diff --git a/doc/Expr.adoc b/doc/Expr.adoc
index 5567ef2..bbfd53f 100644
--- a/doc/Expr.adoc
+++ b/doc/Expr.adoc
@@ -1875,13 +1875,21 @@ TIP: Iterators built on custom data-sources can provide additional named operato
 There are also some infixed operators that can be used with iterators. They are defined as follows.
 
 * <<_cat,cat operator>>
-* <<_diget,digest operator>>
+* <<_digest,digest operator>>
 * <<_filter,filter operator>>
 * <<_groupby,groupby operator>>
 * <<_map,map operator>>
 //* <<_reduce,reduce operator>>: applies a binary expression cumulatively to the elements of the iterator, from left to right, to reduce the iterator to a single value.
 //* <<_zip,zip operator>>: takes two or more iterators and returns a list of tuples, where the i-th tuple contains the i-th element from each of the input iterators.
 
+
+==== Automatic variables in operators
+At each iteration, the following automatic variables are available for use in the expression of the [blue]`digest`, [blue]`filter`, [blue]`groupby`, and [blue]`map` operators.
+
+* `$_`: the current element of the iterator.
+* `$__`: the index of the current element in the iterator, starting from 0.
+* `$_#`: the number of elements already visited in the iterator, starting from 0.
+
 ---
 
 ==== [blue]`cat` operator
diff --git a/operator-digest.go b/operator-digest.go
index 78f2540..64a8230 100644
--- a/operator-digest.go
+++ b/operator-digest.go
@@ -45,7 +45,7 @@ func evalDigest(ctx kern.ExprContext, opTerm *scan.Term) (v any, err error) {
 	for item, err = it.Next(); err == nil; item, err = it.Next() {
 		ctx.SetVar("_", item)
 		ctx.SetVar("__", it.Index())
-		ctx.SetVar("_#", it.Count())
+		ctx.SetVar("#", it.Count())
 		if rightValue, err = opTerm.Children[1].Compute(ctx); err == nil {
 			if rightValue == nil {
 				break
@@ -53,7 +53,7 @@ func evalDigest(ctx kern.ExprContext, opTerm *scan.Term) (v any, err error) {
 				lastValue = rightValue
 			}
 		}
-		ctx.DeleteVar("_#")
+		ctx.DeleteVar("#")
 		ctx.DeleteVar("__")
 		ctx.DeleteVar("_")
 		if err != nil {
diff --git a/operator-filter.go b/operator-filter.go
index a9a6797..f8ee9a7 100644
--- a/operator-filter.go
+++ b/operator-filter.go
@@ -110,9 +110,9 @@ func (it *filterIterator) Next() (item any, err error) {
 	for attempt, err = it.itSrc.Next(); err == nil; attempt, err = it.itSrc.Next() {
 		ctx.SetVar("_", attempt)
 		ctx.SetVar("__", it.Index())
-		ctx.SetVar("_#", it.Count())
+		ctx.SetVar("#", it.Count())
 		result, err = it.expr.Compute(ctx)
-		ctx.DeleteVar("_#")
+		ctx.DeleteVar("#")
 		ctx.DeleteVar("__")
 		ctx.DeleteVar("_")
 
diff --git a/operator-groupby.go b/operator-groupby.go
index 70535a4..9130e95 100644
--- a/operator-groupby.go
+++ b/operator-groupby.go
@@ -62,7 +62,7 @@ func evalGroupBy(ctx kern.ExprContext, opTerm *scan.Term) (v any, err error) {
 	for item, err = it.Next(); err == nil; item, err = it.Next() {
 		ctx.SetVar("_", item)
 		ctx.SetVar("__", it.Index())
-		ctx.SetVar("_#", it.Count())
+		ctx.SetVar("#", it.Count())
 
 		var sItemKey string
 
@@ -71,7 +71,11 @@ func evalGroupBy(ctx kern.ExprContext, opTerm *scan.Term) (v any, err error) {
 				sItemKey = strconv.Itoa(int(it.Index()))
 			} else if d.HasKey(sKey) {
 				if keyValue, exists := d.GetItem(sKey); exists {
-					sItemKey = fmt.Sprintf("%v", keyValue)
+					if s, ok := keyValue.(string); ok {
+						sItemKey = s
+					} else {
+						sItemKey = fmt.Sprintf("%v", keyValue)
+					}
 				} else {
 					sItemKey = "_"
 				}
@@ -92,7 +96,7 @@ func evalGroupBy(ctx kern.ExprContext, opTerm *scan.Term) (v any, err error) {
 		ls.AppendItem(item)
 		values.SetItem(sItemKey, ls)
 
-		ctx.DeleteVar("_#")
+		ctx.DeleteVar("#")
 		ctx.DeleteVar("__")
 		ctx.DeleteVar("_")
 	}