Logical constructs in C - if..else & switch..case statements

if(<expression>)
{
    <statement block>
}

OR

if(<expression>)
{
    <statement block 1>
}
else
{
    <statement block 2>
}

#include <stdio.h>
main()
{
    int min, x, y;
    printf("\nProvide two numbers:");
    scanf("%d %d", &x, &y);
    fflush(stdin);

    /* Computes minimum of x and y */
    if(x < y)
        min = x;
    else
        min = y;
    printf("%d is the minimum of %d and %d\n", min, x, y);
}

if(<expression1>) {     <statement block1> } else if(<expression2>) {     <statement block2> } else {     <statement block3> }

if(<expression1>) {     if(<expression2>)     {         <statement block1>     }     else     {         <statement block2>     } } else {     <statement block3> }

Note that these two examples are not the same.

#include <stdio.h>
#define  TRUE  1
#define  FALSE 0

main()
{
	int EQUAL, min, max, x, y;
	printf("\nProvide two numbers:");
	scanf("%d %d", &x, &y);
	fflush(stdin);

	/* Compare the two numbers x and y */
	if(x == y)
	{
		EQUAL = TRUE;
		min = max = x;
	}
	else
	{
		EQUAL = FALSE;
		if(x > y)
		{
			min = y; 
			max = x; 
		}
		else
		{
			min = x; 
			max = y; 
		}
	}

	if(EQUAL)
		printf("Both x and y are equal\n");
	else
		printf("Minimum: %d, Maximum: %d \n", min, max);
}

1. Identify the mistake in the following C program.

	#include <stdio.h>
	main()
	{
		float basic;
		printf("Enter basic: ");
		scanf("%f", &basic);
		fflush(stdin);
		if(basic = 0)
			printf("Invalid Input\n");
		else
			printf("Basic is %.2f", basic);
	}


2. Predict the output of the following C program.

	#include <stdio.h>
	main()
	{
		int a, b, c;
		scanf("%d %d %d", &a, &b, &c);
		if(a > b)
		{
			a += b;
			a ++;
		}
		if(a > c)
			a *= c;
		else
			c -= (a + b);
	
		printf("%d %d %d\n", a, b, c);
	}

switch <expression> 
{
	case <expression 1>
		<statement block 1>
		break;

	case <expression 2>
		<statement block 2>
		break;

	case <expression 3>
		<statement block 3>
		break;
	...

	case <expression n>
		<statement block n>
		break;

	default:
		//When none of the above expressions evaluate to true
		<statement block n+1>
		break;
}

/* Program having nested if statements */
#include <stdio.h>
main()
{
	char category;
	printf("Enter Category: ");
	category = getchar();
	fflush(stdin);

	if(category == 'B')
	{
		printf("B.TECH students\n");
		/* Statements for B.TECH Processing will go here */
	}
	else if(category == 'M')
	{
		printf("M.Sc. Student \n");
		/* Statements for M.Sc Processing will go here */
	}
	else if(category == 'T')
	{
		printf("M.TECH Student \n");
		/* Statements for M.TECH Processing will go here */
	}
	else if(category == 'P')
	{
		printf("Ph. D. Student \n");
		/* Statements for Ph.D Processing will go here */
	}
	else
	{
		printf("ERROR\n");
		/* Statements for Error Processing will go here */
	}
}


/*
  Such programs with multiple condition checks can be better implemented with
  switch-case construct. The advantage is better readability of the code.
*/
#include <stdio.h>
main()
{
	char category;
	printf("Enter Category: ");
	category = getchar();
	fflush(stdin);

	switch(category)
	{
		case 'B' :
			printf("B.TECH students\n");
			/* B.TECH Processing */
			break;
		
		case 'M' :
			printf("M.Sc. Student \n");
			/* M.Sc Processing */
			break;
	
		case 'T' :
			printf("M.TECH Student \n");
			/* M.TECH Processing */
			break;

		case 'P' :
			printf("Ph. D. Student \n");
			/* Ph. D. Processing */
			break;
		
		default :
			printf("ERROR\n");
			/* Error Processing */
	}
}