feature/count (#13837)

lib/pure/collections/sequtils.nim

@@ -600,6 +600,25 @@ template keepItIf*(varSeq: seq, pred: untyped) =
       inc(pos)
   setLen(varSeq, pos)
 
+since (1, 1):
+  template countIt*(s, pred: untyped): Natural =
+    ## Returns a count of all the items that fulfilled the predicate.
+    ##
+    ## The predicate needs to be an expression using
+    ## the ``it`` variable for testing, like: ``countIt(@[1, 2, 3], it > 2)``.
+    ##
+    runnableExamples:
+      let numbers = @[-3, -2, -1, 0, 1, 2, 3, 4, 5, 6]
+      iterator iota(n: int): int =
+        for i in 0..<n: yield i
+      assert numbers.countIt(it < 0) == 3
+      assert countIt(iota(10), it < 2) == 2
+
+    var result = 0
+    for it {.inject.} in s:
+      if pred: result += 1
+    result
+
 proc all*[T](s: openArray[T], pred: proc(x: T): bool {.closure.}): bool =
   ## Iterates through a container and checks if every item fulfills the
   ## predicate.