how to use api.Compiler().ConstantValue() ?

[YurunChen]

I tried to put frontend. Variable to *big. int, and then convert the data to fr. Element, I found api. Compiler(). ConstentValue() did not successfully convert my value to *big. Int。 I'm confused about the process and want to ask how this is going to be converted. Also, if you can, please check the Usage of fr.Element.

type Circuit struct {
	Mx   [][]frontend.Variable `gnark:"public"`
	R    frontend.Variable
	chal []frontend.Variable `gnark:"public"`
	tag  []frontend.Variable `gnark:"public"`
}

func (circuit *Circuit) Define(api frontend.API) error {

	Mx := make([][]*fr.Element, len(circuit.Mx))
	for i := 0; i < len(circuit.Mx); i++ {
		Mx[i] = make([]*fr.Element, len(circuit.Mx[i]))
		for j := 0; j < len(circuit.Mx[i]); j++ {
			// TODO why the v is nil? how to solve it?
			v, result := api.Compiler().ConstantValue(circuit.Mx[i][j])
			api.Println(v, result)
			Mx[i][j].SetBigInt(v)
		}
	}

	x := fr.Element{1}
	sigma := &x
	sigma.SetOne()
	PkSum := make([]*fr.Element, len(circuit.Mx))
	for i := range PkSum {
		PkSum[i].SetUint64(0)
	}
	tag := make([]*fr.Element, len(circuit.tag))
	for i, v := range circuit.tag {
		t, _ := api.Compiler().ConstantValue(v)
		tag[i].SetBigInt(t)
	}
	chal := make([]*fr.Element, len(circuit.chal))
	for i, v := range circuit.chal {
		t, _ := api.Compiler().ConstantValue(v)
		chal[i].SetBigInt(t)
	}
	for i, _ := range circuit.chal {
		t := tag[i].Mul(tag[i], chal[i])
		sigma = sigma.Mul(sigma, t)
		for j := 0; j < len(circuit.Mx); j++ {
			v := chal[i].Mul(chal[i], Mx[i][j])
			PkSum[i].Add(PkSum[i], v)
		}
	}

	return nil
}

[gbotrel]

Mx[i] = make([]*fr.Element, len(circuit.Mx[i]))
		for j := 0; j < len(circuit.Mx[i]); j++ {
			// TODO why the v is nil? how to solve it?
			v, result := api.Compiler().ConstantValue(circuit.Mx[i][j])

--> that's a Go question rather than specific to this lib; you initalize a slice of []*fr.Element ; the default value is nil for pointers. Not sure what you're trying to do here; if you need to manipulate constants they don't need to be frontend.Variable they can be int, fr.Element, big.Int ,...

[YurunChen]

I want to use frontend. Variable does some arithmetic operations on finite fields like G1, G2, GT, Zr, but I don't know how to use these interfaces in the circuit. You know, when I input values from the outside to the circuit, their types change to frontend. variables, so I don't know how to manipulate them.

[hussein-aitlahcen]

Arithmetic operations in the circuit are done via the api interface, like api.Add(x, y) etc...
Similarly, control flow must also be done using the api if it's based on the input (somehow "dynamic"), api.Select(condition, trueBranch, falseBranch)