Add slice as row dynamically to 2d slice - go

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:
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)
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\n***", matrix, "\n***")
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.

Answer 2

You're adding all to the first row, and you need to add to the next row, see:
Try this (with minimum change to your code: made count a pointer):
package main
import (
"fmt"
)
func main() {
triangleNumber()
}
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)
fmt.Println(matrix)
return 0
}
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(count, 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)
}
output:
4
[[1 2 3] [1 2 4] [1 3 4] [2 3 4]]
Also for your special case, this works too:
matrix[*count] = []int{sofar[0], sofar[1], sofar[2]}
instead of:
matrix[*count] = append(matrix[*count], sofar[0], sofar[1], sofar[2])