Go - Maps

• Declared: map[KeyType]ValueType
• Ex. m := make(map[string]int)
  m["a"]=1
  m{"b"]=2
• Ex. m := map[string]int {
     "a":1,
     "b":2, // Please notice every entry need to be end with comma
  }
  
• Key type must be comparable using ==, array can be used as a key to declare a map, but slice is not acceptable.
• Ex. array can be used as key
  
• Ex. compile error when use slice as a key
  
• Empty map, length of nil map is 0 as well, but you can't put entry to nil map
• m := make(map[string]int)
  m := map[string]int{}
  
• Ex. Delete entry
  m := map[string]int {
      "a":1,
      "b":2
  }
  delete(m, "a")
  
• add 1 to key
  
• add 1 to entry but key not exist, value will start from 0 because there is still an assignment
  
• Can't get entry's address, because entry address may change after rehash.
  Will compile error when trying to get entry's address.
  Ex. keyP := &m["k"]
  
• Iterate map by range, the order is random by design intent. To prevent client code depends on the implementation
  
• If we want to follow order to iterate, need sort key and iterate key to get value
  
• Zero value of map is nil
  
• Store a value in map's zero value will panic!! Must allocate the map before using it
  
• Get value from a nil map will get zero value of the type. 
  
• Check key exist or not
  
• m = make(map[string]map[string]int)