Programonic

Before you start

Swift conditionals and loops.

Handle values that might be missing with if let.
Predict the sample output and explain the state changes.
Complete the practice task and check the boundary cases.

The concept

An optional type, written Int?, either holds a value or holds nil. Int("42") returns an Int? because the text may not be a number. Swift forces you to deal with the possibility of nil before using the value.

if let number = Int(text) tries the conversion and, only if it succeeds, binds the unwrapped Int to number for the first block. Otherwise the else block runs.

Code example

Swift

let values = ["42", "x7"]
for text in values {
    if let number = Int(text) {
        print("number \(number + 1)")
    } else {
        print("not a number: \(text)")
    }
}

Output

number 43
not a number: x7

Example details

Follow the code

Open the walkthrough to inspect the example one step at a time.

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1let values = ["42", "x7"]

2for text in values {

3 if let number = Int(text) {

4 print("number \(number + 1)")

5 } else {

6 print("not a number: \(text)")

7 }

8}

Follow the example

Start to inspect the source, values, and output. These prepared steps explain the published sample; grouped loop steps describe multiple iterations.

View the complete dry-run table

Prepared trace of the published program. Some rows group repeated iterations.
What happens	State after step	Output so far
Two strings, only one of which is a number.	`—`	—
For "42" the conversion succeeds and number is 42.	`text = "42", number = 42`	—
Print the number plus one.	`text = "42", number = 42`	number 43
For "x7" the conversion gives nil, so the else block runs.	`text = "x7"`	number 43 not a number: x7

Behind the scenes

How Swift processes this code

Swift compiles the program; its top-level statements run in source order.

Cost and common mistakes

O(n) over the values; each conversion is linear in the text length.

Force unwrapping with ! crashes when the value is nil; prefer if let, guard let, or ??.
Int(" 42") with a space also returns nil; the text must be exactly a number.

Try it yourself

Give failed conversions a default of 0.

Reveal answer and reasoning

Use let number = Int(text) ?? 0 and print number + 1 for every entry.

Swift Optionals