Type Conversion and Casting in C

In C, values of one data type can be converted to another. There are two kinds of conversions: implicit (automatic) and explicit (casting).

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Implicit Type Conversion (Promotion)

The compiler automatically converts a smaller type to a larger type to prevent data loss.

Hierarchy (low → high):

char → short → int → long → float → double → long double

int a = 5;
float b = 2.5;
float result = a + b;   /* a is promoted to float: 7.5 */

Integer promotion in expressions:

char c = 'A';   /* 65 */
int n = c + 1;  /* c promoted to int → 66 */

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Explicit Type Conversion (Casting)

You force a conversion using the cast operator (type).

int a = 7, b = 2;
float result = (float)a / b;   /* 3.5, not 3 */

double d = 9.99;
int i = (int)d;   /* i = 9, decimal truncated */

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Common Pitfalls

Integer division:

int a = 7, b = 2;
float result = a / b;          /* WRONG: 3.0 (integer division first) */
float correct = (float)a / b;  /* RIGHT: 3.5 */

Overflow:

int big = 1000000;
short s = (short)big;   /* overflow — undefined behaviour */

Signed/unsigned mismatch:

unsigned int u = 10;
int i = -1;
if (i < u)   /* WARNING: -1 compared as large unsigned number */
    printf("i is less");

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Conversion Table