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Basic Swift debugging using print()

We're going to start with the absolute easiest debugging technique, which is the print() function. This prints a message into the Xcode debug console that can say anything you want, because users won't see it in the UI. The "scattershot" approach to bug fixing is to litter your code with calls to print() then follow the messages to see what's going on.

You'll meet lots of people telling you how bad this is, but the truth is it's the debugging method everyone starts with – it's easy, it's natural, and it often gives you enough information to solve your problem. Use it with Swift's string interpolation to see the contents of your variables when your app is running.

We’ve used print() several times already, always in its most basic form:

print("I'm inside the viewDidLoad() method!")

By adding calls like that to your various methods, you can see exactly how your program flowed.

However, print() is actually a bit more complicated behind the scenes. For example, you can actually pass it lots of values at the same time, and it will print them all:

print(1, 2, 3, 4, 5)

This is known as a “variadic function” – a function that accepts any number of parameters – and it’s something I go into a lot more in my book Pro Swift. Here, though, it’s worth adding that print()’s variadic nature becomes much more useful when you use its optional extra parameters: separator and terminator.

The first of these, separator, lets you provide a string that should be placed between every item in the print() call. Try running this code:

print(1, 2, 3, 4, 5, separator: "-")

That should print “1-2-3-4-5”, because the separator parameter is used to split up each item passed into print().

The second optional parameter, terminator, is what should be placed after the final item. It’s \n by default, which you should remember means “line break”. If you don’t want print() to insert a line break after every call, just write this:

print("Some message", terminator: "")

Notice how you don’t need to specify separator if you don’t want to.

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