Basic Syntax

Decision

Boolean

By default, C takes 0 as False, none 0 as True

#include <stdio.h>

int main()
{
	if (1)
		printf("True\n"); // print out
	else
		printf("Flase\n");

	return 0;
}

C99 introduces the bool type to C, define true/false in stdbool.h

#include <stdio.h>
#include <stdbool.h>

int main()
{
	bool b = true;

	if (b)
		printf("True\n");
	else
		printf("False\n");

	b = (10%2 == 0);

	if (b)
		printf("Even\n");
	else
		printf("Odd\n");

	return 0;
}

Braces { and } are used to group declarations and statements together into a compound statement, or block, so that they are syntactically equivalent to a single statement

#include <stdio.h>

int main()
{
	int age = 20;

	if (age >= 21)
	{//start block
		printf("You are allowed to drink ...");
	}//end block
	else
	{
		printf("You are not allowed to drink ...");
	}

	return 0;
}

If there is only one line of code, the curly braces can be ignored

#include <stdio.h>

int main()
{
	int age = 20;

	if (age >= 21)
		printf("You are allowed to drink ...");
	else
		printf("You are not allowed to drink ...");

	return 0;
}

else sentence is not necessary

#include <stdio.h>

int main()
{
	int age = 20;

	if (age >= 21)
		printf("You are allowed to drink ...\n");

	return 0;
}

if ... else if ... else

#include <stdio.h>

int main()
{
	int num = 20;

	if (num < 0)
		printf("Negative Number ...\n");
	else if (num >= 0)
		printf("Non-negative Number ...\n");
	else
		printf("Unknown Number ...\n");

	return 0;
}

Nested if ... else ...

#include <stdio.h>

int main()
{
	int num = 20;

	if (num >= 0)
	{
		if (num > 0)
			printf("Natural Number ...\n");
		else
			printf("Zero ...\n");
	}
	else
		printf("Negative Number ...\n");

	return 0;
}

Relational Operators

==, !=, >, < >=, <=

Logical Operators

&&, and

||, or

#include <stdio.h>

int main()
{
	int num = 20;

	// and
	if (num >= 0 && num%2 == 0)
		printf("Qualified Number ...\n");

	if ((num >= 0) && (num%2 == 0))
		printf("Qualified Number ...\n");

	// or
	if (num >= 0 || num%2 == 0)
		printf("Qualified Number ...\n");

	if ((num >= 0) || (num%2 == 0))
		printf("Qualified Number ...\n");

	// not
	if (!(num%2 == 1))
		printf("Qualified Number ...\n");

	return 0;
}

Relational Operators

<, <=, > >=

Switch

#include <stdio.h>

int main()
{
	int option;

	printf("Enter an option (1-3):\n");
	scanf("%d", &option);

	switch(option)
	{
		case 1:
			printf("Install Software ...\n");
			break;
		case 2:
			printf("Unstall Software ...\n");
			break;
		case 3:
			printf("Readme\n");
			break;
		default:
			printf("Exit ...\n");
	}

	return 0;
}

Reference

Operators Precedence in C