math: Add cumprod and cumproded

lib/pure/math.nim

@@ -1145,6 +1145,37 @@ 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:
+  ## * `prod proc <#sum,openArray[T]>`_
+  ## * `cumproded proc <#cumproded,openArray[T]>`_ for a version which
+  ##   returns cumproded 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:
+  ## * `prod proc <#prod,openArray[T]>`_
+  ## * `cumprod proc <#cumprod,openArray[T]>`_ for the in-place version
+  runnableExamples:
+    let a = [1, 2, 3, 4]
+    doAssert cumproded(a) == @[1, 2, 6, 24]
+  result = @[]
+  let xLen = x.len
+  if xLen == 0:
+    return @[]
+  result.setLen(xLen)
+  result[0] = x[0]
+  for i in 1 ..< xLen: result[i] = result[i-1] * x[i]
+
 func cumsummed*[T](x: openArray[T]): seq[T] =
   ## Returns the cumulative (aka prefix) summation of `x`.
   ##
@@ -1353,3 +1384,4 @@ 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 ]
+      doAssert counts.cumproded == [ 1, 2, 6, 24 ]
+
+    block: # cumprod float seq return
+      let counts = [ 1.0, 2.0, 3.0, 4.0 ]
+      doAssert counts.cumproded == [ 1.0, 2.0, 6.0, 24.0 ]
+
+    block: # cumprod int in-place
+      var counts = [ 1, 2, 3, 4 ]
+      counts.cumprod
+      doAssert counts == [ 1, 2, 6, 24 ]
+
+    block: # cumprod float in-place
+      var counts = [ 1.0, 2.0, 3.0, 4.0 ]
+      counts.cumprod
+      doAssert counts == [ 1.0, 2.0, 6.0, 24.0 ]
+
   block: # ^ compiles for valid types
     doAssert: compiles(5 ^ 2)
     doAssert: compiles(5.5 ^ 2)
@@ -525,3 +544,4 @@ when not defined(js) and not defined(danger):
 
       doAssertRaises(OverflowDefect):
         discard sum(x)
+