Programming Fundamentals › Collections

Slicing

Taking a sub-range of a sequence.

Also known as: slice, sublist, substring

Slicing takes a range of elements from a sequence (a list, string or array) and gives you a new one.

nums = [10, 20, 30, 40, 50]
nums[1:4]      # [20, 30, 40]    from index 1 up to, but not including, 4
nums[:2]       # [10, 20]        the first two
nums[3:]       # [40, 50]        from index 3 to the end
nums[-2:]      # [40, 50]        the last two
nums[::2]      # [10, 30, 50]    every second item
nums[::-1]     # [50, 40, 30, 20, 10]   reversed

"hello"[1:3]   # "el"

The rule: [start:stop] includes start and excludes stop. That makes stop - start the length, and a[:n] + a[n:] the whole list again (off-by-one errors).

In JavaScript

nums.slice(1, 4);       // [20, 30, 40]
nums.slice(-2);         // [40, 50]
"hello".slice(1, 3);    // "el"

Don’t confuse slice (returns a copy of a part) with splice (changes the array in place).

Good to know

• Out-of-range slices don’t fail. In Python, nums[10:20] is just [], which can hide bugs.
• A slice is a copy in Python lists (shallow). Changing the slice doesn’t change the original, but nested objects are shared (shallow vs deep copy).
• nums[:] makes a shallow copy.
• Slicing strings by position can cut a character in half when text has combining marks or emoji (Unicode).
• Pagination is slicing: items[(page - 1) * size : page * size].