如何确定哪个条件为真？

Question

Say I have a few conditions inside a if statement:

if(x < 0 || x > 10 || x == 5)
{

}

If either x is greater than 10 or less than 0 or equal to 5, I want the program to stop working.

if(x < 0 || x > 10 || x == 5)
{
    stop(); // PSEUDO CODE
}

However , I want the program to say something depending on which condition was true. Something like this:

if(x < 0 || x > 10 || x == 5)
{
    if(x < 0)
    {
        printf("your number was less than 0");
    }

    if(x > 10)
    {
        printf("your number was greater than 10");
    }

    if(x == 5)
    {
        printf("wow, your number is equal to 5!");
    }

    stop(); // PSEUDO CODE
}

That's a horrible way of doing it because it unnecessarily checks for the conditions twice; how can I do the same in a more efficient way?

Answer 1

Why make things complicated? Just remove the outer check, and put stop() inside both of the inner ones:

if(x < 0)
{
    printf("your number was less than 0");
    stop();
}

if(x > 10)
{
    printf("your number was greater than 10");
    stop();
}

You can even use an else if if you want.

Answer 2

Save the boolean result in a variable and reuse it inside. Redundancy is ok sometimes. Just check/count the number of executions if it's possible to be lessen down.

bool isXNegative = x < 0;
bool isXMoreThan = x > 10;
bool isXFive = (x == 5);

//for specific conditions
if(isXNegative)
{
    printf("your number was less than 0");
}

if(isXMoreThan)
{
    printf("your number was greater than 10");
}

if(isXFive)
{
    printf("wow, your number is equal to 5!");
}

//for combination of the conditions.
if(isXNegative || isXMoreThan || isXFive)
{
    //common code here for all the conditions.
    stop(); // PSEUDO CODE
}
//else if(another condition combination here...){...}
//else {...}

Answer 3

You can set a flag in the failure cases, then check the flag afterward to do the cleanup.

int do_stop = 0;
if(x < 0)
{
    printf("your number was less than 0");
    do_stop = 1;
}

if(x > 10)
{
    printf("your number was greater than 10");
    do_stop = 1;
}

if(x == 5)
{
    printf("wow, your number is equal to 5!");
    do_stop = 1;
}

if (do_stop) {
    stop();
}

Answer 4

Assuming you do want to be complicated, save some keystrokes and take advantage of short-circuiting, you can assign expression results to variables in the if condition like so:

  int c1, c2, c3;
  if ((c1 = x < 0) || (c2 = x > 10) || (c3 = x == 5)) {
    if (c1) {
      printf("your number was less than 0\n");
    } else if (c2) {
      printf("your number was greater than 10\n");
    } else {
      printf("wow, your number is equal to 5!\n");
    }
  }

Just be aware that if the condition short-circuits (say x < 0 ), then the value of c2 and c3 is not initialized.

I don't actually advocate writing your code this way, but this language feature can be useful and I didn't see it mentioned.