Hi. I'm trying to understand gonum, but it's generating a seg fault.
I don't understand why

package main

import (
"fmt"
ct "github.com/daviddengcn/go-colortext"
ctfmt "github.com/daviddengcn/go-colortext/fmt"
gomat "gonum.org/v1/gonum/mat"
"math/rand/v2"
"os"
)

const aRows = 3
const aCols = aRows
const bRows = aRows

func goNumQRTest() {
// Will look to solve AX = B, for X

initialVal := float64(randRange(1, 10))
increment := float64(randRange(1, 5))

initX := make([]float64, aCols)
initX[0] = initialVal
initX[1] = initialVal + increment
initX[2] = initialVal + 2*increment

X := gomat.NewVecDense(bRows, initX)
fmt.Printf(" X:\n%.4g\n\n", gomat.Formatted(X))

// Now need to assign coefficients in matrix A
initA := make([]float64, aRows*aCols)

for i := range initA {
initA[i] = float64(randRange(1, 20))
}

A := gomat.NewDense(aRows, aCols, initA)
fmt.Printf(" A:\n%.4g\n\n", gomat.Formatted(A))

var B gomat.Dense
B.Mul(A, X)
fmt.Printf(" B:\n%.4g\n\n", gomat.Formatted(&B))

// try QR stuff
var qr gomat.QR
var qrSoln *gomat.Dense
qr.Factorize(A)

// this function call panics w/ invalid memory reference or nil
// pointer dereference.
err := qr.SolveTo(qrSoln, false, &B)
if err != nil {
ctfmt.Printf(ct.Red, false, " Error from qr Solve To
is %s. Bye-Bye\n", err)
os.Exit(1)
}
fmt.Printf(" Soluton by gonum QR factorization is:\n %v\n\n",
gomat.Formatted(qrSoln))

} // end gonumQRTest

// MAIN PROGRAM

func main() {
goNumQRTest()

}

func randRange(minP, maxP int) int {
if maxP < minP {
minP, maxP = maxP, minP
}
return minP + rand.IntN(maxP-minP)
}

On 2024-03-18, Rob <usenet@drrob1.com> wrote:
>
> // this function call panics w/ invalid memory reference or nil
> // pointer dereference.
> err := qr.SolveTo(qrSoln, false, &B)
> if err != nil {
> ctfmt.Printf(ct.Red, false, " Error from qr Solve To
> is %s. Bye-Bye\n", err)
> os.Exit(1)
> }
> fmt.Printf(" Soluton by gonum QR factorization is:\n %v\n\n",
> gomat.Formatted(qrSoln))
>
> } // end gonumQRTest
>

I just overead very fast so double check this,

var qrSoln gomat.Dense // Change this line, remove pointer.

// Initialize qrSoln
qrSoln = *gomat.NewDense(0, 0, nil) // Initialize qrSoln with an empty dense matrix

....
err := qr.SolveTo(&qrSoln, false, &B) // Pass the address of qrSoln

The qrSoln pointer is not initialized, so it's pointing to nil, causing a panic
when you try to dereference it.
https://pkg.go.dev/github.com/changkun/gomat#NewDense

Hope it helps.

Happy Hacking
ReK2

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On Tue, 19 Mar 2024 02:40:28 -0000 (UTC), rek2 hispagatos
<rek2@hispagatos.org.invalid> wrote:

>On 2024-03-18, Rob <usenet@drrob1.com> wrote:
>>
>> // this function call panics w/ invalid memory reference or nil
>> // pointer dereference.
>> err := qr.SolveTo(qrSoln, false, &B)
>> if err != nil {
>> ctfmt.Printf(ct.Red, false, " Error from qr Solve To
>> is %s. Bye-Bye\n", err)
>> os.Exit(1)
>> }
>> fmt.Printf(" Soluton by gonum QR factorization is:\n %v\n\n",
>> gomat.Formatted(qrSoln))
>>
>> } // end gonumQRTest
>>
>
>var qrSoln gomat.Dense // Change this line, remove pointer.
>
> qrSoln = *gomat.NewDense(0, 0, nil) // Initialize qrSoln with an empty dense matrix
>
>The qrSoln pointer is not initialized, so it's pointing to nil, causing a panic
>when you try to dereference it.
>https://pkg.go.dev/github.com/changkun/gomat#NewDense
>
>Happy Hacking
>ReK2

gomat.NewDense(0,0,nil) is not allowed, as the zero dimensions
panicked. But now it's working.

Thanks.