C BASICS

Taking user input

#include <stdio.h>
int main(){
	int a;
	char s[10];
	scanf("%d", &a);
	scanf("%s",s);	
	printf("%d", a);
	return 0;
}

Alternatively getchar can also be used to get one single character from user but this is not recommended due to security vulnerabilities.

Format specifier	Data type
%d	int
%ld	long
%lld	long long
%lf	double
%Lf	long double
%f	float
%c	char
%s	string(array of char)
%hi	short
%hu	unsigned short
%p	pointer(or memory address)
%u	unsigned int
%o	octal
%x	hexadecimal
”%.2f” is used to print float numbers up to 2 decimal places.
”%6d” means increased width for integer.

Macros

Using for variables

#include <stdio.h>
#define MAX 10

int main(){
	int n=MAX;
	printf("Max is: %d", n);
	return 0;
}

Using for functions

#include <stdio.h>
#define AREA(l, b) (l*b)

int main() {
	int l, b;
	scanf("%d %d", &l, &b);
	printf("Area is: %d", AREA(l, b));
	return 0;
}

Can also chain macros, one macro calling another macro is valid

Multi line macro

#include <stdio.h>
#define ELE 1, \
			2, \
			3
int main() {
	int arr[] = { ELE };
	...
	return 0;
}

Pre defined Macros

__FILE__ - Current file name __LINE__ - Current line number __DATE__ - Compilation date (as string) __TIME__ - Compilation time (as string)

Preprocessor Directives

1. ifdef and ifndef

Used to include a section of code if a certain macro is defined or not defined. Syntax:

#define DEBUG 1
#ifdef DEBUG
	printf("DEBUG mode is enabled");
#endif
#ifndef DEBUG
	printf("DEBUG mode is not enabled");
#endif

(endif is used to signify ending of ifdef ifndef blocks)

2. if, elif and else

Same as if elif else from main program.

#if macro_condition  
   statements  
#elif macro_condition  
   statements  
#else
   statements  
#endif

3. error

error is used to abort compilation process and throw error.

#ifndef SOMEMACRO
	#error SOMEMACRO not found!

Compilation steps

1. Pre processing

Handles directives like include and define

2. Compilation

Translates C code to Assembly

3. Assembly

Assembler converts assembly code into pure binary code.

4. Linking

Linker merges all the object code from multiple modules into one.

Comments

//this is a single line comment
/* this
is a multi line
comment */

Variable naming rules

1. Start with letter or underscore
2. Can have digits, letters or underscores
3. Case sensitive
4. No whitespaces

Variable

1. Global
2. Local
3. Static
4. Register

// Global Variable
int globalVar;
void function() {
	// Local Variable
	int localVar;
	// Static Variable
	static int staticVar = 0;
	staticVar++;
	// Register Variable
	for(register int i = 0; i < 10; i++) {
	// Fast access within loop
	}
}

Data types

long double can be 12 or 16 bytes.

Operators

When comparing unsigned int with signed, the signed int is automatically converted to unsigned for comparison purposes.

Escape sequences

Sequence	Function
\n	New line
\t	Tab space
\“	To print double quotes
\‘	To print single quotes

Switch statement

switch is used to execute one condition from multiple conditions.

int var = 1;
switch(var) {
	case 1:
		printf("Var is 1");
		break;
	case 2:
		printf("Value of var is 2");
		break;
	default:
		printf("Value of var is unknown");
		break;
}

Pointers

A pointer (*) provides a reference to an entity of the referenced type. Also used as indirection or dereferencing operator.

int a=17;
int *p_a=&a //returns the memory address of variable a

Use * again on a pointer to get the value.

int value=*p_a;

Use & to turn value into pointer(by getting memory address) Pointers can be used to reference arrays too. By default zeroth index is returned.

int a[10];
int i=*a; //value of 0th element in a
int j=*(a+1) //value of 2nd elmeent in a