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How to check multiple conditions - a free tutorial

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How to check multiple conditions

Paul Hudson    @twostraws   

Swift gives us && and || for checking multiple conditions at the same time, and when used with just two conditions they are fairly straightforward.

As an example, imagine we were running a forum where users could post messages, and delete messages any messages they owned. We might write code like this:

if isOwner == true || isAdmin == true {
    print("You can delete this post")
}

Where things get more confusing is when we want to check several things. For example, we could say that regular users can delete messages only we allowed them, but admins can always delete posts. We might write code like this:

if isOwner == true && isEditingEnabled || isAdmin == true {
    print("You can delete this post")
}

But what is that trying to check? What order are the && and || checks executed? It could mean this:

if (isOwner == true && isEditingEnabled) || isAdmin == true {
    print("You can delete this post")
}

That says “if we’re the owner and editing is enabled you can delete the post, or if you’re an admin you can delete the post even if you don’t own it.” That makes sense: folks can delete their own posts if editing is allowed, but admins can always delete posts.

However, you might also read it like this:

if isOwner == true && (isEditingEnabled || isAdmin == true) {
    print("You can delete this post")
}

And now it means something quite different: “if you’re the owner of the post, and either editing is enabled or you’re the admin, then you can delete the post.” This means admins can’t delete posts they don’t own, which wouldn’t make sense.

Obviously Swift doesn’t like ambiguity like this, so it will always interpret the code as if we had written this:

if (isOwner == true && isEditingEnabled) || isAdmin == true {
    print("You can delete this post")
}

However, honestly it’s not a nice experience to leave this to Swift to figure out, which is why we can insert the parentheses ourselves to clarify exactly what we mean.

There is no specific advice on this, but realistically any time you mix && and || in a single condition you should almost certainly use parentheses to make the result clear.

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