04 - Stuck in the loop with you

Covered in this module:

for
range

loops

Go only has a single loop construct - for. The Go compiler does allow forever loops:

for {}

for

A condition can be provided to make the loop behave like a while loop you would see in other languages:

i := 0
for i < 10 {
    fmt.Println(i)
    i++
}

You can also provide before, condition, and after clauses separated by semicolons.

for i := 0; i < 10; i++ {
    fmt.Println(i)
}

factors := []int{10, 20, 30, 40, 50}
product := 1

for i := 0; i < len(factors); i++ {
    product *= factors[i]
}
fmt.Println("Product is", product)

If you use the before/condition/after loop the semicolons are required but it is not required to have something in all three sub-clauses.

"Wait", you say, "there has to be a more concise way to loop over a slice, right?"

range

Yep

factors := []int{10, 20, 30, 40, 50}
product := 1

for _, factor := range factors {
    product *= factor
}
fmt.Println("Product is", product)

Range returns two parameters for iterable types: the index, and the value.

The for loop will automatically loop over all elements of the provided iterable:

factors := []int{10, 20, 30, 40, 50}
product := 1

for ind, factor := range factors {
    fmt.Println("I factored index", ind)
    product *= factor
}
fmt.Println("Product is", product)

prints

I factored index 0
I factored index 1
I factored index 2
I factored index 3
I factored index 4
Product is 12000000

As you can see, using range with a slice yields index and value. It does the same with a string, but the values aren’t characters. They are runes which is an alias for the int32 type, each representing a unicode code point:

str := "my cool string"

for ind, r := range str {
    fmt.Printf("Index %v is the rune %v\n", ind, r)
}

prints

Index 0 is the rune 109
Index 1 is the rune 121
Index 2 is the rune 32
Index 3 is the rune 99
Index 4 is the rune 111
Index 5 is the rune 111
Index 6 is the rune 108
Index 7 is the rune 32
Index 8 is the rune 115
Index 9 is the rune 116
Index 10 is the rune 114
Index 11 is the rune 105
Index 12 is the rune 110
Index 13 is the rune 103

You can use string(<rune value>) to cast a rune to a string or use the fmt verb %c in your format string

advanced note

The Go blog has a great post that explains the differences between strings, bytes, runes, and characters in Go

You can also range over maps. For maps, range yields the keys and values:

m := map[string]string{"A": "Alfa", "B": "Bravo", "C": "Charlie"}

for key, value := range m {
    fmt.Printf("Key %v holds value %v\n", key, value)
}

prints

Key C holds value Charlie
Key A holds value Alfa
Key B holds value Bravo

If you don’t need the key/index, you can throw it out by replacing with underscore _. If you don’t need the value, you can omit it and the comma-space. Its important to remember that maps are unordered, so if you are transforming a map to a slice or printing from a map the keys will be returned in an unpredictable order!

Hands on!

In the repo, open the file ./basics/04loops.go
Complete the TODOs
Run make 04 from project root (alternatively, type go run ./04loops.go)
Example implementation available on solutions branch