panic: runtime error: index out of range
panic: runtime error: index out of range in Go means you attempted to access a Go slice or array at an index that is less than zero or greater than or equal to its length (len).
How do I fix panic: runtime error: index out of range?
To fix panic: runtime error: index out of range in Go, locate the line causing the error and check the values right before it runs. Most occurrences happen because of indexing at len(slice) instead of len(slice) - 1. Apply a defensive check or safe fallback, make sure collections or objects exist before access, and verify return values from upstream functions.
What does panic: runtime error: index out of range mean?
Go performs bounds checks on all slice and array index operations. Unlike C or C++, accessing beyond the slice length does not silently corrupt memory or return undefined; it triggers a runtime panic immediately.
Why does panic: runtime error: index out of range happen?
1. Indexing at len(slice) instead of len(slice) - 1
Go slices are 0-indexed. The last valid element is at index len(slice) - 1.
Breaks
items := []string{"a", "b"}
last := items[len(items)] // panics with index [2] with length 2!Works
items := []string{"a", "b"}
last := items[len(items)-1]2. Accessing an empty slice
An empty slice has len 0, so any index access panics.
Breaks
var queue []int
first := queue[0] // panics!Works
if len(queue) > 0 {
first := queue[0]
_ = first
}How to find it in your own code
Always verify len(slice) > 0 before accessing indices. When looping with an explicit counter, ensure i < len(slice) rather than i <= len(slice), or prefer the idiomatic 'for i, val := range slice' syntax.
Try it: fix this bug
mediumpanic: runtime error: index out of range [-1] with length 4
package main
import "fmt"
func getHashBucket(hash int, numBuckets int) int {
// In Go, -5 % 4 evaluates to -1
idx := hash % numBuckets
buckets := make([]int, numBuckets)
return buckets[idx]
}
func main() {
fmt.Println(getHashBucket(-5, 4))
}Still not sure why yours breaks?
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