# CSC 151 Reference

The main reference for the Racket language and its libraries is the The Racket Reference available on the Racket website. Here, we summarize some of the parts of the language and libraries we use in this course. If there are other procedures you’d like to see in this document, please let us know.

## Keywords

(and ex0 ex1 ...) — Evaluate each expression until you reach #f (in which case the result is #f) or you run out of expressions, in which case the result is the last expression.

(cond [guard0 exp0a exp0b ...] [guard1 exp1a exp1b ...] ... [else alternative]) — Evaluate each guard in turn until one holds (is not false). Then evaluate the corresponding expressions and return the value of the last one. If no guards hold, evaluate the alternative and return its value.

(define name exp) — Evaluate exp and then assign the value to name.

(if guard true-part false-part) — Evaluate the guard. If it holds (is not #f), evaluate the true-part and returns its value. Otherwise, evaluate the false-part and return its value.

(lambda (params) body) — A procedure with the given formal parameters and body. When applied, we substitute the arguments (actuals) for the parameters in the body.

(or ex0 ex1 ...) — Evaluate each expression until you reach a truish case (in which case the result is the last truish value) or you run out of expressions, in which case the result is #f.

## Procedures

(+ v0 v1 v2 ...) — Add numbers together.

(- v0) — Compute the opposite of v0. Read as “negate”.

(- v0 v1 v2 ...) — Subtract each of v1, v2, … from v0.

(* v0 v1 v2 ...) — Multiply numbers

(/ v0) — Compute one divided by v0.

(/ v0 v1 v2 ...) — Divide v0 by each of v1, v2, ….

(< num0 num1 ...) — Determine if the numbers are in strictly increasing order.

(<= num0 num1 ...) — Determine if the numbers are in non-decreasing order.

(= num0 num1 ...) — Determine if all the numbers are equal.

(>= num0 num1 ...) — Determine if the numbers are in non-increasing order.

(> num0 num1 ...) — Determine if the numbers are in strictly decreasing order.

### A

(above img0 img1 ...) — Create a new image by stacking all of the images vertically, centered. img0 is at the top.

(above/align alignment img0 img1 ...) — Create a new image by stacking all of the images vertically, aligned as specified ('left, 'center, or 'right). img0 is at the top.

(abs num) — Take the absolute value of a number.

(append lst0 lst1 ...) — Combine lists together into a single list.

(apply proc lst) — Apply proc to the elements of lst en masse. (apply proc (list v0 v1 v2 v3 ...)) is the same as (proc v0 v1 v2 v3 ...).

### B

(beside img0 img1 ...) — Create a new image by combining all of the images horizontally, centered. img0 is at left.

(beside/align alignment img0 img1 ...) — Create a new image by combining all of the images horizontally, aligned as specified ('top, 'bottom, or 'center). img0 is at left.

(blue-component color) — Extract the blue component from a color, whether given by name (e.g., purple) or by formula (e.g., (color 10 90 30)).

### C

(car lst) — Get the first element of a non-empty list.

(cdr lst) — Get all but the first element of a non-empty list.

(ceiling num) — “Round up”.

(char? val) — Determine if val is a character.

(char<? ch0 ch1 ...) — Determine if the characters are in strictly increasing order (by collating sequence).

(char<=? ch0 ch1 ...) — Determine if the characters are in non-decreasing order (by collating sequence).

(char=? ch0 ch1 ...) — Determine if the characters are all the same.

(char>=? ch0 ch1 ...) — Determine if the characters are in non-increasing order (by collating sequence).

(char>? ch0 ch1 ...) — Determine if the characters are in strictly decreasing order (by collating sequence).

(char-alphabetic? char) — Determine if char is an alphabetic character.

(char-ci<? letter0 letter1 ...) — Determine if the letters are in strictly increasing order, ignoring case.

(char-ci<=? letter0 letter1 ...) — Determine if the letters are in non-decreasing order, ignoring case.

(char-ci=? letter0 letter1 ...) — Determine if the letters are equal, ignoring case.

(char-ci>=? letter0 letter1 ...) — Determine if the letters are in non-increasing order, ignoring case.

(char-ci>? letter0 letter1 ...) — Determine if the letters are in strictly increasing order, ignoring case.

(char->integer char) — Find the collating sequence number of char.

(char-lower-case? char) — Determine if char is a lowercase letter.

(char-numeric? char) — Determine if char is a numeric character. (A digit, so to speak.)

(char-upper-case? char) — Determine if char is an uppercase letter.

(char-whitespace? char) — Determine if char is a whitespace character, such as space, tab, or return.

(circle radius mode color) — Create the circle that matches the description. mode can be 'solid or 'outline or a number between 0 and 255, which represents the opacity. color can also be a pen.

(complex? val) — Determine if val is a complex number (one that can have an imaginary part).

