feature/count (#13837)

2 files changed, 20 insertions, 1 deletions

diff --git a/changelog.md b/changelog.md
index 7dccc20b9..32f6e0a1b 100644
--- a/changelog.md
+++ b/changelog.md
@@ -124,9 +124,9 @@ echo f
   through an SSL-wrapped `Socket`/`AsyncSocket`.
 - Added `distinctBase` overload for values: `assert 12.MyInt.distinctBase == 12`
 - Added `browsers.openDefaultBrowser` without URL, implements IETF RFC-6694 Section-3.
+- Added `sequtils.countIt`, allowing for counting items using a predicate.
 - Added `jsconsole.trace`, `jsconsole.table`, `jsconsole.exception` for JavaScript target.
 
-
 ## Library changes
 
 - `asyncdispatch.drain` now properly takes into account `selector.hasPendingOperations`
diff --git a/lib/pure/collections/sequtils.nim b/lib/pure/collections/sequtils.nim
index e32c784c6..0cc91b0d0 100644
--- a/lib/pure/collections/sequtils.nim
+++ b/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.