Section 4

Conditional Expressions and Functions

For many problems, computer programs must deal with different situations in different ways. A game program may have to determine whether an object's speed is in some range or whether it is located in some specific area of the screen. For an engine control program, a condition may describe whether or when a valve is to be opened. To deal with conditions, we need to have a way of saying a condition is true or false; we need a new class of values, which, by convention, are called BOOLEAN (or truth) values. This section introduces booleans, expressions that evaluate to Booleans, and expressions that compute values depending on the boolean result of some evaluation.

4.1  Booleans and Relations


Consider the following problem statement:

Company XYZ & Co. pays all its employees $12 per hour. A typical employee works between 20 and 65 hours per week. Develop a program that determines the wage of an employee from the number of hours of work, if the number is within the proper range.
The italic words highlight the new part (compared to section 2.3). They imply that the program must deal with its input in one way if it is in the legitimate range, and in a different way if it is not. In short, just as people need to reason about conditions, programs must compute in a conditional manner.

Conditions are nothing new. In mathematics we talk of true and false claims, which are conditions. For example, a number may be equal to, less than, or greater than some other number. If x and y are numbers, we state these three claims about x and y with

  1. x = y: ``x is equal to y'';

  2. x < y: ``x is strictly less than y'';

  3. x > y: ``x is strictly greater than y''.

For any specific pair of (real) numbers, exactly one of these claims holds. If x = 4 and y = 5, the second claim is a true statement, and the others are false. If x = 5 and y = 4, however, the third claim is true, and the others are false. In general, a claim is true for some values of the variables and false for others.

In addition to determining whether an atomic claim holds in a given situation, it is sometimes important to determine whether combinations of claims hold. Consider the three claims above, which we can combine in several ways:

  1. x = y      and     x < y      and     x > y

  2. x = y      or     x < y      or     x > y

  3. x = y      or     x < y .

The first compound claim is false because no matter what numbers we pick for x and y, two of the three claims are false. The second compound claim, however, always holds no matter what numbers we pick for x and y. Finally, the third kind of compound claim is the most important of all, because it is true in some cases and false in others. For example, it holds when x = 4, y = 4 and x = 4, y = 5, but it is false if x = 5 and y = 3.

Like mathematics, Scheme has ``words'' for expressing truth and falsity, for stating atomic claims, for combining claims into compound claims, and for expressing that a claim is true or false. The ``word'' for true is true and the ``word'' for false is false. If a claim concerns the relationship between two numbers, it can typically be expressed with a RELATIONAL OPERATION, for example, =, <, and >.

Translating the three mathematical claims from above follows our well-known pattern of writing a left parenthesis, followed by the operator, its arguments, and a right parenthesis:

  1. (= x y): ``x is equal to y'';

  2. (< x y): ``x is strictly less than y''; and

  3. (> x y): ``x is strictly greater than y''.

We will also encounter <= and >= as relational operators.

A Scheme expression that compares numbers has a result just like any other Scheme expression. The result, however, is true or false, not a number. That is, when an atomic Scheme claim about two numbers is true, it evaluates to true. For example,

  (< 4 5) 
= true

Similarly, a false claim evaluates to false:

  (= 4 5) 
= false

Expressing compound conditions in Scheme is equally natural. Suppose we want to combine (= x y) and (< y z) so that the compound claim holds if both conditions are true. In Scheme we would write

(and (= x y) (< y z))

to express this relationship. Similarly, if we want to formulate a compound claim that is true if (at least) one of two claim holds, we write

(or (= x y) (< y z))

Finally, when we write something such as

(not (= x y))

we state that we wish the negation of a claim to be true.13

Compound conditions, like atomic conditions, evaluate to true or false. Consider the following compound condition:

(and (= 5 5) (< 5 6))

It consists of two atomic claims: (= 5 5) and (< 5 6). Both evaluate to true, and therefore the evaluation of the and-expression continues as follows:

= (and true true)
= true

The last step follows because, if both parts of an and-expression are true, the entire expression evaluates to true. In contrast, if one of the two claims in an and-expression evaluates to false, the and-expression evaluates to false:

  (and (= 5 5) (< 5 5))
= (and true false)
= false

The evaluation rules for or and not are similarly intuitive.

The next few sections will explain why programming requires formulating conditions and reasoning about them.

Exercise 4.1.1.   What are the results of the following Scheme conditions?

