C Data Types

Choose a representation, inspect sizes with sizeof, and convert values without pretending sizes are universal.

A type is a representation, not a label

A C type decides which values an object can hold, which operators apply, and how those values are stored on this implementation. It is not a Python-style runtime tag you can query after the fact. Choose the type that matches the problem: a count, a character code, a measurement, or a yes/no flag.

Later lessons add arrays, pointers, and structures. This lesson introduces the scalar types you will use constantly. Detailed collection behavior belongs in C Arrays and C Strings.

Integer types have minimum ranges, not universal sizes

The usual whole-number type is int. short and long exist when you need a smaller or larger integer, and signed versus unsigned changes whether negative values are representable. The C standard requires minimum ranges—for example int must cover at least −32767 through 32767—but it does not require int to be 32 bits on every machine.

Do not write production code that assumes int is four bytes because that happened to be true on one laptop. When a width is a requirement, use a fixed-width type from <stdint.h> such as int32_t, or document the limit you actually need.

char is an integer type

A character literal such as 'A' has an integer value—the character's code. Printing with %c shows the glyph; printing with %d shows the code. That is why C has no built-in string object: text is an array of char values ending in \0, taught in C Strings.

#include <stdio.h>

int main(void) {
  char grade = 'A';

  printf("Grade letter: %c\n", grade);
  printf("Grade code: %d\n", grade);
  return 0;
}

Compile this locally. The lesson runner used in earlier pages understands int, double, and character arrays, not a standalone char object. The language rule is the same: char is an integer type.

float and double approximate real numbers

float and double store binary floating-point values. They are appropriate for measurements and rates, not for money that must be exact to a decimal cent. double is the usual beginner choice for fractional quantities. Binary floating-point cannot represent every decimal fraction exactly, so later numeric work needs a deliberate rounding or decimal policy.

sizeof reports this implementation

sizeof(int) is the size in bytes of an int on the compiler and target you are using. It is a useful measurement, not a portable constant you should hard-code into a protocol. Compile the snippet locally and compare it with a friend’s machine before treating a size as a file format.

#include <stdio.h>

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

sizeof(char) is 1 by definition. The others are implementation-defined. Use %zu for a size_t value such as the result of sizeof.

Convert on purpose

C converts operands in mixed expressions using integer promotions and the usual arithmetic conversions. A common surprise is integer division: 7 / 2 is 3 because both operands are int. Writing 7 / 2.0 makes one operand floating-point, so the result can keep a fraction. An explicit cast such as (double)total / count documents the conversion. Casts do not make overflow safe; they only reinterpret or convert a value according to the language rules.

#include <stdio.h>

int main(void) {
  int whole_pages = 7;
  int groups = whole_pages / 2;
  double hours = 7 / 2.0;

  printf("Integer groups: %d\n", groups);
  printf("Hours: %f\n", hours);
  return 0;
}

Edit the program, predict its output, then run it.

bool is a small integer; C has no None

C11 and later provide bool through <stdbool.h>, with true and false. Historically, C treated zero as false and nonzero as true—the rule C If Else still depends on. There is no Python None. Absence is modeled with a sentinel, a flag, or a null pointer once you reach pointers.

Inspect types on your compiler

Save the sizeof program as types.c and compile with warnings enabled. The printed sizes are facts about that build, not about every C implementation.

Independent lab: report mixed types

Write a small program that prints an int count, a double duration, and a char grade. Predict the printf placeholders before you run it. Then add a sizeof line for int and explain why that number might differ on another computer.

Lesson review

• A type decides representation and operations, not a runtime object tag.
• int, char, float, and double have standard minimums; exact widths are implementation-defined unless you use a fixed-width type.
• sizeof measures this compiler and target.
• Integer division discards a fraction; convert deliberately when the fraction matters.
• C strings are character arrays, taught next in the strings lesson.