Smalltalk cheat sheet

Idiom ID Imports Comments Links

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		Smalltalk
1	Print Hello World Print a literal string on standard output	Transcript cr; show: 'Hello World!' The Transcript is a standard-output-like object in the Smalltalk environment. FYI; In classic Smalltalk, message cr alone does not output line breaks (it requires followed by an endEntry message i.e. Transcirpt cr ; entEntry.), so there was a convention to output a line break firstly then output the text by using show: method in which includes the endEntry process. Hence the cr message is sending firstly in the idiom. Demo
2	Print Hello 10 times Loop to execute some code a constant number of times	10 timesRepeat: [Transcript showln: 'Hello'].
3	Create a procedure Like a function which doesn't return any value, thus has only side effects (e.g. Print to standard output)	methodWithoutReturn self doSomething. [Transcript showln: 'something'] value. Note that this is how it typically looks in a Smalltalk code browser and belongs a class of some sort. Moreover, line 4, `[Transcript showln: 'something'] value.` creates an anonymous procedure and invokes it directly via value
4	Create a function Create a function which returns the square of an integer	[:anInteger \| anInteger squared]
5	Create a 2D Point data structure Declare a container type for two floating-point numbers x and y	Object subclass: #Point instanceVariableNames: 'x y' classVariableNames: '' poolDictionaries: '' category: 'Graphics-Primitives' This is an existing class. Instances are created as 3@4
6	Iterate over list values Do something with each item x of an array-like collection items, regardless indexes.	items do: [:x \| x doSomething]
7	Iterate over list indexes and values Print each index i with its value x from an array-like collection items	items withIndexDo: [:item :index \| Transcript showln: 'item: ' , item asString , ' index: ' , index asString].
8	Initialize a new map (associative array) Create a new map object x, and provide some (key, value) pairs as initial content.	x := Dictionary newFrom: { #a -> 1. #b -> Object new}. Alternate: { #a -> 1. #b -> Object new} asDictionary Demo
9	Create a Binary Tree data structure The structure must be recursive because left child and right child are binary trees too. A node has access to children nodes, but not to its parent.	Object subclass: #TreeNode instanceVariableNames: 'left right' classVariableNames: '' poolDictionaries: '' category: 'Trees'. Use the IDE to generate accessors for left and right.
10	Shuffle a list Generate a random permutation of the elements of list x	x shuffled.
11	Pick a random element from a list The list x must be non-empty.	x atRandom Demo
12	Check if list contains a value Check if the list contains the value x. list is an iterable finite container.	list includes: x. Demo
13	Iterate over map keys and values Access each key k with its value x from an associative array mymap, and print them.	mymap keysAndValuesDo: [ :k :x \| Transcript cr; show: 'Key = ', k printString; show: ', Value = ', x printString] Demo
14	Pick uniformly a random floating point number in [a..b) Pick a random number greater than or equals to a, strictly inferior to b. Precondition : a < b.	\| rand \| rand := Random new. rand next * (b - a) + a Demo
15	Pick uniformly a random integer in [a..b] Pick a random integer greater than or equals to a, inferior or equals to b. Precondition : a < b.	(a to: b) atRandom Demo
19	Reverse a list Reverse the order of the elements of the list x. This may reverse "in-place" and destroy the original ordering.	X reversed. Demo
21	Swap values Swap the values of the variables a and b	[\| temp \| temp := a. a := b. b := temp] value
22	Convert string to integer Extract the integer value i from its string representation s (in radix 10)	i := s asInteger. Demo
26	Create a 2-dimensional array Declare and initialize a matrix x having m rows and n columns, containing real numbers.	x := Matrix rows: m columns: n element: 1.0
28	Sort by a property Sort the elements of the list (or array-like collection) items in ascending order of x.p, where p is a field of the type Item of the objects in items.	items sorted: [:x :y \| x p <= y p].
29	Remove item from list, by its index Remove i-th item from list items. This will alter the original list or return a new list, depending on which is more idiomatic. Note that in most languages, the smallest valid value for i is 0.	items removeAt: i. Assuming items is an OrderedCollection. Other SequenceableCollections do not provide this.
31	Recursive factorial (simple) Create the recursive function f which returns the factorial of the non-negative integer i, calculated from f(i-1)	f := [:i \| i = 0 ifTrue: [1] ifFalse: [i * (f value: i - 1)]].
