | decimal numbers, ~7 digits of precision | 4 bytes | %f |
| double | decimal numbers, ~15 digits of precision | 8 bytes | %f |
Integer types can be unsigned too (unsigned int), which trades negative numbers for twice the positive range. For decimals, prefer double over float unless you have a reason - the extra precision is almost always worth it.
Sizes and sizeof
Every type occupies a fixed number of bytes, and the sizeof operator tells you how many. Sizes can differ between platforms (the C standard only guarantees minimums), which is exactly why sizeof exists: never hard-code a size you can ask the compiler for. sizeof produces a value of type size_t, which you print with %zu.
1#include <stdio.h>
2
3int main(void) {
4 printf("char: %zu byte\n", sizeof(char));
5 printf("int: %zu bytes\n", sizeof(int));
6 printf("double: %zu bytes\n", sizeof(double));
7 return 0;
8}char: 1 byte int: 4 bytes double: 8 bytes
Characters are numbers
A char is a one-byte integer. Character literals in single quotes, like 'A', are just a friendly way of writing that character's code - 65 for 'A' in ASCII. That means you can do arithmetic on characters:
1#include <stdio.h>
2
3int main(void) {
4 char letter = 'A';
5 printf("%c is %d\n", letter, letter);
6 printf("Next letter: %c\n", letter + 1);
7 return 0;
8}A is 65 Next letter: B
Constants
If a value should never change, say so with const: const double PI = 3.14159;. The compiler will refuse any later assignment to it, which documents your intent and catches mistakes. You will also see #define PI 3.14159 in older code - a preprocessor text substitution with no type. Prefer const for values.
Key takeaways
Every variable has a type that fixes its size and meaning. Declare it, and initialize it - uninitialized locals hold garbage.
Use
intfor whole numbers,doublefor decimals,charfor single characters (which are small integers).Ask for sizes with
sizeof(printed with%zu) instead of assuming them, and mark values that never changeconst.
Lesson quiz
6 questions · pass with 5 correct · up to 50 XP
Passing this quiz completes the lesson and keeps your streak going. Questions you miss come back in review sessions later.
Practice: write real C
These run for real. Write the program, press Run tests, and read what the compiler tells you - learning to read compiler messages is half of learning C.
Profile card
Create variables for a person and print them. Use an int named age set to 21, a double named gpa set to 3.8, and a char named initial set to 'K'. Print exactly:
Initial: K
Age: 21
GPA: 3.8Print the GPA with one digit after the decimal point (%.1f).
- Prints the profile
Your code is compiled with gcc 14 (-Wall -Wextra) and run on Compiler Explorer (godbolt.org), a free public service.
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