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04_maps.slide
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# maps
////////////////////////////////////////////////////////////////////////////////////
## what is map?
Go provides a built-in **map** type that implements a **hash table**
**Hash table** is a data structure that stores data in **key-value pairs**.
[Learn more in go blog](https://go.dev/blog/maps)
////////////////////////////////////////////////////////////////////////////////////
## declaring a map
## declaring a map
**syntax:**
map[KeyType]ValueType
- KeyType: any type that is comparable (i.e. not slice, map, function)
- ValueType: any type at all, including another map!
- key is unique
## declaring a map
**example: map of integer to string**
.play -numbers _concepts/04_maps/01declaring/1declareOnly.go /^func main/,/^}/
////////////////////////////////////////////////////////////////////////////////////
## initializing a map
## initializing a map
**initialize multiple key value pairs**
example 1:
.play -numbers _concepts/04_maps/02initializing/1declareAndInitialize_inSeparateLines_Example1.go /^func main/,/^}/
: declaration and initialization is made in separate lines
## initializing a map
**initialize multiple key value pairs**
another example: map of phone number to person name:
.play -numbers _concepts/04_maps/02initializing/1declareAndInitialize_inSeparateLines_Example2.go /^func main/,/^}/
: along with that, in this example, we have broken down each key value pairs into separate lines, for easy readability
## initializing a map
**initialize multiple key value pairs**
_declare and initialize in same line:_
.play _concepts/04_maps/02initializing/2declareAndInitialize_inSameLine.go /START OMIT/,/END OMIT/
_declare and initialize in same line, inferring datatype:_
.play _concepts/04_maps/02initializing/3declareAndInitialize_inSameLineInferringDatatype.go /START OMIT/,/END OMIT/
_short hand form:_
.play _concepts/04_maps/02initializing/4declareAndInitialize_inSameLineShortHandForm.go /START OMIT/,/END OMIT/
: like in the case of declaration and initialization of any variable, we can do in different variations
## initializing a map
**initialize individual key value pairs**
_empty initialization:_
.play -numbers _concepts/04_maps/02initializing/5a_empty_initialization.go /START OMIT/,/END OMIT/
_initializing individual key value pairs, after empty initializing:_
syntax: `map_var[key]=new_value`
.play -numbers _concepts/04_maps/02initializing/5b_initializing_individual_key_value_pairs.go /START OMIT/,/END OMIT/
_reinitialize and "later-initialize" a key:_
.play -numbers _concepts/04_maps/02initializing/5c_reinitialize_key.go /START OMIT/,/END OMIT/
## initializing a map
**using make**
syntax:
= make(map[KeyType]ValueType)
examples:
var mapVar map[KeyType]ValueType = make(map[KeyType]ValueType)
:
var mapVar = make(map[KeyType]ValueType)
:
mapVar := make(map[KeyType]ValueType)
////////////////////////////////////////////////////////////////////////////////////
## writing individual element
## writing individual element
like we have seen earlier...
.play -numbers _concepts/04_maps/05accessingIndividualElement/4writing.go /START OMIT/,/END OMIT/
## writing individual element
**overwriting individual element**
- key is unique
.play -numbers _concepts/04_maps/05accessingIndividualElement/5writing_overwriting.go /START OMIT/,/END OMIT/
////////////////////////////////////////////////////////////////////////////////////
## retrieving a key value pairs
## retrieving a key value pairs
**reading individual value given the key**
syntax:
map_var[key]
**reading individual value given the key**
.play -numbers _concepts/04_maps/05accessingIndividualElement/1a_reading_with_hardcoding_key.go /START OMIT/,/END OMIT/
**reading value into a variable**
.play -numbers _concepts/04_maps/05accessingIndividualElement/1b_reading_into_var.go /START OMIT/,/END OMIT/
**using a variable as a key**
.play -numbers _concepts/04_maps/05accessingIndividualElement/1c_reading_with_variable_key.go /START OMIT/,/END OMIT/
## retrieving a key value pairs
**with existing key with empty value**
.play -numbers _concepts/04_maps/05accessingIndividualElement/2reading_existingKeyWithEmptyValue.go /START OMIT/,/END OMIT/
## retrieving a key value pairs
**with non existing key**
.play -numbers _concepts/04_maps/05accessingIndividualElement/3reading_withNonExistingKey.go /START OMIT/,/END OMIT/
////////////////////////////////////////////////////////////////////////////////////
## exists check
## exists check
**find if the key already exists in the map**
as we noticed in the previous example, both of the cases (mentioned below) return same value, (ie 0). so how to differenciate between these two cases
- case 1: key exists but with zero value
- case 2: key doesn't exist in the map
map returns a optional second value, of type bool, which indicates if the given key exists or not
syntax:
value, exists := map_variable[key]
if exists is true, given key is in the map_variable
if exists is false, given key is not in the map_variable
## exists check
.play -numbers _concepts/04_maps/05accessingIndividualElement/6keyExistsCheck.go /START OMIT/,/END OMIT/
////////////////////////////////////////////////////////////////////////////////////
## assigning
## assigning
**assigning with another map literal**
.play -numbers _concepts/04_maps/04assigning/1withAnotherMap.go /START OMIT/,/END OMIT/
**assigning with another map variable**
.play -numbers _concepts/04_maps/04assigning/2withAnotherMapVariable.go /START OMIT/,/END OMIT/
## TODO
////////////////////////////////////////////////////////////////////////////////////
////////////////////////////////////////////////////////////////////////////////////
////////////////////////////////////////////////////////////////////////////////////
////////////////////////////////////////////////////////////////////////////////////
## un-initialized map
**default value**
- map is a reference (pointer) type
- default value is nil
example:
var m map[int]string
// m = map[int]string{1: "one"}
// m = map[int]string{}
// m = make(map[int]string)
if m == nil {
fmt.Println("map is nil")
} else {
fmt.Println("map is not nil", m)
}
## un-initialized map
**writing into un-initialized map**
var m map[int]string // this is un-initialized map
m[1] = "ONE"
fmt.Println(m) // what is the output?
