How to copy a map?

Question

I'm trying to copy the contents of a map ( amap ) inside another one (aSuperMap) and then clear amap so that it can take new values on the next iteration/loop. The issue is that you can't clear the map without to clear its reference in the supermap as well. Here is some pseudo code.

for something := range fruits{         aMap := make(map[string]aStruct)         aSuperMap := make(map[string]map[string]aStruct)          for x := range something{             aMap[x] = aData             aSuperMap[y] = aMap             delete(aMap, x)     } //save aSuperMap   saveASuperMap(something)  } 

I've also tried some dynamic stuff but obviously it throws an error (cannot assign to nil)

aSuperMap[y][x] = aData 

The question is how can I create an associative map ? In PHP I simply use aSuperMap[y][x] = aData. It seems that golang doesn't have any obvious method. If I delete delete(aMap, x) its reference from the super map is deleted as well. If I don't delete it the supermap ends up with duplicate data. Basically on each loop it gets aMap with the new value plus all the old values.

Answer 1

You are not copying the map, but the reference to the map. Your delete thus modifies the values in both your original map and the super map. To copy a map, you have to use a for loop like this:

for k,v := range originalMap {   newMap[k] = v } 

Here's an example from the now-retired SO documentation:

// Create the original map originalMap := make(map[string]int) originalMap["one"] = 1 originalMap["two"] = 2  // Create the target map targetMap := make(map[string]int)  // Copy from the original map to the target map for key, value := range originalMap {   targetMap[key] = value } 

Excerpted from Maps - Copy a Map. The original author was JepZ. Attribution details can be found on the contributor page. The source is licenced under CC BY-SA 3.0 and may be found in the Documentation archive. Reference topic ID: 732 and example ID: 9834.