Flashcards for week 5

Due: 8 p.m. on Wednesday, 20 February 2019

Create ten questions and answers on topics we have explored in the past week (e.g., Boolean values, conditionals, preconditions and postconditions, precondition checking, and testing).

Send your questions and answers to rebelsky@grinnell.edu.

Title your email CSC 151.01 Flashcards for week 5 (YOUR NAME).

Include your questions and answers in the body of the email.

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Submitted questions

Here you can find some of the questions and answers that have been submitted so far (at least those that Sam had time to add).

Big picture

What is the difference between a keyword and a procdure?

Answer For procedures, all the parameters are evaluated, then the procedure is applied. For keywords, custom orders of evaluation are possible. We also cannot use keywords in procedures like map.

What keywords have we learned?

Answer and, or, not, lambda, if, cond, define, when. Maybe a few others.

Boolean values and Boolean operators

What is the difference between (and real? positive? number) and (and (real? number) (positive? number))?

Answer The first expression returns number, the second returns #t or #f

What is the order of parameters for tally?

Answer (tally lst pred?)

What is the order of parameters for filter?

Answer (filter pred? lst)

What is the format for and?

Answer (and exp1 exp2 ...)

How does and work?

Answer It evaluates each expression in turn. If any of the expressions evaluates to false (#f), it returns false. Otherwise, it returns the value of the last expression.

What is (and)?

Answer #t

What is the format for or?

Answer (or exp1 exp2 ...)

How does or work?

Answer It evaluates each expression. If any is truish (that is, not false), returns the first one. Otherwise, it returns false.

What is (or)?

Answer #f

What is the function of the procedure (tally lst pred?)?

Answer It counts the number of elements of a list for which a certain predicate holds.

What is the function of the procedure (filter pred? lst)?

Answer It extracts the elements of a list for which a certain predicate holds.

What does the procedure all do?

Answer (all pred? lst) determines whether the predicate (pred?) holds for all elements in a list (lst).

What is a predicate?

Answer A procedure that returns true or false.

What is Racket’s custom for naming predicates?

Answer Predicates traditionally end in a question mark.

What is the difference between tally and filter?

Answer tally counts the number of times the predicate holds in a list, filter returns a list of those values the predicate holds for. Also as a general note, the parameters for tally and filter are switched: (tally lst pred?) (filter pred? lst)

What does conjoin do?

Answer It combines two unary predicates. This is useful with filter and other similar operations.

What is the difference between (regexp-match? regexp str) and (regexp-match-exact? regexp str)?

Answer The difference is that while (regexp-match? regexp str) finds whether or not a regexp appears at any point in a string, (regexp-match-exact? regexp str) finds if the regexp matches the entire string exactly.

What is the difference between not and negate?

Answer not reverses a value, returning #t or #f, while negate reverses the predicate, returning anew predicate.

What is the purpose of conjoin?

Answer Similarly to negate’s relationship to not, conjoin allows us to use the equivalent of an and expression in combining two predicate into a new predicate.

Conditionals

What is the format for if?

Answer (if condition consequent alternative)

What does Racket do with an if expression do?

Answer It evaluates the condition. If the condition is #t (or anything other than #f), Racket evaluates the consequent. If the condition is #f, Racket evaluates the alternative.

How does a cond expression work?

Answer It works its way through the cond clauses, evaluating the guard at the beginning of each one, until it reaches a guard that succeeds (one that does not have #f as its value). Then it evaluates the corresponding consequents and returns the value of the last consequent.

What is the difference between a cond expression and a when expression?

Answer The when expression is an alternative-free conditional expression; whereas, a cond procedure is a conditional expression that supports multiple tests and multiple alternatives.

How can one rewrite (if test consequent alternate) with and and or?

Answer In many cases, (or (and test consequent) alternate), although that won’t work if the consequent evaluates to false.

Preconditions and Postconditions

What purpose do preconditions serve?

Answer They document the requirements of inputs to the procedure. E.g. if a list has to be a certain length, specify that in the preconditions

What are some things to consider when writing a precondition?

Answer Does the function depend on values named elsewhere? Are there any restrictions for the parameters? What inputs will and will not work?

What should be in the postconditions section?

Answer A somewhat formal, detailed description of the output and relevant information about how it relates to the input.

How do you verify preconditions in Scheme?

Answer (cond [(precondition-guard-1) (error "message1")] ... [(precondition-guard-n) (error "messagen")] [else (procedure-kernel parameters)])

What are husks and kernels?

Answer They are two separate procedures that make up one larger procedure. A husk check the preconditions for a procedure and interacts with the outside world, while the kernel is the heart of the procedure. They exist to save time for checking preconditions.

Testing

What is the purpose of test suites?

Answer To show that code achieves a desired goal without requiring formal proof. Also to help us clarify desired goals.

What is the difference between a check and a test?

Answer A check is a singular comparison between an actual output and the expected. A test is a group of similar checks.

What is the format for the RackUnit check-=?

Answer (check-= expression expected epsilon optional-message)

What is the epsilon parameter in check-=?

Answer The amount by which expression and expected can differ without producing an error.

How do you make a test case?

Answer (test-case description check-1 ... check-n)

Why use a test suite over a test case?

Answer Suites do not run immediately and can run many cases. You can also get some nice summaries from test suites.

Why do we need to write in the definition pane before testing?

Answer (require rackunit) and (require rackunit/text-ui)

What is the optional message in (check-= expression expected epsilon optional-message)?

Answer It is printed after an error message when the expression is not equal to expected within epsilon.

(check-= (+ 1.05 0) 1 0.1)

Answer Does not return any message, which means the check passes.

How do you run a test-suite?

Answer (run-tests name-of-the-test-suite). Note that you must use tests, with an s.

How do we define a zero-parameter procedure?

Answer (lambda () ...)

What is the difference between check-equal? and check-=?

Answer check-=? is for comparing numbers (and requires the extra epsilon parameter, while check-equal? is for comparing values other than numbers.

What do we expect from: (check-= (+ 2 5) 8 1)?

Answer Nothing.

What do we expect from: (check-= (+ 2 5) 8 0)?

Answer An error message.

How can you tell if a check succeeded?

Answer There will be no results printed for successful checks, only errors get messages.

Miscellaneous

What is the range of results for (random 10)?

Answer 0 through 9

What other P’s might we document apart from the basic 6 P’s?

Answer Package, Problem, Practicum (or Practica), Philosophy, and Props