Modifying Map During Range Iteration

Why This Happens

Go allows deleting map entries during range iteration and they will not appear in subsequent iterations. However, new entries added during iteration may or may not be visited. For predictable behavior, collect changes in a separate structure and apply them after the loop.

The Problem

func filterMap(m map[string]int) {
    for k, v := range m {
        if v < 0 {
            delete(m, k) // safe but confusing
        }
        m[k+"_copy"] = v // unpredictable: may or may not be visited
    }
}

The Fix

func filterMap(m map[string]int) {
    toDelete := []string{}
    toAdd := map[string]int{}
    for k, v := range m {
        if v < 0 {
            toDelete = append(toDelete, k)
        }
        toAdd[k+"_copy"] = v
    }
    for _, k := range toDelete {
        delete(m, k)
    }
    for k, v := range toAdd {
        m[k] = v
    }
}

Step-by-Step Fix

1. 1

   Identify in-loop modifications

   Find map additions inside range loops that iterate over the same map.

2. 2

   Collect changes separately

   Store additions and deletions in separate collections during iteration.

3. 3

   Apply after the loop

   Apply all collected changes after the range loop completes.