How to Remove Elements from a Slice Within a Loop in Go: What Are the Best Practices?

Removing Slice Elements Within a Loop

Effectively removing elements from a slice within a loop can be tricky. An incorrect but common approach is to use append inside a range-based loop:

<code class="go">for i := range a { // BAD
    if conditionMeets(a[i]) {
        a = append(a[:i], a[i+1:]...)
    }
}</code>

However, this approach would result in loop variables getting out of sync and skipped elements.

Correct Loop-Based Removal

Instead, consider manually decrementing the loop variable after removing an element:

<code class="go">for i := 0; i < len(a); i++ {
    if conditionMeets(a[i]) {
        a = append(a[:i], a[i+1:]...)
        i--
    }
}

Downward Looping for Multiple Removals

If multiple elements may need to be removed, downward looping ensures the shifted elements remain outside the loop's iteration:

<code class="go">for i := len(a) - 1; i >= 0; i-- {
    if conditionMeets(a[i]) {
        a = append(a[:i], a[i+1:]...)
    }
}</code>

Alternate for Many Removals

For extensive removals, consider copying non-removable elements to a new slice, avoiding numerous copy operations:

<code class="go">b := make([]string, len(a))
copied := 0
for _, s := range(a) {
    if !conditionMeets(s) {
        b[copied] = s
        copied++
    }
}
b = b[:copied]</code>

In-Place Removal with Cycling

To perform in-place removal, maintain two indices, assigning non-removable elements while zeroing out removed ones:

<code class="go">copied := 0
for i := 0; i < len(a); i++ {
    if !conditionMeets(a[i]) {
        a[copied] = a[i]
        copied++
    }
}
for i := copied; i < len(a); i++ {
    a[i] = "" // Zero places of removed elements
}
a = a[:copied]</code>

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