How to write a C program to find maximum between three numbers using switch statement?

#include <stdio.h>

int main()
{
    int num1, num2, num3;
    printf("Enter three integers to find maximum: \n");
    scanf("%d%d%d", num1, num2, num3);
    switch (num1 > num2)
    {
        switch (num1 > num3)
        {
        case 1:
            printf("%d is maximum", num1);
            break;
            switch (num2 > num1)
            {
                switch (num2 > num3)
                {
                case 1:
                    printf("%d is maximum", num2);
                    break;
                default:
                    printf("%d is maximum", num3);
                }
            }
        }
    }
}

This is the switch statement program I wrote in VS Code to find maximum between 3 numbers, but it doesn't work. I declared 3 variables num1, num2, num3 then I compared num1 to num2 and num3, Similarly num2 to num1 and num3, num3 is in default.

Can we write maximum between 3 numbers?

There are missing case or default clauses in your switch statement bodies.

Note also that you must pass the addresses of the target variables to scanf() and you should test that this scanf succeeded at converting 3 integers: it returns the number of successful conversions.

Here is a modified version:

#include <stdio.h>

int main(void) {
    int num1, num2, num3;

    printf("Enter three integers to find maximum: \n");

    if (scanf("%d%d%d", &num1, &num2, &num3) != 3) {
        printf("invalid input\n");
        return 1;
    }

    switch (num1 > num2) {
    case 1:
        switch (num1 > num3) {
        case 1:
            printf("%d is maximum\n", num1);
            break;
        default:
            printf("%d is maximum\n", num3);
            break;
        }
    default:
        switch (num2 > num3) {
        case 1:
            printf("%d is maximum\n", num2);
            break;
        default:
            printf("%d is maximum\n", num3);
            break;
        }
    }
    return 0;
}

And here is an alternative with a single switch statement:

#include <stdio.h>

int main(void) {
    int num1, num2, num3;

    printf("Enter three integers to find maximum: \n");

    if (scanf("%d%d%d", &num1, &num2, &num3) != 3) {
        printf("invalid input\n");
        return 1;
    }

    switch ((num1 >= num2) + 2 * (num1 >= num3) + 4 * (num2 >= num3)) {
    case 3:
    case 7:
        printf("%d is maximum\n", num1);
        break;
    case 4:
    case 6:
        printf("%d is maximum\n", num2);
        break;
    case 0:
    case 1:
        printf("%d is maximum\n", num3);
        break;
    default:
        printf("CPU is broken\n");
        break;
    }
    return 0;
}

Finally, abusing the rules, here is a simplified version still using a switch statement, that is easy to extend to a larger set of numbers:

#include <stdio.h>

int main(void) {
    int num1, num2, num3, max;

    printf("Enter three integers to find maximum: \n");

    if (scanf("%d%d%d", &num1, &num2, &num3) != 3) {
        printf("invalid input\n");
        return 1;
    }

    switch (max = num1,
            max = (max >= num2) * max + (max < num2) * num2,
            max = (max >= num3) * max + (max < num3) * num3)
    {
    default:
        printf("%d is maximum\n", max);
        break;
    }
    return 0;
}

For completeness, the switch statement here above can be rewritten with an empty body:

    switch (max = num1,
            max = (max >= num2) * max + (max < num2) * num2,
            max = (max >= num3) * max + (max < num3) * num3)
        /* empty body */;

    printf("%d is maximum\n", max);

For starters the arguments of this call of scanf

scanf("%d%d%d", num1, num2, num3);

are incorrect. You need to write

scanf("%d%d%d", &num1, &num2, &num3);

And in general you should check whether the input was successful as for example

if ( scanf("%d%d%d", &num1, &num2, &num3) == 3 )
{
    //...
}
else
{
    //...
}

Secondly the user can enter at least two equal each other numbers that are gretaer than the third number. You have to take that into account.

To make it simpler to write switch statements it will be useful to perform the task using if statements. They can look the following way

	if (!( num1 < num2 ) && !( num1 < num3 ))
	{
		printf( "%d is maximum\n", num1 );
	}
	else if ( !( num2 < num3 ) )
	{
		printf( "%d is maximum\n", num2 );
	}
	else
	{
		printf( "%d is maximum\n", num3 );
	}

So now it is not hard to rewrite the if statements using switch statements. They can look the following way

	switch (!( num1 < num2 ) && !( num1 < num3 ))
	{
	case 1:
		printf( "%d is maximum\n", num1 );
		break;

	case 0:
		switch (!( num2 < num3 ))
		{
		case 1:
			printf( "%d is maximum\n", num2 );
            break;

		case 0:
			printf( "%d is maximum\n", num3 );
            break;
		}
	}

Instead of using the constants 0 and 1 in the case labels you can use named constants false and true by means of including header <stdbool.h> as for example

#include <stdbool.h>

//...

	switch (!( num1 < num2 ) && !( num1 < num3 ))
	{
	case true:
		printf( "%d is maximum\n", num1 );
		break;

	case false:
		switch (!( num2 < num3 ))
		{
		case true:
			printf( "%d is maximum\n", num2 );
            break;

		case false:
			printf( "%d is maximum\n", num3 );
            break;
		}
	}

Of course there are other ways to write the switch statements but this way is enough clear.

Here is a demonstration program.

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

int main( void )
{
	int num1, num2, num3;

	printf( "Enter three integers to find maximum: " );
	scanf( "%d%d%d", &num1, &num2, &num3 );

	if (!( num1 < num2 ) && !( num1 < num3 ))
	{
		printf( "%d is maximum\n", num1 );
	}
	else if ( !( num2 < num3 ) )
	{
		printf( "%d is maximum\n", num2 );
	}
	else
	{
		printf( "%d is maximum\n", num3 );
	}

	switch (!( num1 < num2 ) && !( num1 < num3 ))
	{
	case true:
		printf( "%d is maximum\n", num1 );
		break;

	case false:
		switch (!( num2 < num3 ))
		{
		case true:
			printf( "%d is maximum\n", num2 );
            break;

		case false:
			printf( "%d is maximum\n", num3 );
            break;
		}
	}
}

Its output might look like

Enter three integers to find maximum: 3 2 3
3 is maximum
3 is maximum

The if statements are included in the demonstration program only to show that the results of using if statements and switch statements will be the same. Of course in your program you should remove the if statements.