Added pick operator

pkg/yqlib/operator_pick_test.go

@@ -0,0 +1,33 @@
+package yqlib
+
+import (
+	"testing"
+)
+
+var pickOperatorScenarios = []expressionScenario{
+	{
+		description:    "Pick keys from map",
+		subdescription: "Note that the order of the keys matches the pick order and non existent keys are skipped.",
+		document:       "myMap: {cat: meow, dog: bark, thing: hamster, hamster: squeek}\n",
+		expression:     `.myMap |= pick(["hamster", "cat", "goat"])`,
+		expected: []string{
+			"D0, P[], (doc)::myMap: {hamster: squeek, cat: meow}\n",
+		},
+	},
+	{
+		description:    "Pick indices from array",
+		subdescription: "Note that the order of the indexes matches the pick order and non existent indexes are skipped.",
+		document:       `[cat, leopard, lion]`,
+		expression:     `pick([2, 0, 734, -5])`,
+		expected: []string{
+			"D0, P[], (!!seq)::[lion, cat]\n",
+		},
+	},
+}
+
+func TestPickOperatorScenarios(t *testing.T) {
+	for _, tt := range pickOperatorScenarios {
+		testScenario(t, &tt)
+	}
+	documentOperatorScenarios(t, "pick", pickOperatorScenarios)
+}