// move to new slide
var m map[int]string
// m= map[int]string{}
m[1] = "ONE"
fmt.Println(m)
// new slide
: because the map is reference type and default value of un-initialized map is nil, writing to un-initialized map will result in panic
var m map[int]string
if m == nil {
fmt.Println("map in nil")
}
m[1]="one" // results in panic
solution: initialize the map using empty map or using make()
////////////////////////////////////////////////////////////////////////////////////
## map is reference type
**copy the map and edit the copy**
: the values of both maps is same, even though we modified only one of them.
example:
/////// value type ///////
var i int = 10
iCopy := i
iCopy = 20 // change
fmt.Println(i)
fmt.Println(iCopy)
/////// reference type ///////
var m map[int]string = map[int]string{1: "one"}
mCopy := m
mCopy[1] = "ONE"
fmt.Println(m)
fmt.Println(mCopy)
// TODO: example (in later slides for function) of map as parameter to function and value changes visible outside the function body
////////////////////////////////////////////////////////////////////////////////////
## iterate over map
: unlike a slice and array, we cannot use the for loop with index counter as the key for the map can be more than an number and need not be sequential
syntax:
for KeyVariable, ValueVariable := range MapVariable{}
: range on a map returns key and value corresponding to that key in each iteration
: Note: comparing it with a slice, range on slice returns index and value.
: Note: the order in which the key value pairs are retrieved is not guaranteed to be the insertion order
example:
var m = map[int]string{0: "A", 9: "B"}
fmt.Println(m)
for k, v := range m { // initializing key and value
fmt.Println(k, v)
}
for _, v := range m { // initializing value, ignoring key
fmt.Println(v)
}
for k, _ := range m {// initializing key, ignoring value
fmt.Println(k, m[k])
}
for k := range m { // since second var is ignored, we can skip underscore
fmt.Println(k)
}
////////////////////////////////////////////////////////////////////////////////////
## check if key exists
// slide 1
map retrieval results in a zero value when the key is not present.\
var m = map[int]float32{}
m[1] = 0
fmt.Println(m[1]) // retrieving an existing key with zero value
fmt.Println(m[10]) // retrieving a non existing key
Question: how to know if the result is because \
- the key is present in the map with zero value\
- or the key is not present at all?
// slide 2
## check if key exists
**two-value assignment**
syntax:
valVar, boolVar := mapVar[keyVal]
var m = map[int]float32{
1: 0,
}
v1 := m[1]
fmt.Println(v1)
v2, _ := m[1] // retrieving from map using a key returns a second optional value
fmt.Println(v2)
v3, ok := m[1]
fmt.Println(v3, ok)
// slide 2
## check if key exists
: retrieving an existing key with zero value
var m = map[int]float32{}
m[1] = 0
v1, ok := m[1]
// second return value is a bool type indicating if the given key is present in the map or not
fmt.Println(v1, ok)
// slice 3
: retrieving an non existing key also return zero value, but returns false for second return value
var m = map[int]float32{}
m[1] = 0
v1, ok := m[1] // retrieving an existing key with zero value
fmt.Println(v1, ok)
v10, ok := m[10] // retrieving a non existing key
fmt.Println(v10, ok)
////////////////////////////////////////////////////////////////////////////////////
## len()
**returns the number of key value pairs in the map**
// slide 1
syntax: len(MapVariable)\
returns: int
**example:**
var m = map[int]string{1: "one", 2: "two"}
fmt.Println(len(m))
// slide 2
**returns 0 for a nil map**
var m map[int]string
if m == nil {
fmt.Println("map in nil")
}
fmt.Println(len(m))
////////////////////////////////////////////////////////////////////////////////////
## delete()
**delete a key value pair**
// slide 1
syntax: delete(MapVariable, KeyValue)
example:
var m = map[int]float32{1: 1.1, 2: 2.2}
delete(m, 1)
// slide 2
: retrieving by deleted key returns zero value. how do we know for sure that the key value pair is deleted vs re-initialized to zero value?
var m = map[int]float32{1: 1.1, 2: 2.2}
fmt.Println(m[1])
delete(m, 1)
fmt.Println(m[1])
m[2] = 0
fmt.Println(m[2])
// slide 3
: len() returns one less value after delete
var m = map[int]float32{1: 1.1, 2: 2.2}
fmt.Println(len(m))
delete(m, 1)
fmt.Println(len(m))
// slide 4
: the OK bool value will be false for the deleted key
var m = map[int]float32{10: 0.0}
v, ok := m[10]
fmt.Println(v, ok)
delete(m, 10)
v, ok = m[10]
fmt.Println(v, ok)