6 Conditionals and Booleans

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6 Conditionals and Booleans

6.1 Motivating Example: Shipping Costs

6.2 Conditionals: Computations with Decisions

6.3 Booleans

6.3.1 Other Boolean Operations

6.3.2 Combining Booleans

6.4 Asking Multiple Questions

6.5 Evaluating by Reducing Expressions

6.6 Wrapping up: Composing Functions

6.1 Motivating Example: Shipping Costs

In Functions Practice: Cost of pens, we wrote a program (pen-cost) to compute the cost of ordering pens. Continuing the example, we now want to account for shipping costs. We’ll determine shipping charges based on the cost of the order.

Specifically, we will write a function add-shipping to compute the total cost of an order including shipping. Assume an order valued at $10 or less ships for $4, while an order valued above $10 ships for $8. As usual, we will start by writing examples of the add-shipping computation.

Do Now!
Use the is notation from where blocks to write several examples of add-shipping. How are you choosing which inputs to use in your examples? Are you picking random inputs? Being strategic in some way? If so, what’s your strategy?

Here is a proposed collection of examples for add-shipping.

add-shipping(10) is 10 + 4
add-shipping(3.95) is 3.95 + 4
add-shipping(20) is 20 + 8
add-shipping(10.01) is 10.01 + 8

Do Now!
What do you notice about our examples? What strategies do you observe across our choices?

Our proposed examples feature several strategic decisions:

Including 10, which is at the boundary of charges based on the text
Including 10.01, which is just over the boundary
Including both natural and real (decimal) numbers
Including examples that should result in each shipping charge mentioned in the problem (4 and 8)

So far, we have used a simple rule for creating a function body from examples: locate the parts that are changing, replace them with names, then make the names the parameters to the function.

Do Now!
What is changing across our add-shipping examples? Do you notice anything different about these changes compared to the examples for our previous functions?

Two things are new in this set of examples:

The values of 4 and 8 differ across the examples, but they each occur in multiple examples.
The values of 4 and 8 appear only in the computed answers—not as an input. Which one we use seems to depend on the input value.

These two observations suggest that something new is going on with add-shipping. In particular, we have clusters of examples that share a fixed value (the shipping charge), but different clusters (a) use different values and (b) have a pattern to their inputs (whether the input value is less than or equal to 10). This calls for being able to ask questions about inputs within our programs.

6.2 Conditionals: Computations with Decisions

To ask a question about our inputs, we use a new kind of expression called an if expression. Here’s the full definition of add-shipping:

fun add-shipping(order-amt :: Number) -> Number:
  doc: "add shipping costs to order total"
  if order-amt <= 10:
    order-amt + 4
  else:
    order-amt + 8
  end
where:
  add-shipping(10) is 10 + 4
  add-shipping(3.95) is 3.95 + 4
  add-shipping(20) is 20 + 8
  add-shipping(10.01) is 10.01 + 8
end

In an if expression, we ask a question that can produce an answer that is true or false (here order-amt <= 10, which we’ll explain below in Booleans), provide one expression for when the answer to the question is true (order-amt + 4), and another for when the result is false (order-amt + 8). The else in the program marks the answer in the false case; we call this the else clause. We also need end to tell Pyret we’re done with the question and answers.

6.3 Booleans

Every expression in Pyret evaluates in a value. So far, we have seen three types of values: Number, String, and Image. What type of value does a question like order-amt <= 10 produce? We can use the interactions prompt to experiment and find out.

Do Now!
Enter each of the following expressions at the interactions prompt. What type of value did you get? Do the values fit the types we have seen so far?
3.95 <= 10
20 <= 10

The values true and false belong to a new type in Pyret, called Boolean.Named for George Boole. While there are an infinitely many values of type Number, there are only two of type Boolean: true and false.

Exercise
Explain why numbers and strings are not good ways to express the answer to a true/false question.

Exercise
Why did we not enter order-amt <= 10 at the interactions prompt to explore booleans?

6.3.1 Other Boolean Operations

There are many other built-in operations that return Boolean values. Comparing values for equality is a common one: There is much more we can and should say about equality, which we will do later [Re-Examining Equality].

1 == 1

true

1 == 2

false

"cat" == "dog"

false

"cat" == "CAT"

false

In general, == checks whether two values are equal. Note this is different from the single = used to associate names with values in the directory.

The last example is the most interesting: it illustrates that strings are case-sensitive, meaning individual letters must match in their case for strings to be considered equal.This will become relevant when we get to tables later.

