动态将切片作为行添加到2D切片

Question

I'm trying to add the slice sofar to a new row in matrix after each iteration.

func combinations(sofar []int, rest []int, n int, matrix [][]int, count int) {
    if n == 0 {
        //Next two lines problematic
        matrix[count] = append(matrix[count], sofar[0], sofar[1], sofar[2])
        count++
        fmt.Println(sofar)
        } else {
            for i := range rest[:len(rest)] {
                concat := sofar
                concat = append(concat, rest[i])
                combinations(concat, rest[i+1:], n-1, matrix, count)
            }
        }
}

func factorial(x uint) uint {
    if x == 0 {
        return 1
    }
    return x * factorial(x-1)
}

Driver program

func triangleNumber() int {
    sofar := []int{}
    rest := []int{1,2,3,4}
    matrixSize := factorial(4)/(factorial(1)*factorial(3))
    matrix := make([][]int, matrixSize)
    count := 0 
    fmt.Println(matrixSize)
    combinations(sofar, rest, 3, matrix, count)
    return 0
}

triangleNumber()

I want matrix to be;

[1 2 3]
[1 2 4]
[1 3 4]
[2 3 4]

But instead it's all going in the first row. Also is there a way I can get rid of count, and just add the slice sofar to the next row?

Answer 1

Actually, there are a couple of things I note with your program:

1. Append adds to the existing slice at its end (after length), so if you are using append for matrix, you need not allocate a slice of that size (see main in the code below)
2. After you are adding elements to the matrix, it is simply being dumped in your current program. The combinations function needs to return it back so that when future elements (slice of ints) are added, they are actually all there.

I've added some debugs and remodeled your program a bit, see if it makes sense:

package main

import (
    "fmt"
)

func main() {
    triangleNumber()
}
func combinations(sofar []int, rest []int, n int, matrix [][]int, count int) [][]int {
    fmt.Println("Entered with matrix", matrix)
    if n == 0 {
        fmt.Println("Entered with count", count)
        //Next two lines problematic
        matrix = append(matrix, sofar)
        count++
        fmt.Println(sofar)
        fmt.Println("Printing matrix
***", matrix, "
***")
        return matrix
    } else {
        for i := range rest[:len(rest)] {
            concat := sofar
            concat = append(concat, rest[i])
            matrix = combinations(concat, rest[i+1:], n-1, matrix, count)
            fmt.Println("Sending with count", count)
        }
    }
    return matrix
}

func factorial(x uint) uint {
    if x == 0 {
        return 1
    }
    return x * factorial(x-1)
}

func triangleNumber() int {
    sofar := []int{}
    rest := []int{1, 2, 3, 4}
    matrixSize := factorial(4) / (factorial(1) * factorial(3))
    matrix := make([][]int, 0)
    count := 0
    fmt.Println(matrixSize)
    combinations(sofar, rest, 3, matrix, count)
    return 0
}

And as you can see, you can pretty much get rid of count too with this approach (look at the output). There's still some scope for improvement left though, but I guess this addresses what you were asking.

On playground: https://play.golang.org/p/rnCdPcaIG3N

Hope this helps.