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Variables in C

A variable is a named container that occupies a memory location to store a value. That value can be read and changed during program execution.

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Declaration and Initialization

Declaration — reserves memory for the variable: 
int number;
Initialization — declaration + assigning a value: 
int number = 10;
Syntax: 
data_type variable_name;
data_type variable_name = value;

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Naming Conventions

StyleExampleUsed In
Pascal CaseUserNameTypes, structs
Camel CaseuserNameVariables, functions
Snake Caseuser_nameC variables, Python
Kebab Caseuser-nameNot valid in C

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Variable Classification by Data Type

Data TypeKeywordSizeRange
Characterchar1 byte−128 to 127
Integerint4 bytes±2.1 billion
Short Integershort2 bytes−32,768 to 32,767
Long Integerlong4–8 bytesPlatform dependent
Floatfloat4 bytes±3.4 × 10³⁸
Doubledouble8 bytes±1.7 × 10³⁰⁸
Long Doublelong double10–16 bytesEven higher

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Variable Classification by Scope

TypeDeclared InScopeLifetime
LocalA function/blockThat block onlyUntil block ends
GlobalOutside functionsWhole fileWhole program
StaticWith staticLimited, keeps valueUntil program ends
ExternAnother fileGlobalDepends on origin
RegisterWith registerCPU registerUntil block ends

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Format Specifiers

Used with printf() and scanf() to read/print specific types:
SpecifierData TypeExample Output
%dint25
%ffloat5.68
%lfdouble3.14
%ccharA
%sstringHello
printf("%d", 25);       // 25
printf("%.2f", 5.6789); // 5.68
printf("%.2lf", 3.14);  // 3.14
printf("%c", 'A');      // A
printf("%s", "Hello");  // Hello

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Example

#include <stdio.h>

int main() {
    int age = 20;
    float cgpa = 8.75;
    char grade = 'A';

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

    return 0;
}