docs: add lookup example

examples/lookup/lookup.go

@@ -0,0 +1,30 @@
+package lookup
+
+import (
+	"fmt"
+
+	"github.com/consensys/gnark/frontend"
+	"github.com/consensys/gnark/std/lookup"
+)
+
+type LookupExampleCircuit struct {
+	Entries []frontend.Variable
+	Queries []frontend.Variable
+	Results []frontend.Variable
+}
+
+func (c *LookupExampleCircuit) Define(api frontend.API) error {
+	table := lookup.New()
+	for i := range c.Entries {
+		table.Insert(c.Entries[i])
+	}
+	results := table.Lookup(api, c.Queries...)
+	if len(results) != len(c.Results) {
+		return fmt.Errorf("result length %d expected %d", len(results), len(c.Results))
+	}
+	for i := range results {
+		api.AssertIsEqual(results[i], c.Results[i])
+	}
+	table.Commit(api)
+	return nil
+}

examples/lookup/lookup_test.go

@@ -0,0 +1,25 @@
+package lookup
+
+import (
+	"testing"
+
+	"github.com/consensys/gnark/frontend"
+	"github.com/consensys/gnark/test"
+)
+
+func TestLookup(t *testing.T) {
+	assert := test.NewAssert(t)
+
+	assert.ProverSucceeded(
+		&LookupExampleCircuit{
+			Entries: make([]frontend.Variable, 6),
+			Queries: make([]frontend.Variable, 2),
+			Results: make([]frontend.Variable, 2),
+		},
+		&LookupExampleCircuit{
+			Entries: []frontend.Variable{10, 20, 30, 40, 50, 60},
+			Queries: []frontend.Variable{2, 4},
+			Results: []frontend.Variable{30, 50},
+		},
+	)
+}