Sort files, size

In Go programs we sometimes want to act upon the largest files first, and then proceed to the smallest ones. The opposite order is sometimes desired as well.

Custom sort type

We can use the "sort" package, and call "sort.Sort" on the FileInfo array returned by Readdir. With some custom logic, we can implement our sorting logic.

In this example, we specify a directory and assign this path to the "folder" variable. Then we use os.Open and Readdir to get the list of files in that directory.

package main

import (
    "fmt"
    "os"
    "sort"
    "io/fs"
)

// Access Size() in Less() to sort by file size.
type ByFileSize []fs.FileInfo
func (a ByFileSize) Len() int           { return len(a) }
func (a ByFileSize) Less(i, j int) bool { return a[i].Size() > a[j].Size() }
func (a ByFileSize) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }

func main() {
    folder := "/Users/sam/programs/"

    // Read directory.
    dirRead, _ := os.Open(folder)
    dirFiles, _ := dirRead.Readdir(0)

    // Sort by file size.
    sort.Sort(ByFileSize(dirFiles))

    // Write results.
    for dirIndex := range dirFiles {
        fileHere := dirFiles[dirIndex]
        fmt.Println(fileHere.Name(), fileHere.Size())
    }
}
search-perls.txt 204568
merged-perls.txt 49157
...
program.php 196

Originally I wrote the file size sorting method to call os.Size on each string file name from a slice. This is inefficient, as the size is already available through FileInfo.

Sorting files by their size (particularly in descending, low to high order) has been a recurring task in programming. A file's size can indicate its importance.