Thread: 2 assignmentss from one if.

I am doing a program exercise form c a modern approach chapter 5. I resolved the program to work but then have been playing with it, introduced error checking etc.

My question is can one if result in two assignments?
This is my code.

Code:

#include <stdio.h>

int main(void){
    
    int hour, min, myhour;
    char * postfix;


    printf("Print the time in 24hour format: ");
    scanf("%d:%d", &hour, &min);
    
    if ((hour < 25 && hour > 0) && (min < 61 && min > 0 )){
            if (hour > 12)
                myhour = (hour - 12) && postfix = "PM";
            else
                myhour = hour && postfix = "AM";


            printf("The time is %.2d:%.2d %s \n", myhour, min, postfix);
            }
            
            else 
                printf("Your input was invalid \n");


            


            
    return 0;
}

Basically the purpose is to convert 24 hour time to 12 hour time. I introduced

Code:

if ((hour < 25 && hour > 0) && (min < 61 && min > 0 ))

to make sure my input was valid.

But see my if subsequent to that I want to say if hours is greater than 12 minus 12 from the hours given and assign to myhour variable and assign "PM" to my postfix variable. It's not working I am getting an lvalue error.

The error is in your assignment

myhour = (hour - 12) && postfix = "PM";

&& is used for a conjunction of test conditions, and is invalid in the way you used it. You will need two seperate statements for your assignments.

&& is a logical or boolean operator, not an instruction to do more.

If you want two assignments, you need two statements.

if() and else() both are followed by one statement. A compound statement, or block, is 1 statement. 3 examples to help you understand:

Code:

if( a < b )
   a = b ;   
else
   b = a ;

if( a < b )
{                       //start of compound statement
   a = b ;
   b = c ;
}                      //end compound
else
{
   b = a ;
   a = c ;
}

if( a < b ) ;  //<--- Null statement, perfectly legal, most likely an error though
  a = b ;      //<--- Always executes. Not tied to the if.

Originally Posted by grumpy

Firstly, a block (which is what your examples show) is formally a type of statement.

I believe that's what I said.

Even ignoring that, an assignment is an expression, and there can be multiple expressions in a single statement. The trick is using operators with different precedence. For example, grouping in braces, the comma operator, etc.

That is completely valid, but I refrain from indulging someone in abusing the comma operator. There can be nothing clearer than two separate statements for two separate assignments.

If you want two assignments, you need two statements.

a = b = c;

I have updated the program however it is warning that my second else needs a "(" or identifier first.

Code:

#include <stdio.h>

int main(void){
	
	int hour, min, myhour;
	char * postfix;


	printf("Print the time in 24hour format: ");
	scanf("%d:%d", &hour, &min);
	
	if ((hour < 25 && hour > 0) && (min < 61 && min > 0 )){
			if (hour > 12)
				myhour = (hour - 12);
		       		postfix = "PM";
			else
				myhour = hour;
		       		postfix = "AM";


			printf("The time is %.2d:%.2d %s \n", myhour, min, postfix);
			}
			
	else 
		printf("Your input was invalid \n");
			
	return 0;
}

Fully brace all your code until you understand what not bracing means.

Originally Posted by whiteflags

a = b = c;

Man, we're getting picky aren't we? In the context of what the OP is trying to do...

myhour = postfix = "PM"; ????

Originally Posted by Tclausex

Man, we're getting picky aren't we?

myhour = postfix = "PM"; ????

Well that or you're getting sloppy. Your example demonstrates that a and b were assignable, as was c, to either thing. That's not what the OP was doing either. If I made a mistake, so did you.

Originally Posted by laserlight

What do you mean by "bracketed the if"? Working from CommonTater's fix in post #12 to your code, whiteflags means something like this:

I was referring to p82 of the book. The author doesn't use bracketing with the if's as in CommonTater's code.

He advises to make sure that if you use two if's that you should bracket the second if to make sure that the else remains in the outer block.
So:

Code:

if ( ) ... {
               if ( ) ...
                   do  something }
else
     something else