c 1/5 20 XP

C Program Structure

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Quick Reference

Minimal C program:

#include <stdio.h>          // 1. Headers

#define MAX 100              // 2. Constants

void greet(void);            // 3. Prototypes

int main(void)               // 4. Entry point
{
    greet();
    return 0;                // 0 = success
}

void greet(void)             // 5. Definitions
{
    printf("Hello!\n");
}

Common headers:

Header	Provides
`<stdio.h>`	printf, scanf, fopen, fgets
`<stdlib.h>`	calloc, malloc, free, exit, atoi
`<string.h>`	strlen, strcmp, strcpy, memcpy
`<ctype.h>`	isalpha, isdigit, toupper, tolower
`<math.h>`	sqrt, pow, sin (link with -lm)

Format specifiers:

Specifier	Type	printf	scanf
`%d`	int	`%d`	`%d`
`%f`	double	`%f`	`%lf` (!!)
`%c`	char	`%c`	`%c`
`%s`	string	`%s`	`%s`
`%zu`	size_t	`%zu`	—

Java → C translation:

Java	C
`System.out.println(x)`	`printf("%d\n", x);`
`Scanner sc = new Scanner(System.in)`	`scanf("%d", &x);`
`public static void main(String[] args)`	`int main(void)`
`import java.util.*`	`#include <stdio.h>`

Common Pitfalls

Forgetting \n — printf doesn’t add newlines. You must include \n explicitly.
Wrong format specifier — %d for int, %f for double in printf but %lf in scanf. Mismatch = garbage.
Missing & in scanf — scanf("%d", x) crashes. Use scanf("%d", &x).
() vs (void) — Empty () means unspecified args in C. Use (void) for no parameters.
Forgetting headers — Each function needs its header. printf needs <stdio.h>, malloc needs <stdlib.h>.

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