  1. (and (> 4 3) (<= 10 100))

  2. (or (> 4 3) (= 10 100))

  3. (not (= 2 3))    Solution

Exercise 4.1.2.   What are the results of

  1. (> x 3)

  2. (and (> 4 x) (> x 3))

  3. (= (* x x) x)

for (a) x = 4, (b) x = 2, and (c) x = 7/2 ?    Solution

4.2  Functions that Test Conditions


Here is a simple function that tests some condition about a number:

;; is-5? : number  ->  boolean
;; to determine whether n is equal to 5
(define (is-5? n)
  (= n 5))

The function produces true if, and only if, its input is equal to 5. Its contract contains one novel element: the word boolean. Just like number, boolean represents a class of values that is built into Scheme. Unlike number, boolean consists of just two values: true and false.

Here is a slightly more interesting function with a boolean output:

;; is-between-5-6? : number  ->  boolean
;; to determine whether n is between 5 and 6 (exclusive)
(define (is-between-5-6? n)
  (and (< 5 n) (< n 6)))

It consumes a number and produces true if the number is between, but does not include, 5 and 6. One good way to understand the function is to say that it describes the following interval on the number line:



Interval Boundaries: An interval boundary marked with ``('' or ``)'' is excluded from the interval; an interval boundary marked with ``['' or ``]'' is included. 

The following third function from numbers to boolean values represents the most complicated form of interval:

;; is-between-5-6-or-over-10? : number  ->  boolean
;; to determine whether n is between 5 and 6 (exclusive) 
;; or larger than or equal to 10
(define (is-between-5-6-or-over-10? n)
  (or (is-between-5-6? n) (>= n 10)))

The function returns true for two portions of the number line:



The left part of the interval is the portion between, but not including, 5 and 6; the right one is the infinite line starting at, and including, 10. Any point on those two portions of the line satisfies the condition expressed in the function is-between-5-6-or-over-10?.

All three functions test numeric conditions. To design or to comprehend such functions, we must understand intervals and combinations (also known as unions) of intervals. The following exercises practice this important skill.

Exercise 4.2.1.   Translate the following five intervals on the real line into Scheme functions that accept a number and return true if the number is in the interval and false if it is outside:

  1. the interval (3,7]:



  2. the interval [3,7]:



  3. the interval [3,9):



  4. the union of (1,3) and (9,11):



  5. and the range of numbers outside of [1,3].




Exercise 4.2.2.   Translate the following three Scheme functions into intervals on the line of reals:

1. (define (in-interval-1? x)
       (and (< -3 x) (< x 0)))

2. (define (in-interval-2? x)
       (or (< x 1) (> x 2)))

3. (define (in-interval-3? x)
       (not (and (<= 1 x) (<= x 5))))

Also formulate contracts and purpose statements for the three functions.

Evaluate the following expressions by hand:

  1. (in-interval-1? -2)

  2. (in-interval-2? -2)

  3. (in-interval-3? -2)

Show the important steps. Use the pictures to check your results.    Solution

Exercise 4.2.3.   Mathematical equations in one variable are claims about an unknown number. For example, the quadratic equation


is a claim concerning some unknown number x. For x = - 1, the claim holds:


For x = 1, it doesn't, because


and 4 is not equal to 0. A number for which the claim holds is called a solution to the equation.

We can use Scheme to formulate equational conditions as a function. If someone then claims to have a solution, we can use the function to test whether the proposed solution is, in fact, a solution. Our running example corresponds to the function

;; equation1 : number  ->  boolean
;; to determine whether x is a solution for x2  +  2  ·  x  +  1  =  0
(define (equation1 x)
  (= (+ (* x x) (+ (* 2 x) 1)) 0))

When we apply equation1 to some number, we get true or false:

  (equation1 -1)
= true


  (equation1 +1)
= false

Translate the following equations into Scheme functions:

  1. 4 · n + 2 = 62

  2. 2 · n2 = 102

  3. 4 · n2 + 6 · n + 2 = 462

Determine whether 10, 12, or 14 are solutions of these equations.    Solution

Exercise 4.2.4.   Equations are not only ubiquitous in mathematics, they are also heavily used in programming. We have used equations to state what a function should do with examples, we have used them to evaluate expressions by hand, and we have added them as test cases to the Definitions


window. For example, if our goal is to define Fahrenheit->Celsius, we might have added our examples as test cases as follows:

;; test expression:
(Fahrenheit->Celsius 32)
;; expected result:


;; test expression:
(Fahrenheit->Celsius 212)
;; expected result: 

After clicking the Execute button we can compare the two numbers. If they are equal, we know our function works.

As our results become more and more complex, comparing values becomes more and more tedious. Using =, we can instead translate these equations into claims:

(= (Fahrenheit->Celsius 32)


(= (Fahrenheit->Celsius 212)

Now, if all claims evaluate to true, we know that our function works for the specified examples. If we see a false anywhere, something is still wrong.

