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Maps

4:06 with Jay McGavren

Slices are good for storing collections of data, but the only way you can get data back out of a slice is by using a numeric index. Sometimes it's useful to be able to use other values as keys that let you look up items in a collection. Go uses the term "map" to refer to this sort of key-value list.

Sometimes it's useful to be able to use other values as keys that let you look up items in a collection. Go uses the term map to refer to this sort of key-value list.

The idea is that a map "maps" a set of keys to corresponding values. You can use any type you want as a key, as long as the keys are all the same type. If you know the key a value is stored under, you can use the key to retrieve that value.

package main

import "fmt"

func main() {
  ages := map[string]float64{}
  // Like the array/slice syntax, but you can use any value of the type you specified for the keys.
  ages["Alice"] = 12
  ages["Bob"] = 9
  fmt.Println(ages)
}
  • The syntax to retrieve an individual value is just like arrays or slices, too, except you use a key in place of a numeric index.
  fmt.Println(ages["Alice"], ages["Bob"])
  • Initialize map with values:
package main

import "fmt"

func main() {
    ages := map[string]float64{"Alice": 12, "Bob": 9}
    fmt.Println(ages)
}
  • The built-in function delete() can be used to remove a key and its corresponding value:
  delete(ages, "Bob")
  • Maps also support for ... range loops:
package main

import "fmt"

func main() {
    ages := map[string]float64{"Alice": 12, "Carol": 10, "Bob": 9}
    for name, age := range ages {
        fmt.Println(name, age)
    }
}
  • You can ignore the values from the map:
  for name := range ages {
    fmt.Println(name)
  }
  • Or ignore the keys:
  for _, age := range ages {
    fmt.Println(age)
  }

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    Slices are good for storing collections of data, but 0:00
    the only way you can get data back out of a slice is by using a numeric index. 0:03
    Sometimes it's useful to be able to use other values as keys that let you look up 0:07
    items in a collection. 0:11
    Many programing languages support collections like this. 0:13
    In C# and 0:16
    Python they're called dictionaries, in Ruby it's a hash, in Java it's a hash map. 0:17
    Gojes seizes the term map to refer to this sort of key value list. 0:22
    The idea is that a map maps a set of keys to corresponding values. 0:27
    You can use any type you want as a key as long as the keys are all the same type. 0:31
    If you know the key a value is stored under, 0:35
    you can use the key to retrieve that value. 0:37
    Let's create a map that stores the names and ages of some students. 0:40
    So we're gonna specify that it should create a new map, and 0:44
    here we specify the type that should be used for the keys. 0:48
    Since we're using names as keys, we're gonna use a type of string. 0:52
    And since the values will be ages, we'll go ahead and 0:57
    use the value type of float 64. 1:01
    We'll specify that the map should be empty when it starts. 1:04
    And we'll assign the resulting map to the ages variable. 1:07
    Now, let's assign some values to keys within the map, so this is just like 1:13
    the syntax for assigning to an array index except that instead of being limited 1:18
    to numeric indexes, we can use any value of the type we've specified for the key. 1:24
    So we can use any string we want. 1:29
    So we'll create a student with the name of Alice and we'll say that her age is 12. 1:31
    And then we'll assign a second key, Bob. 1:37
    And we'll say that his age is nine. 1:42
    Then let's print out the whole map. 1:44
    So format, printline, ages. 1:46
    Okay let's save that. 1:50
    Try running it. 1:52
    And we see that the result is a map under the Alice key we have an age of 12 and 1:55
    under the Bob key we have an age of 9. 2:00
    The syntax to retrieve an individual value is just like a razor slices two except you 2:02
    use a key in place of the numeric index again. 2:07
    So we can say format Println And 2:10
    let's print the age of Alice, And the age of Bob. 2:18
    Save that, rerun it, and it prints out the values for those two keys. 2:30
    Similar to arrays of slices, 2:36
    we can use a map literal to pre-populate a map with values. 2:38
    We just type them here within the curly braces. 2:42
    So we can say, under the key of Alice, there should be an age of 12. 2:45
    And under the key of Bob. 2:51
    There should be an age of nine. 2:53
    Just separate the key from the value with a colon, and 2:56
    separate multiple key value pairs with commas. 3:00
    Let's try running this. 3:04
    And there's our populated map. 3:07
    And, as with arrays of slices, we can use the for 3:09
    range loop to loop over each item that a map contains. 3:12
    Each key will be assigned to the first variable you provide. 3:16
    And each value will be assigned to the second variable. 3:19
    So if we were to run this for range loop, 3:22
    it would print out a list of each of the names and the corresponding ages. 3:25
    If you don't need to work with the keys 3:31
    from a map you can ignore them by using the blank identifier. 3:34
    So for example we can print just the ages by using the blank 3:37
    identifier in place of the keys. 3:42
    If we were to try running this, it'll print out just the ages. 3:43
    Or if we want to ignore the values, and use just the keys, 3:47
    we can just eliminate the second variable that holds the values. 3:53
    So we can print just the names with this code. 3:58
    Try running that and we get a list of just the names. 4:02

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