math: Add cumprod and cumproded

lib/pure/math.nim

@@ -1145,6 +1145,33 @@ func prod*[T](x: openArray[T]): T =
  result = T(1)
  for i in items(x): result = result * i
 
+func cumprod*[T](x: var openArray[T]) =
+ ## Transforms ``x`` in-place (must be declared as `var`) into its
+ ## product.
+ ##
+ ## See also:
+ ## * `sum proc <#sum,openArray[T]>`_
+ ## * `cumprodmed proc <#cumprodmed,openArray[T]>`_ for a version which
+ ## returns cumprodmed sequence
+ runnableExamples:
+ var a = [1, 2, 3, 4]
+ cumprod(a)
+ doAssert a == @[1, 2, 6, 24]
+ for i in 1 ..< x.len: x[i] = x[i-1] * x[i]
+
+func cumproded*[T](x: openArray[T]): seq[T] =
+ ## Return cumulative (aka prefix) product of ``x``.
+ ##
+ ## See also:
+ ## * `sum proc <#prod,openArray[T]>`_
+ ## * `cumsum proc <#cumprod,openArray[T]>`_ for the in-place version
+ runnableExamples:
+ let a = [1, 2, 3, 4]
+ doAssert cumproded(a) == @[1, 2, 6, 24]
+ result.setLen(x.len)
+ result[0] = x[0]
+ for i in 1 ..< x.len: result[i] = result[i-1] * x[i]
+
 func cumsummed*[T](x: openArray[T]): seq[T] =
  ## Returns the cumulative (aka prefix) summation of `x`.
  ##
@@ -1309,3 +1336,4 @@ func lcm*[T](x: openArray[T]): T {.since: (1, 1).} =
  result = x[0]
  for i in 1 ..< x.len:
  result = lcm(result, x[i])
+

tests/stdlib/tmath.nim

@@ -245,6 +245,25 @@ template main() =
  empty.cumsum
  doAssert empty == @[]
 
+ block: # cumprod
+ block: #cumprod int seq return
+ let counts = [ 1, 2, 3, 4 ]
+ check counts.cumproded == [ 1, 2, 6, 24 ]
+
+ block: # cumprod float seq return
+ let counts = [ 1.0, 2.0, 3.0, 4.0 ]
+ check counts.cumproded == [ 1.0, 2.0, 6.0, 24.0 ]
+
+ block: # cumprod int in-place
+ var counts = [ 1, 2, 3, 4 ]
+ counts.cumprod
+ check counts == [ 1, 2, 6, 24 ]
+
+ block: # cumprod float in-place
+ var counts = [ 1.0, 2.0, 3.0, 4.0 ]
+ counts.cumprod
+ check counts == [ 1.0, 2.0, 6.0, 24.0 ]
+
  block: # ^ compiles for valid types
  doAssert: compiles(5 ^ 2)
  doAssert: compiles(5.5 ^ 2)
@@ -471,3 +490,4 @@ when not defined(js) and not defined(danger):
 
  doAssertRaises(OverflowDefect):
  discard sum(x)
+