Reformulate the test cases for exercises 2.2.1, 2.2.2, 2.2.3, and 2.2.4 as claims.

Testing: Writing tests as claims is good practice, though we need to know more about equality to develop good automatic tests. To do so, we resume the discussion of equality and testing in section 17.8.    Solution

4.3  Conditionals and Conditional Functions


Some banks pay different levels of interest for saving accounts. The more a customer deposits, the more the bank pays. In such arrangements, the interest rate depends on the interval into which the savings amount falls. To assist their bank clerks, banks use interest-rate functions. An interest function consumes the amount that a customer wishes to deposit and responds with the interest that the customer receives for this amount of money.

Our interest rate function must determine which of several conditions holds for the input. We say that the function is a CONDITIONAL FUNCTION, and we formulate the definition of such functions using CONDITIONAL EXPRESSIONS. The general shape of a conditional expression is

  [question answer]
  [question answer])
  [question answer]
  [else answer])
The dots indicate that a cond-expression may contain an arbitrary number of cond-lines. Each cond-line, also called a cond-clause, contains two expressions, called CONDITION and ANSWER. A condition is a conditional expression that involves the parameters; the answer is a Scheme expression that computes the result from the parameters and other data if the conditional expression holds.14

Conditional expressions are the most complicated form of expressions we have encountered and will encounter. It is therefore easy to make mistakes when we write them down. Compare the following two parenthesized expressions:

  [(< n 10) 5.0]
  [(< n 20) 5]
  [(< n 30) true])
  [(< n 10) 30 12]
  [(> n 25) false]
  [(> n 20) 0])
The left one is a valid cond-expression because each cond-line contains two expressions. In contrast, the right one is not a valid cond-expression. Its first line contains three expressions instead of two.

When Scheme evaluates a cond-expression, it determines the value of each condition, one by one. A condition must evaluate to true or false. For the first condition that evaluates to true, Scheme evaluates the corresponding answer, and the value of the answer is the value of the entire cond-expression. If the last condition is else and all other conditions fail, the answer for the cond is the value of the last answer expression.15

Here are two simple examples:

  [(<= n 1000) .040]
  [(<= n 5000) .045]
  [(<= n 10000) .055]
  [(> n 10000) .060])
  [(<= n 1000) .040]
  [(<= n 5000) .045]
  [(<= n 10000) .055]
  [else .060])
If we replace n with 20000, the first three conditions evaluate to false in both expressions. For the expression on the left the fourth condition, (> 20000 10000), evaluates to true and therefore the answer is 0.60. For the expression on the right, the else clause specifies what the result of the entire expression is. In contrast, if n is 10000, the value is .055 because for both expressions, (<= 10000 1000) and (<= 10000 5000) evaluate to false and (<= 10000 10000) evaluates to true.

Exercise 4.3.1.   Decide which of the following two cond-expressions is legal:

  [(< n 10) 20]
  [(> n 20) 0]
  [else 1])
  [(< n 10) 20]
  [(and (> n 20) (<= n 30))]
  [else 1])
Explain why the other one is not. Why is the following illegal?
(cond [(< n 10) 20]
      [* 10 n]
      [else 555]) ;     


Exercise 4.3.2.   What is the value of

  [(<= n 1000) .040]
  [(<= n 5000) .045]
  [(<= n 10000) .055]
  [(> n 10000) .060])

when n is (a) 500, (b) 2800, and (c) 15000?    Solution

Exercise 4.3.3.   What is the value of

  [(<= n 1000) (* .040 1000)]
  [(<= n 5000) (+ (* 1000 .040) 
		  (* (- n 1000) .045))]
  [else (+ (* 1000 .040) 
	   (* 4000 .045)
	   (* (- n 10000) .055))])

when n is (a) 500, (b) 2800, and (c) 15000?    Solution

With the help of cond-expressions, we can now define the interest rate function that we mentioned at the beginning of this section. Suppose the bank pays 4% for deposits of up to $1,000 (inclusive), 4.5% for deposits of up to $5,000 (inclusive), and 5% for deposits of more than $5,000. Clearly, the function consumes one number and produces one:

;; interest-rate : number  ->  number
;; to determine the interest rate for the given amount
(define (interest-rate amount) ...)

Furthermore, the problem statement provides three examples:

  1. (= (interest-rate 1000) .040)

  2. (= (interest-rate 5000) .045)

  3. (= (interest-rate 8000) .050)

Recall that examples are now formulated as boolean expressions when possible.

