Iterating Over Collections
Iterating means visiting every single item in a collection, one after another, usually to do something with each one.
In this page:
Syntax
for (index, element) in array.enumerated() {
// index and element
}
for (key, value) in dictionary {
// each pair
}Iterating an Array with enumerated()
.enumerated() pairs each element with its index, letting you access both together inside a for-in loop.
Example: Iterating an Array with enumerated()
// Declare a constant `colors`, set to `["red", "green", "blue"]`
let colors = ["red", "green", "blue"]
for (index, color) in colors.enumerated() {
// Print "\(index): \(color)" to the console
print("\(index): \(color)")
}Login to try C/C++/Java/PHP code in the editor
Iterating a Dictionary
Iterating a dictionary directly with for-in gives you a tuple of (key, value) for each entry, which can be destructured right in the loop header.
Example: Iterating a Dictionary
// Declare a constant `ages`, set to `["Alice": 30, "Bob": 25]`
let ages = ["Alice": 30, "Bob": 25]
for (name, age) in ages {
// Print "\(name) is \(age)" to the console
print("\(name) is \(age)")
}Login to try C/C++/Java/PHP code in the editor
Related Topics
{# common_mistakes/chapter_summary/browser_support: on Hindi pages the
view already swaps in the hi_ translation fields (or blanks these
out if untranslated), so this renders correctly for both languages
without a lang_code check here. #}
Common Mistakes
- Forgetting that iterating a dictionary gives
(key, value)tuples, not just values. - Using an index-based loop when a direct
for-inover the collection's elements would be simpler and safer. - Not knowing
.enumerated()exists, and manually tracking an index counter variable instead.
Chapter Summary
for-initerates directly over an array's elements, a set's members, or a dictionary's key-value pairs..enumerated()provides both the index and the element together while iterating an array.- Iterating a dictionary yields tuples that can be destructured into separate key and value constants.
- Sets iterate in an unspecified order, unlike arrays which preserve insertion order.
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