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.

  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


Sequences are dynamically allocated (i.e. allocated on the heap, not the stack), but they are immutable unless marked as var. That means

let a = @[1, 2, 3]

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

var b = @[1, 2, 3]

will work without any problems. Sequences passed as “argument by value” are not modifiable. For example, the following will fail to compile.

proc doSomething(mySeq: seq[int]) =
  mySeq[0] = 2  # this is a compile-time error
var testSeq = @[1, 2, 3]

seq arguments can be mutable if they are passed as “argument by reference”, ie. the parameter is annotated with the var or ref:

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

var thisSeq = newSeq[int](10)

assert thisSeq[9] == 999

You can copy a sequence passed as “argument by value” and modify the copy:

proc doSomething(mySeq: seq[int]) =
  var varMySeq = mySeq  # copy the seq
  varMySeq[0] = 999
  assert varMySeq[0] == 999
var testSeq = @[1, 2, 3]
assert testSeq[0] == 1