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Count Instances

Remember that you can check if a key is already present in a map by using the second return value from the index operation.

You can combine an if statement with an assignment operation to use the variables inside the if block:

names := map[string]int{}
missingNames := []string{}

if _, ok := names["Denna"]; !ok {
    // if the key doesn't exist yet,
    // append the name to the missingNames slice
    missingNames = append(missingNames, "Denna")
}

Assignment

Each time a user is sent a message, their username is logged in a slice. We want a more efficient way to count how many messages each user received.

Implement the updateCounts function. It takes as input:

messagedUsers: a slice of strings.
validUsers: a map of string -> int.

It should update the validUsers map with the number of times each user has received a message. Each string in the slice is a username, but they may not be valid. Only update the message count of valid users.

So, if "benji" is in the map and appears in the slice 3 times, the key "benji" in the map should have the value 3.