1d array from 2d array

Getting Help Code Review
1

I need to extract a column from a 2d array into a 1d array.
I tried search the internet but not successful.
Can someone please help?
Thanks

2

Do you have a slice of rows or a slice of columns?

If it is a slice of colums, you can just do foo[n]. If it is a slice of rows, you need to use a loop and build a resulting slice with the individual column.

3

thanks for your quick reply.
following is the info.

input : x [ ][ ] float64 {{1,2,3},{4,5,6},{7,8,9}}
desired output y [ ] float64 eg: [2,5,8]

4 
package main

import "log"

func main() {
  var input = [][]float64{{1,2,3},{4,5,6},{7,8,9}}
  var result = []float64{}
 
  for _, arr := range input {
    for _, item := range arr {
      result = append(result, item)
    }
  }

  log.Println(result)
}

This gets you all the values in a single slice, going back to Nobbz’s answer. If you want help with this, its done like this:

package main

import "log"

func main() {
  var input = [][]float64{{1,2,3},{4,5,6},{7,8,9}}
  var row = 1

  var result = []float64{}
  for _, column := range input {
    result = append(result, column[row])
  }

  log.Println(result)
}
5 
package main

import "fmt"

func column(x [][]float64, col int) []float64 {
	y := make([]float64, 0, len(x))
	for _, row := range x {
		if col < 0 || col >= len(row) {
			return nil
		}
		y = append(y, row[col])
	}
	return y[0:len(y):len(y)]
}

func main() {
	x := [][]float64{{1, 2, 3}, {4, 5, 6}, {7, 8, 9}}
	fmt.Println(x)
	y := column(x, 1)
	fmt.Println(y)
}

[[1 2 3] [4 5 6] [7 8 9]]
[2 5 8]
1 Like
6

thanks guys for your quick reply. it works. but I was thinking if I can take the transpose of the input and read the roll. Will that work?

7

The XY problem is asking about your attempted solution rather than your actual problem. Read The XY Problem.

If the actual problem is transposing a matrix:

package main

import "fmt"

func transpose(x [][]float64) [][]float64 {
	if len(x) == 0 {
		return [][]float64{}
	}
	rows, cols := len(x[0]), len(x)
	y := make([][]float64, rows)
	for c, xrow := range x {
		if len(xrow) != rows {
			return nil
		}
		if c >= cols {
			return nil
		}
		for r := range xrow {
			if r >= rows {
				return nil
			}
			if y[r] == nil {
				y[r] = make([]float64, cols)
			}
			y[r][c] = x[c][r]
		}
	}
	return y
}

func main() {
	tests := []struct {
		x [][]float64
	}{
		{[][]float64{{1, 2, 3}, {4, 5, 6}, {7, 8, 9}}},
		{[][]float64{{1, 2}}},
		{[][]float64{{1, 2}, {3, 4}}},
		{[][]float64{{1, 2}, {3, 4}, {5, 6}}},
	}
	for _, tt := range tests {
		x := tt.x
		fmt.Println("x", x)
		y := transpose(x)
		fmt.Println("y", y)
		fmt.Println()
	}
}

x [[1 2 3] [4 5 6] [7 8 9]]
y [[1 4 7] [2 5 8] [3 6 9]]

x [[1 2]]
y [[1] [2]]

x [[1 2] [3 4]]
y [[1 3] [2 4]]

x [[1 2] [3 4] [5 6]]
y [[1 3 5] [2 4 6]]
8

Hi petrus
thanks for your advise and help.
very much appreciated.

1 Like