meet it

Maps grow, change and shrink while your program runs.

To add a key or change its value, assign to it: stock["apples"] = 12. A new key makes the map grow, and a key that is already there gets its old value replaced. removes a key, and len(stock) counts the keys.

Ask for a key that is not there and Go does not crash. It hands back the for the type: 0 for an int, "" for a string and false for a bool.

To start with an empty map, use : stock := make(map[string]int). A map declared with var stock map[string]int was never made, and storing anything in it causes a panic.

package main
import "fmt"
func main() {
	stock := make(map[string]int)
	stock["apples"] = 12
	stock["pears"] = 4
	stock["apples"] = 10
	delete(stock, "pears")
	fmt.Println(stock, len(stock))
	fmt.Println(stock["plums"])
}

Deleting a key that is not there does nothing, so delete is always safe to call.

guess first

Lunch vote! What does the map hold now?

Guess before you run it. Wrong guesses are how you learn, and nothing is marked.

package main
import "fmt"
func main() {
	votes := map[string]int{"pizza": 3, "pasta": 2}
	votes["pasta"] = votes["pasta"] + 2
	votes["soup"] = votes["soup"] + 1
	fmt.Println(votes)
}

your turn

Your turn. Sort out the lost property box.

On line 5, add a lost item of your own and the name of its owner. On line 6, delete either umbrella or scarf, because its owner came to collect it. Then run it.

Your code

output

press run to see what happens

fix it

The scoreboard crashes. Can you fix it?

Broken code to fix

output

press run to see what happens

The program panics on line 5, the first time it stores something. Was the map ever made?
A map declared with var is not ready to hold anything. You need make to build it.
Change line 4 to scores := make(map[string]int), then run.

nailed it

your maps can grow and shrink. :D

m[key] = value adds a key or replaces its value.
delete removes a key, and len counts the keys.
A missing key gives the zero value, such as 0 or "".

new words in your collection

zero value
make
delete

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