The body of the function must be a cond-expression that distinguishes the three cases mentioned in the problem statement. Here is a sketch:

    [(<= amount 1000) ...]
    [(<= amount 5000) ...]
    [(> amount 5000) ...])

Using the examples and the outline of the cond-expression, the answers are easy:

(define (interest-rate amount)
    [(<= amount 1000) 0.040]
    [(<= amount 5000) 0.045]
    [(> amount 5000) 0.050]))

Since we know that the function requires only three cases, we can also replace the last condition with else:

(define (interest-rate amount)
    [(<= amount 1000) 0.040]
    [(<= amount 5000) 0.045]
    [else 0.050]))

When we apply interest-rate to an amount, say, 4000, the calculation proceeds as usual. Scheme first copies the body of the function and replaces amount by 4000:

  (interest-rate 4000)
= (cond
    [(<= 4000 1000) 0.040]
    [(<= 4000 5000) 0.045]
    [else 0.050])
= 0.045 

The first condition is false but the second one is true, so the result is 0.045 or 4.5%. The evaluation would proceed in the same manner if we had used the variant of the function with (> amount 5000) instead of else.

4.4  Designing Conditional Functions

Developing conditional functions is more difficult than designing a plain function. The key is to recognize that the problem statement lists cases and to identify the different cases. To emphasize the importance of this idea, we introduce and discuss a design recipe for designing conditional functions. The new recipe introduces a new step, DATA ANALYSIS, which requires a programmer to understand the different situations that the problem statement discusses. It also modifies the Examples and the Body steps of the design recipe in section 2.5:

Data Analysis and Definition:
After we determine that a problem statement deals with distinct situations, we must identify all of them. The second step is a DATA DEFINITION, an idea that we will explore a lot more.

For numeric functions, a good strategy is to draw a number line and to identify the intervals that correspond to a specific situation. Consider the contract for the interest-rate function:

  ;; interest-rate : number  ->  number
  ;; to determine the interest rate for the given amount >= 0
  (define (interest-rate amount) ...)

It inputs non-negative numbers and produces answers for three distinct situations:



For functions that process booleans, the cond-expression must distinguish between exactly two situations: true and false. We will soon encounter other forms of data that require case-based reasoning.

Function Examples:
Our choice of examples accounts for the distinct situations. At a minimum, we must develop one function example per situation. If we characterized the situations as numeric intervals, the examples should also include all borderline cases.

For our interest-rate function, we should use 0, 1000, and 5000 as borderline cases. In addition, we should pick numbers like 500, 2000, and 7000 to test the interiors of the three intervals.

The Function Body -- Conditions:
The function's body must consist of a cond-expression that has as many clauses as there are distinct situations. This requirement immediately suggests the following body of our solution:

(define (interest-rate amount)
    [... ...]
    [... ...]
    [... ...]))

Next we must formulate the conditions that characterize each situation. The conditions are claims about the function's parameters, expressed with Scheme's relational operators or with our own functions.

The number line from our example translates into the following three conditions:

  1. (and (<= 0 amount) (<= amount 1000))

  2. (and (< 1000 amount) (<= amount 5000))

  3. (< 5000 amount)

Adding these conditions to the function produces a better approximation of the final definition:

(define (interest-rate amount)
    [(and (<= 0 amount) (<= amount 1000)) ...]
    [(and (< 1000 amount) (<= amount 5000)) ...]
    [(> amount 5000) ...]))

At this stage, a programmer should check that the chosen conditions distinguish inputs in an appropriate manner. Specifically, if some input belongs to a particular situation and cond-line, the preceding conditions should evaluate to false and the condition of the line should evaluate to true.

The Function Body -- Answers:
Finally, it is time to determine what the function should produce for each cond-clause. More concretely, we consider each line in the cond-expression separately, assuming that the condition holds.

In our example, the results are directly specified by the problem statement. They are 4.0, 4.5, and 5.0. In more complicated examples, we may have to determine an expression for each cond-answer following the suggestion of our first design recipe.

Hint: If the answers for each cond-clause are complex, it is good practice to develop one answer at a time. Assume that the condition evaluates to true, and develop an answer using the parameters, primitives, and other functions. Then apply the function to inputs that force the evaluation of this new answer. It is legitimate to leave ``...'' in place of the remaining answers.


When the definition is complete and tested, a programmer might wish to check whether the conditions can be simplified. In our example, we know that amount is always greater than or equal to 0, so the first condition could be formulated as

(<= amount 1000)

Furthermore, we know that cond-expressions are evaluated sequentially. That is, by the time the second condition is evaluated the first one must have produced false. Hence we know that the amount is not less than or equal to 1000, which makes the left component of the second condition superfluous. The appropriately simplified sketch of interest-rate is as follows:

(define (interest-rate amount)
    [(<= amount 1000) ...]
    [(<= amount 5000) ...]
    [(> amount 5000) ...]))

