Error with api.Select in a loop

[jeljelih]

I would like to call api.Select inside a for loop and I obtain the following error:
ERR parsing circuit error="assertIsBoolean failed: constant is not 0 or 1\nfrontend.parseCircuit.func2\n\tcompile.go:117\nr1cs.(*builder).AssertIsBoolean\n\tapi_assertions.go:52\nr1cs.(*builder).Select\n\tapi.go:440\nmain.(*MyCircuit).Define\n\tfirstCircuit.go:26\n"

This is my code:
`package main

import (
"github.com/consensys/gnark-crypto/ecc"
"github.com/consensys/gnark/backend/groth16"
"github.com/consensys/gnark/frontend"
"github.com/consensys/gnark/frontend/cs/r1cs"
)

// MyCircuit defines a simple circuit
type MyCircuit struct {
// struct tags on a variable is optional
// default uses variable name and secret visibility.
Temp1 frontend.Variable
Temp2 frontend.Variable
}

// Define declares the circuit constraints
func (circuit *MyCircuit) Define(api frontend.API) error {
temp := api.Select(api.Cmp(3, circuit.Temp1), circuit.Temp2, 0)
api.Println(temp)

for i := 0; i < 10; i++ {
	temp1 := api.Select(api.Cmp(3, i), circuit.Temp2, 0)
	api.Println(temp1)
}
return nil

}

func main() {

// compiles our circuit into a R1CS
var circuit MyCircuit
ccs, _ := frontend.Compile(ecc.BN254.ScalarField(), r1cs.NewBuilder, &circuit)

// groth16 zkSNARK: Setup
pk, vk, _ := groth16.Setup(ccs)

// witness definition
assignment := MyCircuit{
	Temp1: 2,
	Temp2: 35,
}
witness, _ := frontend.NewWitness(&assignment, ecc.BN254.ScalarField())
publicWitness, _ := witness.Public()

// groth16: Prove & Verify
proof, _ := groth16.Prove(ccs, pk, witness)
groth16.Verify(proof, vk, publicWitness)

}`

[ivokub]

Cmp returns either -1, 0 or 1 depending if x < y, x == y or x > y, but select assumes that input is either 0 or 1.

[jeljelih]

Thanks for your reply @ivokub
The behavior I am looking for is:

• for values below circuit.Limit, I want to select Temp1 value
• for values above circuit.Limit, I want to select Temp2 value

[ivokub]

And for value equaling circuit.Limit?

[jeljelih]

for value equaling circuit.Limit, I want to select Temp1 value

[ivokub]

Then it is going to be a bit more difficult. Essentially what you want to do is to have masked lists:

1. mask if i < Limit. Say this mask is mask_Less
2. mask if i == Limit. Sa this mask is mask_isEq
3. mask if i > Limit, call it mask_More

Then you set the value of the variable as mask_less * Temp1 + mask_isEq * Temp1 + mask_more * Temp2.

To compute the mask values, you can use api.Cmp and api.IsZero methods. Note that api.Cmp is expensive and you can optimise the computation of mask by using std/math/cmp gadget which provides bounded comparator implementation (as we know the limit on i.

[aybehrouz]

When binary output is needed, there is also some generic comparison methods in the cmp package. They may come in handy sometimes. As I remember the generated bit by bit comparison circuit is 1/3 smaller than api.Cmp.