Slice Bounds Out of Range

Why This Happens

Go slices are zero-indexed and accessing an index equal to or greater than the slice length causes a runtime panic. This often happens in loops with off-by-one errors, or when assuming a slice has elements without checking its length first.

The Problem

func main() {
    s := []int{1, 2, 3}
    fmt.Println(s[3]) // panic: index out of range [3] with length 3
}

The Fix

func main() {
    s := []int{1, 2, 3}
    if len(s) > 3 {
        fmt.Println(s[3])
    } else {
        fmt.Println("index out of bounds")
    }
}

Step-by-Step Fix

1. 1

   Identify the access

   Find the line in the stack trace where the out-of-range access happens and note the index and length.

2. 2

   Check the data source

   Determine why the slice has fewer elements than expected. It may be an empty result from a database query or API call.

3. 3

   Add bounds checking

   Always check len(slice) before accessing by index, especially for user-supplied indices or parsed data.