# Lab 0.2:

## More Practice with Scheme

The big idea for this lesson is deceptively simple. It's that we can take the value returned by one function and use it as an argument to another function. By “hooking up” two functions in this way, we invent a new, third function. For example, let's say we have a function that adds the letter s to the end of a word:

``add-s(“run”) = “runs”``

and another function that puts two words together into a sentence:

``sentence(“day”, “tripper”) = “day tripper”``

We can combine these to create a new function that represents the third person singular form of a verb:

``third-person(verb) = sentence(“she”, add-s(verb))``

That general formula looks like this when applied to a particular verb:

``third-person(“sing”) = “she sings”``

The way we say it in Scheme is

``````(define (third-person verb)

We know that this idea probably doesn't look like much of a big deal to you. It might seem obvious. Nevertheless, it will turn out that we can express a wide variety of computational algorithms by linking functions together in this way. This linking is what we mean by “functional programming.”

## finish this before section

This week, most of the reading is meant to improve your understanding of concepts that you saw in the first lab. So, you may want to skim parts that you already know. You can use these readings as a reference when you're finishing the the rest of the lab.

# Exercise 1.

The expression `(+ 8 2)` has the value 10. It is a compound expression made up of three atoms.

For this problem, write five other Scheme expressions whose values are also the number ten:

• An atom
• Another compound expression made up of three atoms
• A compound expression made up of four atoms
• A compound expression made up of an atom and two compound subexpressions
• Any other kind of expression

# Exercise 2.

Let's build a repertoire of functions to deal with words and sentences. We'll give you the second procedure from the previous lab.

• a. Write a procedure `first-two` that takes a word as its argument, returning a two-letter word containing the first two letters of the argument.
• b. Write a procedure `two-first` that takes two words as arguments, returning a two-letter word containing the first letters of the two arguments.
• c. Now write a procedure `two-first-sent` that takes a two-word sentence as argument, returning a two-letter word containing the first letters of the two words.

# Exercise 3.

Write a predicate `teen?` that returns true if its argument is between 13 and 19.

# Exercise 4.

Write a procedure `indef-article`. See examples below:

# Exercise 5.

Write a procedure `insert-and` that takes a sentence of items and returns a new sentence with an `and` in the right place.

# Exercise 6.

Write a procedure `query` that turns a statement into a question by swapping the first two words and adding a question mark to the last word:

# Exercise 7.

Write a procedure `european-time` to convert a time from American AM/PM notation into European 24-hour notation. Also write `american-time`, which does the opposite:

# Exercise 8.

Write a procedure `describe-time` that takes a number of seconds as its argument and returns a more useful description of that amount of time:

# Exercise 9.

The following program doesn't work. Why not? Fix it.

``````(define (superlative adjective word)