Sometimes, we also want to compare strings to determine their alphabetical order. Here are several examples:

"a" < "b"

true

"a" >= "c"

false

"that" < "this"

true

"alpha" < "beta"

true

which is the alphabetical order we’re used to; but others need some explaining:

"a" >= "C"

true

"a" >= "A"

true

These use a convention laid down a long time ago in a system called ASCII.Things get far more complicated with non-ASCII letters: e.g., Pyret thinks "Ł" is > than "Z", but in Polish, this should be false. Worse, the ordering depends on location (e.g., Denmark/Norway vs. Finland/Sweden).

Do Now!
Can you compare true and false? Try comparing them for equality (==), then for inequality (such as <).

In general, you can compare any two values for equality (almost; if you’re on top of everything and want to read something more challenging, look at Comparing Functions); for instance:

"a" == 1

false

If you want to compare values of a specific kind, you can use more specific operators:

num-equal(1, 1)

true

num-equal(1, 2)

false

string-equal("a", "a")

true

string-equal("a", "b")

false

Why use these operators instead of the more generic ==?

Do Now!

Try

num-equal("a", 1)
string-equal("a", 1)

Therefore, it’s wise to use the type-specific operators where you’re expecting the two arguments to be of the same type. Then, Pyret will signal an error if you go wrong, instead of blindly returning an answer (false) which lets your program continue to compute a nonsensical value.

There are even more Boolean-producing operators, such as:

wm = "will.i.am"

string-contains(wm, "will")

true

Note the capital W.

string-contains(wm, "Will")

false

In fact, just about every kind of data will have some Boolean-valued operators to enable comparisons.

6.3.2 Combining Booleans

Often, we want to base decisions on more than one Boolean value. For instance, you are allowed to vote if you’re a citizen of a country and you are above a certain age. You’re allowed to board a bus if you have a ticket or the bus is having a free-ride day. We can even combine conditions: you’re allowed to drive if you are above a certain age and have good eyesight and—either pass a test or have a temporary license. Also, you’re allowed to drive if you are not inebriated.

Corresponding to these forms of combinations, Pyret offers three main operations: and, or, and not. Here are some examples of their use:

(1 < 2) and (2 < 3)

true

(1 < 2) and (3 < 2)

false

(1 < 2) or (2 < 3)

true

(3 < 2) or (1 < 2)

true

not(1 < 2)

false

6.4 Asking Multiple Questions

Shipping costs are rising, so we want to modify the add-shipping program to include a third shipping level: orders between $10 and $30 ship for $8, but orders over 30 ship for $12. This calls for two modifications to our program:

We have to be able to ask another question to distinguish situations in which the shipping charge is 8 from those in which the shipping charge is 12.
The question for when the shipping charge is 8 will need to check whether the input is between two values.

We’ll handle these in order.

The current body of add-shipping asks one question: order-amt <= 10. We need to add another one for order-amt <= 30, using a charge of 12 if that question fails. Where do we put that additional question?

An expanded version of the if-expression, using else if, allows you to ask multiple questions:

fun add-shipping(order-amt :: Number) -> Number:
  doc: "add shipping costs to order total"
  if order-amt <= 10:
    order-amt + 4
  else if order-amt <= 30:
    order-amt + 8
  else:
    order-amt + 12
  end
where:
  ...
end

At this point, you should also add where examples that use the 12 charge.

How does Pyret determine which answer to return? It evaluates each question expression in order, starting from the one that follows if. It continues through the questions, returning the value of the answer of the first question that returns true. Here’s a summary of the if-expression syntax and how it evaluates.

if <Boolean expression to check>:
  <expression if first expression is true>
else if <another Boolean expression to check>:
  <expression if first expression is false and second expression is true>
else:
  <expression if both expressions are false>
end

A program can have multiple if else cases, thus accommodating an arbitrary number of questions within a program.

Do Now!
The problem description for add-shipping said that orders between 10 and 30 should incur an 8 charge. How does the above code capture “between”?

This is currently entirely implicit. It depends on us understanding the way a if evaluates. The first question is order-amt <= 10, so if we continue to the second question, it means order-amt > 10. In this context, the second question asks whether order-amt <= 30. That’s how we’re capturing “between”-ness.

Do Now!
How might you modify the above code to build the “between 10 and 30” requirement explicitly into the question for the 8 case?

Remember the and operator on booleans? We can use that to capture “between” relationships, as follows:

(order-amt > 10) and (order-amt < 30)

Do Now!
Why are there parentheses around the two comparisons? If you replace order-amt with a concrete value (such as 20) and leave off the parenthesis, what happens when you evaluate this expression in the interactions window?

Here is what add-shipping look like with the and included:

fun add-shipping(order-amt :: Number) -> Number:
  doc: "add shipping costs to order total"
  if order-amt <= 10:
    order-amt + 4
  else if (order-amt > 10) and (order-amt < 30):
    order-amt + 8
  else:
    order-amt + 12
  end
where:
  add-shipping(10) is 10 + 4
  add-shipping(3.95) is 3.95 + 4
  add-shipping(20) is 20 + 8
  add-shipping(10.01) is 10.01 + 8
  add-shipping(30) is 30 + 12
end

Both versons of add-shipping support the same examples. Are both correct? Yes. And while the first part of the second question (order-amt > 10) is redundant, it can be helpful to include such conditions for three reasons:

They signal to future readers (including ourselves!) the condition covering a case.
They ensure that if we make a mistake in writing an earlier question, we won’t silently get surprising output.
They guard against future modifications, where someone might modify an earlier question without realizing the impact it’s having on a later one.

6.5 Evaluating by Reducing Expressions

In How Functions Evaluate, we talked about how Pyret reduces expressions and function calls to values. Let’s revisit this process, this time expanding to consider if-expressions. Suppose we want to compute the wages of a worker. The worker is paid $10 for every hour up to the first 40 hours, and is paid $15 for every extra hour. Let’s say hours contains the number of hours they work, and suppose it’s 45:

hours = 45

Suppose the formula for computing the wage is

if hours <= 40:
  hours * 10
else if hours > 40:
  (40 * 10) + ((hours - 40) * 15)
end

Let’s now see how this results in an answer, using a step-by-step process that should match what you’ve seen in algebra classes:The first step is to substitute the hours with 45.

if 45 <= 40:
  45 * 10
else if 45 > 40:
  (40 * 10) + ((45 - 40) * 15)
end

Next, the conditional part of the if expression is evaluated, which in this case is false.

=>  if false:
      45 * 10
    else if 45 > 40:
      (40 * 10) + ((45 - 40) * 15)
    end

Since the condition is false, the next branch is tried.

=>  if false:
      45 * 10
    else if true:
      (40 * 10) + ((45 - 40) * 15)
    end

Since the condition is true, the expression reduces to the body of that branch. After that, it’s just arithmetic.

=>  (40 * 10) + ((45 - 40) * 15)

=>  400 + (5 * 15)
=>  475

This style of reduction is the best way to think about the evaluation of Pyret expressions. The whole expression takes steps that simplify it, proceeding by simple rules. You can use this style yourself if you want to try and work through the evaluation of a Pyret program by hand (or in your head).

6.6 Wrapping up: Composing Functions

We started this chapter wanting to account for shipping costs on an order of pens. So far, we have written two functions:

pen-cost for computing the cost of the pens
add-shipping for adding shipping costs to a total amount

What if we now wanted to compute the price of an order of pens including shipping? We would have to use both of these functions together, sending the output of pen-cost to the input of add-shipping.

Do Now!
Write an expression that computes the total cost, with shipping, of an order of 10 pens that say "bravo".

There are two ways to structure this computation. We could pass the result of pen-cost directly to add-shipping:

add-shipping(pen-cost(10, "wow!"))

Alternatively, you might have named the result of pen-cost as an intermediate step:

pens = pen-cost(10, "wow!")
add-shipping(pens)

Both methods would produce the same answer.

Exercise
Manually evaluate each version. Where are the sequences of evaluation steps the same and where do they differ across these two programs?

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1	Introduction
2	Acknowledgments
3	Getting Started
4	Naming Values
5	From Repeated Expressions to Functions
6	Conditionals and Booleans
7	Introduction to Tabular Data
8	From Tables to Lists
9	Processing Lists
10	Introduction to Structured Data
11	Collections of Structured Data
12	Recursive Data
13	Interactive Games as Reactive Systems
14	Examples, Testing, and Program Checking
15	Functions as Data
16	Predicting Growth
17	Sets Appeal
18	Halloween Analysis
19	Sharing and Equality
20	Graphs
21	State, Change, and More Equality
22	Algorithms That Exploit State
23	Processing Programs: Parsing
24	Processing Programs: A First Look at Interpretation
25	Interpreting Conditionals
26	Interpreting Functions
27	Reasoning about Programs: A First Look at Types
28	Safety and Soundness
29	Parametric Polymorphism
30	Type Inference
31	Mutation: Structures and Variables
32	Objects: Interpretation and Types
33	Control Operations
34	Pyret for Racketeers and Schemers
35	Glossary

6.1	Motivating Example: Shipping Costs
6.2	Conditionals: Computations with Decisions
6.3	Booleans
6.4	Asking Multiple Questions
6.5	Evaluating by Reducing Expressions
6.6	Wrapping up: Composing Functions