Remove elements in slice

Question

I am complete newbie in Golang, I am trying to remove elements in one slice based on the elements in another slice. e.g.

input slice : urlList := []string{"test", "abc", "def", "ghi"}

elements to remove slice : remove := []string{"abc", "test"}

expected output slice : urlList := []string{"def", "ghi"}

This is what I tried.

func main() {

    urlList := []string{"test", "abc", "def", "ghi"}
    remove := []string{"abc", "test"}
loop:
    for i, url := range urlList {
        for _, rem := range remove {
            if url == rem {
                urlList = append(urlList[:i], urlList[i+1:]...)
                continue loop
            }
        }
    }
    for _, v := range urlList {
        fmt.Println(v)
    }
}

But it's not working as I expected. I don't know what I am missing.

Answer 1

The problem is that when you remove an element from the original list, all subsequent elements are shifted. But the range loop doesn't know that you changed the underlying slice and will increment the index as usual, even though in this case it shouldn't because then you skip an element.

And since the remove list contains 2 elements which are right next to each other in the original list, the second one ("abc" in this case) will not be checked and will not be removed.

One possible solution is not to use range in the outer loop, and when you remove an element, you manually decrease the index i-- because continuing with the next iteration it will get auto-incremented:

urlList := []string{"test", "abc", "def", "ghi"}
remove := []string{"abc", "test"}

loop:
for i := 0; i < len(urlList); i++ {
    url := urlList[i]
    for _, rem := range remove {
        if url == rem {
            urlList = append(urlList[:i], urlList[i+1:]...)
            i-- // Important: decrease index
            continue loop
        }
    }
}

fmt.Println(urlList)

Output:

[def ghi]

Note:

Since the outer loop contains nothing after the inner loop, you can replace the label+continue with a simple break:

urlList := []string{"test", "abc", "def", "ghi"}
remove := []string{"abc", "test"}

for i := 0; i < len(urlList); i++ {
    url := urlList[i]
    for _, rem := range remove {
        if url == rem {
            urlList = append(urlList[:i], urlList[i+1:]...)
            i-- // Important: decrease index
            break
        }
    }
}

fmt.Println(urlList)

Try it on Go Playground.

Alternative

An alternative to this would be for the outer loop to go downward, so no need to manually decrease (or increase) the index variable because the shifted elements are not affected (already processed due to the downward direction).

Answer 2

Maybe it is simpler to create a new slice, that only contains your desired elements, for example:

package main

import "fmt"

func main() {
    urlList := []string{"test", "abc", "def", "ghi"}
    remove := []string{"abc", "test"}

    new_list := make([]string, 0)

    my_map := make(map[string]bool, 0)
    for _, ele := range remove {
        my_map[ele] = true
    }

    for _, ele := range urlList {
        _, is_in_map := my_map[ele]
        if is_in_map {
            fmt.Printf("Have to ignore : %s\n", ele)
        } else {
            new_list = append(new_list, ele)    
        }
    }

    fmt.Println(new_list)

}

playground

Result:

Have to ignore : test
Have to ignore : abc
[def ghi]