(cons x xs) — Add x to the front of the list xs.

### D

(denominator num) — Compute the denominator of the internal representation of a real number.

(drop lst n) — Create a new list consisting of all but the first n elements of lst.

### E

(eq? val0 val1) — Determine if the two values have the same location in the computer’s memory. (We generally use the equal? procedure instead of the eq? procedure.)

(equal? val0 val1) — Determine if the two values are equal.

(error str ...) — Report an error and stop the computation immediately.

(exact? val) — Determine if val is an exact number.

(expt base power) — Compute base to the power‘th power or basepower

(even? i) — Determine if the integer i is even.

### F

(fetch-page url) — Fetch a Web page and return its contents as SXML. [csc151www]

(file->lines file) — Read a file into a list of strings, one string per line in the file.

(file->string file) — Read a file into a string.

(file->words file) — Read a file into a list of strings, one string per “word” in the file.

(file->xml file) — Read an XML file into an SXML structure. [csc151www]

(filter pred? lst) — Select the elements of lst for which pred? holds.

(floor num) — “Round down”.

(for-each proc! lst) — Apply proc! to each element of lst in sequence. Called only for the side effect; returns nothing.

### G

(green-component color) — Extract the green component from a color, whether given by name (e.g., purple) or by formula (e.g., (color 10 90 30)).

### H

(hash->list hash) — Get all of the key/value pairs from the hash table as a list.

(hash-has-key? hash key) — Determine if a hash table contains a particular key.

(hash-keys hash) — Extract all of the keys from hash.

(hash-map hash proc) — Apply proc, a procedure of two parameters, to each key/value pair in hash, creating a new list of the results. Note that hash-map expects parameters in the opposite order from that of map.

(hash-ref hash key) — Look up key in the hash table and return the corresponding value. Reports an error if the key is not in the table.

(hash-ref hash key default) — Look up key in the hash table and return the corresponding value. Returns default if the key is not in the table.

(hash-remove hash key) — Return a new immutable hash table that differs from hash only in that it no longer contains a key/value pair for the give key. hash-remove can only be used with immutable hash tables.

(hash-remove! hash key) — Remove the key/value pair with the specified key from the hash table. hash-remove! may only be used with mutable hash tables.

(hash-set hash key value) — Creates a new immutable hash table that now associates the given value with the given key. hash-set must be used only with immutable hash tables.

(hash-set! hash key value) — Update the hash table so that future calls to hash-ref will return value for key. (Alternately, store the key/value pair in the hash table.) hash-set! must be used only with mutable hash tables.

(hash-values hash) — Extract all of the values from hash.

### I

(imag-part num) — Extract the imaginary part of a complex number.

(index-of lst val) — Find an index of val in lst. (Note: In some versions, you’ll find the two parameters reversed.)

(indexes-of lst val) — Get a list of all of the indices of val in lst.

(inexact? val) — Determine if val is an inexact number.

(integer? val) — Determine if val is an integer.

(integer->char i) — Find the character associated with i in the collating sequence.

### L

(length lst) — Determine how many values appear in a list. (Do not use this to determine if a list has zero elements or if a list has one element.)

(lines->file lines file) — Save all the values in lines (a list of strings) to a file, one string per line.

(list v0 v1 ...) — Create a list consisting of the values v0, v1, etc.

(list? val) — Determine if val is a list.

(list-ref lst pos) — Find the value at the given position in lst.

(list->string list-of-characters) — Build a string from a list of characters.

(list->vector lst) — Convert a list to a vector.

### M

(make-hash) — Create a new, empty, hash table.

(make-hash list-of-key-value-pairs) — Create a new hash table initialized with the given key/value pairs.

(make-list n val) — Make a list of n copies of val.

(make-vector n val) — Make a vector of n copies of val.

(map proc lst) — Apply proc to each element of lst, individually, creating a new list of the same length. If lst has the form '(v0 v1 ...), the result has the form ((proc v0) (proc v1) ...).

(map proc lst1 lst2 lst3 ...) — Apply proc to the corresponding elements of each list (which all must have the same length) creating a new list of the same length. If lsti has the form '(vi0 vi1 vi2 ...), the result has the form ((proc v10 v20 v30 ...) (proc v11 v21 v31) (proc v12 v22 v32 ...)).

(max v0 v1 v2 ...) — Find the largest of the specified values.

(max v0 v1 v2 ...) — Find the smallest of the specified values.

(member? val lst) — Determine whether or not val appears in lst.

### N

(negative? num) — Determine if the real number num is negative.

(number->string num) — Convert a number to its string/textual representation.

(not val) — Compute the logical opposite of val. (not #f) is #t. (not anything-else) is #f.

(numerator num) — Compute the numerator of the internal representation of a real number.

### O

(o f1 f2 f3 ...) — Create a new procedure that applies each of the other procedures, one by one, from right to left. (An alternate name for compose.)

