Hash Maps in Go

Hash Tables, better known as Hash Maps, are just collections of key-value pairs. In other words, they are pieces of data (values) mapped to unique identifiers called keys.

Hash Maps were designed to give us a way of, given a key, associating a value with it for very quick lookups.

In Go, the Hash Map is implemented as a built-in map type.

Creation:

You create an empty Hash Map by declaring a map[keyType]valueType, where the keyType and valueType are the data types of the keys and values.

Let's declare a Hash Map that maps text values representing digits to their numerical value (e.g. "two" => 2). We wil map strings to integers, so the two types are string and int. We will name the map digitValue

digitValue := map[string]int{}

Creating an empty map takes O(1) time.

Initialization:

You can also add some key-value pairs in your map at creation:

digitValue := map[string]int{
	"zero": 0,
	"one":  1,
}

Accessing values:

A value is retrieved from a map by specifying its corresponding key in square brackets ([]):

digitValue := map[string]int{
	"zero": 0,
	"one":  1,
}

// Accessing  values:
fmt.Println(digitValue["zero"]) // prints 0
fmt.Println(digitValue["one"])  // prints 1

fmt.Println(digitValue["three"]) // prints 0

As you can see, if you refer to a key that is not in the map, Go returns a zero value (0 for numbers, empty string for strings). Unlike some other languages, accessing a non-existing key in Go does not add it to the map.

Accessing a value from a map takes O(1) time.

Adding an entry:

Adding an entry to an existing map is simply a matter of assigning a new key and value, via the assignment operator:

digitValue := map[string]int{
	"zero": 0,
	"one":  1,
}

// Adding new entries:
digitValue["three"] = 3

//The new Hash Map:
// {
//	"zero": 0,
//  "one":  1,
//	"three": 3
//}

Adding a new entry to a map takes O(1) time.

Updating an entry:

If you want to update an entry, you can just assign a new value to an existing key:

digitValue := map[string]int{
	"zero": 0,
	"one":  1,
}

// Updating entries:
digitValue["zero"] = 5

//The new Hash Map:
// {
//	"zero": 5,
//  "one":  1
//}

Updating an entry from a map takes O(1) time.

Deleting entries:

The delete() built-in function allows you to remove a key from a map:

digitValue := map[string]int{
	"zero": 0,
	"one":  1,
}

// Removing an entry
delete(digitValue, "zero")

//The new Hash Map:
// {
//  "one":  1
//}

If you try to delete a key that is not in the map, it won't do anything.

Deleting an entry from a map takes O(1) time.

Checking if a key exists:

You can check if a key exists in a map using the comma-ok idiom:

digitValue := map[string]int{
	"zero": 0,
	"one":  1,
}

_, ok := digitValue["zero"]
fmt.Println(ok) // prints true since the key exists

_, ok = digitValue["three"]
fmt.Println(ok) // prints false since the key does not exist in the map

Checking if a key exists in the map is O(1) time.

Iterating over the map keys:

If we want to iterate over keys of the map, we can use the for loop along with the range keyword:

digitValue := map[string]int{
	"zero": 0,
	"one":  1,
}

for key := range digitValue {
    fmt.Println(key, "is", digitValue[key])
}

// This will print:
// one is 1
// zero is 0

Notice the order is not the same as initiated. map keeps the data in random order

Iterating over a map takes O(n) time.

Iterating using both the key and the value for simplicity:

If we want to iterate over both keys and values of the map, we can use the for loop along with the range keyword as follows:

digitValue := map[string]int{
	"zero": 0,
	"one":  1,
}

for key, val := range digitValue {
    fmt.Println(key, "is", val)
}

// This will print:
// one is 1
// zero is 0

Notice the order is not the same as initiated. map keeps the data in random order

Iterating over a map takes O(n) time.

Assignment
Follow the Coding Tutorial and let's practice with maps.

Hint
Look at the examples above if you get stuck.