nim-by-example/content/seqs.md

title
Seqs

Seqs

Seqs, abbreviated from "sequence", provide dynamically expandable storage.

There are two ways to create seqs, with the @ operator and with the newSeq[T](n: int) method. Once a seq is created, it can be modified using the add(item: T), delete(idx: int). The length of a seq can be found through len: int, and the maximum index through high: int. The standard items: T and pairs: tuple[i: int, v: T] iterators are also available.

var
  a = @[1, 2, 3]
  b = newSeq[int](3)

for i, v in a:
  b[i] = v*v

for i in 4..100:
  b.add(i * i)

b.delete(0)  # takes «O(n)» time
b = a[0] & b  # Same as original b

Immutability

While sequences are dynamiclly allocated, they are still immutable.

let a = @[1, 2, 3]
a.add(4)

will fail to compile because a cannot be assigned to. However,

var b = @[1, 2, 3]
b.add(4)

will work without any problems. If wanted, a seq can be passed to a method with the var or ref annotation to make it modifiable, so

proc foo(mySeq: var seq[int]) =
  mySeq[9] = 999

var thisSeq = newSeq[int](10)
foo(thisSeq)

assert thisSeq[9] == 999