(odd? i) — Determine if the integer i is odd.

(overlay img0 img1 img2 ...) — Overlay images on top of each other. [2htdp/image]

(overlay/align halign valign img0 img1 img2 ...) — Overlay images on top of each other. [2htdp/image]

### P

(pen color width style cap join) — Create a pen for use in drawing circles and squares and such.

(positive? num) — Determine if the real number num is positive.

(procedure? val) — Determine if val is a procedure.

### Q

(quotient dividend divisor) — Integer division. Divide dividend by divisor, giving the whole number result.

### R

(random) — Generate an unpredictable real number between 0 (inclusive) and 1 (exclusive).

(random n) — Generate an unpredictable integer between 0 (inclusive) and n (exclusive).

**(random lower upper) — Generate an unpredictable integer between lower (inclusive) and upper (exclusive).

(range n) — Create the list '(0 1 2 ... n-1).

(range start finish) — Create a list of the integers from start (inclusive) to finish (exclusive).

(range start finish offset) — Create a list of the integers from start (inclusive) to finish (exclusive), adding offset each time.

(rational? val) — Determine if val is a real number.

(read) – Prompt the user for input and return that input as a Scheme value.

(read-line) — Prompt the user for input and return that input as a string.

(real? val) — Determine if val is a real number.

(real-part num) — Extract the real part of a complex number.

(rectangle width height mode color) — Create a rectangle of the specified characteristics. mode can be 'solid or 'outline or a number between 0 and 255, which represents the opacity. color can also be a pen.

(red-component color) — Extract the red component from a color, whether given by name (e.g., purple) or by formula (e.g., (color 10 90 30)).

(reduce proc lst) — Reduce a list to a single value by repeatedly combining neighboring elements with proc.

(reduce-left proc lst) — Reduce a list to a single value by repeatedly combining neighboring elements with proc, moving from left to right.

(reduce-right proc lst) — Reduce a list to a single value by repeatedly combining neighboring elements with proc, moving from right to left.

(regexp-match* regexp string) — Make a list of all the portions of string that match the regular expression.

(remainder dividend divisor) — Remainder from integer division. Give what remains after dividing dividend by divisor.

(rex-any-char) — Generate a regular expression that matches any single character.

(rex-any-of rex1 rex2 ... rexn) — Generate a regular expression that matches any of rex1 or rex2 or ….

(rex-char-antiset str) — Generate a regular expression that matches any character not in str.

(rex-char-range start finish) — Generate a regualr expression that matches any character that falls between start and finish, inclusive. (rex-char-range #\a #\z) matches any lowercase letter.

(rex-char-set str) — Generate a regular expression that matches any character in str.

(rex-concat rex1 rex2 ... rexn) — Generate a regular expression that matches a string that begins with a substring that matches rex1, then has a subsequent substring that matches rex2, and so on.

(rex-empty) — Generate a regular expression that matches the empty string. (Primarily useful to make optional parts of a string.)

(rex-end-of-string) — Generate a regular expression that matches the end of a string.

(rex-find-matches rex str) — Find all of the matches of a regular expression in a string.

(rex-matches? rex str) — Determine if a regular expression matches a string.

(rex->regexp rex) — Convert a rex-style regular expression to one of the most standard Racket-style regular expression.

(rex-repeat rex) — Generate a regular expression that matches one or more repetitions of rex.

(rex-repeat-0 rex) — Generate a regular expression that matches zero or more repetitions of rex.

(rex-split-string rex str) — Split a string at every instance of rex.

(rex-start-of-string) — Generate an expression that matches the start of a string.

(rex-string str) — Generate a regular expression that exactly matches str. (rex-string str) — Generate a regular expression that exactly matches str.

(round num) — Round a number toward the closest integer. Halves round toward the closest even integer.

### S

(section proc val-or-hole val-or-hole ..._) - Create a new procedure by selectively filling in some values as inputs to proc.

(sort lst compare?) - Sort the given list by comparing neighboring values with compare?.

(sqr num) — Compute the square of a number.

(sqrt num) — Compute the square root of a number.

(square size mode color) — Create a square of the specified characteristics. mode can be 'solid or 'outline or a number between 0 and 255, which represents the opacity. color can also be a pen.

(string ch0 ch1 ...) — Convert the sequence of characters into a string.

(string? val) — Determine if val is a string.

(string<? str0 str1 ...) — Determine if the strings are in strictly increasing order, using character comparison at each position.

(string<=? str0 str1 ...) — Determine if the strings are in non-decreasing order, using character comparison at each position.

(string=? str0 str1 ...) — Determine if the strings are all the same.

(string>=? str0 str1 ...) — Determine if the strings are in non-increasing order, using character comparison at each position.

(string>=? str0 str1 ...) — Determine if the strings are in strictly decreasing order, using character comparison at each position.