Figure 6 summarizes these suggestions on the design of conditional functions. Read it in conjunction with figure 4 and compare the two rows for ``Body.'' Reread the table when designing a conditional function!

Phase Goal                 Activity


to determine the distinct situations a function deals with

inspect the problem statement for distinct situations [curriculum-Z-G-D-4.gif] enumerate all possible situations


to provide an example per situation

choose at least one example per situation [curriculum-Z-G-D-4.gif] for intervals or enumerations, the examples must include borderline cases

Body (1)

to formulate a conditional expression

write down the skeleton of a cond expression, with one clause per situation [curriculum-Z-G-D-4.gif] formulate one condition per situation, using the parameters [curriculum-Z-G-D-4.gif] ensure that the conditions distinguish the examples appropriately

Body (2)

to formulate the answers for the cond-clauses

deal with each cond-line separately [curriculum-Z-G-D-4.gif] assume the condition holds and develop a Scheme expression that computes the appropriate answer for this case

Figure 6:  Designing the body of a conditional function
 (Use with the recipe in figure 4 (pg. 5)) 

Exercise 4.4.1.   Develop the function interest. Like interest-rate, it consumes a deposit amount. Instead of the rate, it produces the actual amount of interest that the money earns in a year. The bank pays a flat 4% for deposits of up to $1,000, a flat 4.5% per year for deposits of up to $5,000, and a flat 5% for deposits of more than $5,000.    Solution

Exercise 4.4.2.   Develop the function tax, which consumes the gross pay and produces the amount of tax owed. For a gross pay of $240 or less, the tax is 0%; for over $240 and $480 or less, the tax rate is 15%; and for any pay over $480, the tax rate is 28%.

Also develop netpay. The function determines the net pay of an employee from the number of hours worked. The net pay is the gross pay minus the tax. Assume the hourly pay rate is $12.

Hint: Remember to develop auxiliary functions when a definition becomes too large or too complex to manage.    Solution

Exercise 4.4.3.   Some credit card companies pay back a small portion of the charges a customer makes over a year. One company returns

  1. .25% for the first $500 of charges,

  2. .50% for the next $1000 (that is, the portion between $500 and $1500),

  3. .75% for the next $1000 (that is, the portion between $1500 and $2500),

  4. and 1.0% for everything above $2500.

Thus, a customer who charges $400 a year receives $1.00, which is 0.25 · 1/100 · 400, and one who charges $1,400 a year receives $5.75, which is 1.25 = 0.25 · 1/100 · 500 for the first $500 and 0.50 · 1/100 · 900 = 4.50 for the next $900.

Determine by hand the pay-backs for a customer who charged $2000 and one who charged $2600.

Define the function pay-back, which consumes a charge amount and computes the corresponding pay-back amount.    Solution

Exercise 4.4.4.   An equation is a claim about numbers; a quadratic equation is a special kind of equation. All quadratic equations (in one variable) have the following general shape:


In a specific equation, a, b and c are replaced by numbers, as in




The variable x represents the unknown.

Depending on the value of x, the two sides of the equation evaluate to the same value (see exercise 4.2.3). If the two sides are equal, the claim is true; otherwise it is false. A number that makes the claim true is a solution. The first equation has one solution, - 1, as we can easily check:


The second equation has two solutions: + 1 and - 1.

The number of solutions for a quadratic equation depends on the values of a, b, and c. If the coefficient a is 0, we say the equation is degenerate and do not consider how many solutions it has. Assuming a is not 0, the equation has

  1. two solutions if b2 > 4 · a · c,

  2. one solution if b2 = 4 · a · c, and

  3. no solution if b2 < 4 · a · c.

To distinguish this case from the degenerate one, we sometimes use the phrase proper quadratic equation.

Develop the function how-many, which consumes the coefficients a, b, and c of a proper quadratic equation and determines how many solutions the equation has:

(how-many 1 0 -1) = 2
(how-many 2 4 2) = 1

Make up additional examples. First determine the number of solutions by hand, then with DrScheme.

How would the function change if we didn't assume the equation was proper?    Solution

13 In truth, the operations and and or are different from not, which is why they are typeset in different fonts. We ignore this minor difference for now.

14 The use of brackets, that is, [ and ], in place of parentheses is optional, but it sets apart the conditional clauses from other expressions and helps people read functions.

15 If the cond-expression has no else clause and all conditions evaluate to false, an error is signaled in Beginning Student Scheme.