如何读取数组或切片的子集

Question

In Go, I can now read an excel file and put it into a slice.

I know also how to read the value of a specific cell.

But I would like now to read a subset of the initial slice, so basically read only rows 10 to 15 and columns 23 to 25 for example.

My code below does not do that, it reads rows 35 and 36 and all columns. How can I read only columns 23 to 25 into df2?

package main

import (
    "fmt"

    "github.com/360EntSecGroup-Skylar/excelize"
)

func main() {
    xlsx, err := excelize.OpenFile("/media/Snaps/test.xlsm")
    if err != nil {
        fmt.Println(err)
        return
    }

    rows := xlsx.GetRows("ticker")

    df2 := rows[10:15][23:25]
    fmt.Println(df2)
}

Answer 1

The key here I think is to stop thinking of it as rows and colums; in your code, you're operating on slices of slices. You can slice the outer level to get the rows you want, then iterate those to get the columns you want from each row. If you just take slices, though, all of the data you're not using will still be held in memory as part of the arrays backing the slices. For memory efficiency, you'll probably want to copy the desired data to completely new slices with new backing arrays:

    rows := xlsx.GetRows("ticker")

    df2 := make([][]string, 5) // I don't know what data type the cells are, assuming string, adjust the number to the number of rows you want to avoid unnecessary allocations)
    idx := 0
    for _, row := range rows[10:15] { // iterate over desired rows
        df2row := make([]string, 2) // again, assuming 2 columns of string cells, adjust as needed
        copy(df2row, row[23:25]) // copy desired columns to new row slice
        df2[idx] = df2row
        idx++
    }

    fmt.Println(df2)

There's an excellent, detailed rundown of slice operations here: https://blog.golang.org/go-slices-usage-and-internals

Here's a runable playground example with generated dummy data: https://play.golang.org/p/wRKcTpnTUA2