I don't understand the guard statement

Question

Ok, in this video, you write a guard statement:

guard firstChoice != nil && secondChoice != nil else { return self }

And you explain this guard statement this way:

"So here we're checking if the first choice is nil, and the second choice. If neither of them are nil, that means they already have choices. So we simply want to return self, that is, the page that we're calling this method on. "

So it sounds to me that what you're saying is that if neither choice is nil–and the boolean condition in the guard statement is 'true'–then we want to return 'self'–execute the code in the else statement.

But I don't think that's how guard statements work. In a guard statement, the coding in the 'else' block only executes when the boolean condition is NOT true. Is that not correct? Am I totally off the wall here?

Answer 1 · 2019-01-25T05:53:44Z

January 25, 2019 5:53am

Hi, Mary.

I do not know what video are you talking about, but you are right about the way guard statements work. If the conditional statement of the guard is false, then the procedure in the else statement runs, otherwise program skips the else statement and works as usual.

So in the case of the code piece you gave, else statement will NOT run only in case if BOTH of the firstChoise and secondChoice are NOT equal to nil. On the other hand, else statement will run if SOME of the variables ARE equal to nil.

firstChoice | secondCoice | Result      |
     nil          |         nil          |  ELSE       |
     nil          |      not nil      |   ELSE      |
  not nil       |        nil          |   ELSE      |
  not nil       |     not nil      |NOT ELSE |

So I think the goal of the code's author is not reached.

The better solution would be:

guard firstChoice == nil || secondChoice == nil else { return self }

So basically it is the negation of the condition.

I hope you will understand. If any questions, do not hesitate to ask.

Answer 2 · 2019-01-25T14:25:47Z

January 25, 2019 2:25pm

Thank you very much! That's exactly what I thought.

Answer 3 · 2019-01-25T16:09:02Z

January 25, 2019 4:09pm

I am happy to help you.

It worths to note that using simple if statement is much more recommended in such cases.

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Mary Kenkel

Mary Kenkel

I don't understand the guard statement

3 Answers

Alikhan Oitan

Alikhan Oitan

Mary Kenkel

Mary Kenkel

Alikhan Oitan

Alikhan Oitan