(string>? str0 str1 ...) — Determine if the strings are in strictly decreasing order, using character comparison at each position.

(string-append str0 str1 ...) — Combine strings together into a single string.

(string-ci<? str0 str1 ...) — Determine if the strings are in strictly increasing order, using character comparison at each position.

(string-ci<=? str0 str1 ...) — Determine if the strings are in non-decreasing order, using character comparison at each position and ignoring case.

(string-ci=? str0 str1 ...) — Determine if the strings are all the same, ignoring case.

(string-ci>=? str0 str1 ...) — Determine if the strings are in non-increasing order, using character comparison at each position and ignoring case.

(string>=? str0 str1 ...) — Determine if the strings are in strictly decreasing order, using character comparison at each position and ignoring case.

(string>? str0 str1 ...) — Determine if the strings are in strictly decreasing order, using character comparison at each position.

(string-length str) — Determine how many characters are in str.

(string->file str) — Save a string into a file.

(string->list str) — Convert a string into a list of characters.

(string->number str) — Convert a string to the corresponding number. Returns #f if it cannot be converted.

(string->xml str) — Convert an XML string into an SXML structure. [csc151www]

(string-prefix? str pre) — Determine if pre is a prefix of str (that is, it falls at the beginning of str).

(string-ref str pos) — Find the character at the given position in str.

(string-replace str old new) — Replace all copies of old in str with new.

(string-split str) — Split str into a list of strings at sequences of spaces.

(string-split str separator) — Split str into a list of strings, splitting at every instance of separator.

(string-suffix? str suf) — Determine if suf is a suffix of str (that is, it falls at the end of str).

(substring str start) — Extract the substring of str starting at start and finishing at the end of the string.

(substring str start finish) — Extract the substring of str starting at index start and finishing immediately before index finish.

### T

(take lst n) — Create a new list consisting of the first n elements of lst.

(tally pred? lst) — Count how many elements of lst meet pred?.

(tally-value lst val) — Count how many times val appears in lst.

(test-equal? str exp expected) — Test whether the expression evaluates to the expected value. If so, print nothing. If so, print str and some other information.

(test-false str exp) — Test whether the expression evaluates to false. If so, print nothing. If not, print str and some other information.

(test-true str exp) — Test whether the expression evaluates to true. If so, print nothing. If not, print str and some other information.

(triangle edge style color) — Create an equilateral triangle of the specified edge length. Style is as in other shapes.

(truncate num) — Round a number by cutting off anything after the decimal. (Rounds toward zero.)

### V

(vector v0 v1 v2 ... vn) — Create a vector of the specified elements.

(vector-fill! vec val) — Fill a vector with identical values.

(vector-length vec) — Determine how many values are in a vector.

(vector-ref vec pos) — Extract the value of vec at the given position.

(vector-set! vec pos newval) — Set the value of vec at the given position.

(vector->list vec) — Convert a vector to a corresponding list.

### Z

(zero? num) — Determines if the number is zero.

## Syntax Reference

Here is a concise description of the subset of the Racket programming language we use in this course. This description is given in a variant of Extended Backnus-Naur Form (EBNF).

Note that the description is not entirely consistent with the formal definition of the language; in particular, we only distinguish between two syntactic classes: top-level declarations and expressions.

<top-level> ::= (define <id> <expr>)
| (require <module>)
| <expr>

<expr> ::= <number>
| <string>
| #t
| #f
| #px<string>
| '(<expr1> ... <exprk>)
| x
| (<expr> <expr> ... <expr>)
| (lambda (<id1> ... <idk>) <expr>)
| (<let-kind> ([<id1> <expr1> ... <idk> <exprk>]) <expr>)
| (if <expr1> <expr2> <expr3>)
| (cond [<expr11> <expr21>] ... [<expr1k> <expr2k>])
| (begin <expr1> ... <exprk>)

<let-kind> ::= let | let* | letrec

<id> ::= a set of characters with no whitespace in between
incl. letters, numbers, and most symbols


Throughout, you may interchange any bracket symbols—parentheses ((...)), square brackets ([...]), and curly brackets ({...})—as long each open-bracket is closed by the same kind of bracket. By convention, parentheses are used in most circumstances with the exception being the pairs of binding-expression pairs for let expressions and expression-expression pairs for cond expressions.

Single-line comments in Racket are lines of text prepended with semi-colons (;).

Multi-line comments in Racket are blocks of text that begin with hash-pipe (#|) and end in pipe-hash (|#).

## API Reference

Equally important as the syntax of a language is its standard library. The standard library are the modules bundled with the language that require no additional installation to use. These libraries typically include the functions we use to manipulate the built-in types of the language, e.g., numbers, strings, and lists, as well as perform fundamental operations such as file input-output.

Below are the API documentation for each of the built-in types we use in Racket: