Given a string of digits, output all the contiguous substrings of length n in that string in the order that they appear.

For example, the string "49142" has the following 3-digit series:

  • "491"
  • "914"
  • "142"

And the following 4-digit series:

  • "4914"
  • "9142"

And if you ask for a 6-digit series from a 5-digit string, you deserve whatever you get.

Note that these series are only required to occupy adjacent positions in the input; the digits need not be numerically consecutive.

Define two functions: (Two? Yes, sometimes we ask more out of Go.)

All returns a list of all substrings of s with length n.

All(n int, s string) []string

UnsafeFirst returns the first substring of s with length n.

UnsafeFirst(n int, s string) string

At this point you could consider this exercise complete and move on.

But wait, maybe you ask a reasonable question: Why is the function called Unsafe First? If you are interested, read on for a bonus exercise.

Bonus exercise:

Once you get go test passing, try go test -tags asktoomuch. This uses a build tag to enable a test that wasn't enabled before. Build tags allow for the selection of files to be used by the package. In our case it will control which test files are used. You can read more about those at the Go documentation.

You may notice that you can't make this asktoomuch test happy. We need a way to signal that in some cases you can't take the first N characters of the string. UnsafeFirst can't do that since it only returns a string.

To fix that, let's add another return value to the function. Define

First(int, string) (first string, ok bool)

and test with go test -tags first.

The ok bool second return argument is a common and idiomatic pattern in Go. For example you see it in Map lookups and type assertions.

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