32	Integer exponentiation by squaring Create function exp which calculates (fast) the value x power n. x and n are non-negative integers.	exp: n n = 0 ifTrue: [^ 1]. n = 1 ifTrue: [^ self]. n%2 = 0 ifTrue: [^ self * self exp: n/2]. ^ self * (self * self exp: n-1/2) This would be a method of Integer. Integers unlimited
38	Extract a substring Find substring t consisting in characters i (included) to j (excluded) of string s. Character indices start at 0 unless specified otherwise. Make sure that multibyte characters are properly handled.	t := s copyFrom: i to: j - 1. In Smalltalk, collections (including String) use 1-based indexing.
39	Check if string contains a word Set boolean ok to true if string word is contained in string s as a substring, or to false otherwise.	ok := s includesSubstring: word.
41	Reverse a string Create string t containing the same characters as string s, in reverse order. Original string s must remain unaltered. Each character must be handled correctly regardless its number of bytes in memory.	t := s reversed. Demo
43	Break outer loop Look for a negative value v in 2D integer matrix m. Print it and stop searching.	\| negativeValue \| negativeValue := [ :matrix \|\| ctx \| ctx := thisContext sender. matrix do: [ :row \| row do: [ :value \| (value < 0) ifTrue: [ ctx resumeWith: value ]. ] ] ] value: m. "ex. m := #((1 2 -1 3))" Transcript show: negativeValue; cr. Normally, you would use non-local return inside a method containing just the outer loop. thisContext allows access and manipulation of the call stack. It can be used to implement continuations, which is what the example does.
45	Pause execution for 5 seconds Sleep for 5 seconds in current thread, before proceeding with the next instructions.	5 seconds wait.
46	Extract beginning of string (prefix) Create string t consisting of the 5 first characters of string s. Make sure that multibyte characters are properly handled.	s first: 5
47	Extract string suffix Create string t consisting in the 5 last characters of string s. Make sure that multibyte characters are properly handled.	s last: 5
48	Multi-line string literal Assign to variable s a string literal consisting in several lines of text, including newlines.	s := 'Will this compile? Oh yes, it will!'.
49	Split a space-separated string Build list chunks consisting in substrings of input string s, separated by one or more space characters.	chunks := s substrings
50	Make an infinite loop Write a loop that has no end clause.	[] repeat.
51	Check if map contains key Determine whether the map m contains an entry for the key k	m includesKey: k.
52	Check if map contains value Determine whether the map m contains an entry with the value v, for some key.	m includes: v.
53	Join a list of strings Concatenate elements of string list x joined by the separator ", " to create a single string y.	y := x joinSeparatedBy: ', '. Alternative implementation: ', ' join: #('abc' 'def' 'ghi') Demo
54	Compute sum of integers Calculate the sum s of the integer list or array x.	s := x sum.
55	Convert integer to string Create the string representation s (in radix 10) of the integer value i.	s := i asString. Demo
57	Filter list Create the list y containing the items from the list x that satisfy the predicate p. Respect the original ordering. Don't modify x in-place.	y := x select: [:item \| item p]. Demo
58	Extract file content to a string Create string lines from the content of the file with filename f.	\| lines \| lines := f asFilename readStream upToEnd.
59	Write to standard error stream Print the message "x is negative" to standard error (stderr), with integer x value substitution (e.g. "-2 is negative").	" implementation: Visual Works " OS.Stderr nextPutAll: x asString; nextPutAll: ' is negative'; nextPut: Character cr.
66	Big integer exponentiation Calculate the result z of x power n, where x is a big integer and n is a positive integer.	z := x raisedTo: n. Alternative implementation: z := x ** n.
71	Echo program implementation Basic implementation of the Echo program: Print all arguments except the program name, separated by space, followed by newline. The idiom demonstrates how to skip the first argument if necessary, concatenate arguments as strings, append newline and print it to stdout.	" Implementation: Visual Works " \| args arg sep \| args := CEnvironment commandLine readStream. args next. " skip executable name " sep := [args peek isNil not ifTrue: [' '] ifFalse: ['']]. [ (arg := args next) isNil ] whileFalse: [ Transcript show: arg; show: (sep value). ]. Transcript cr. " cr -> carriage return; ie. newline " Needs to be run in headless mode so that Transcript prints to stdout.
78	"do while" loop Execute a block once, then execute it again as long as boolean condition c is true.	[ " do something " c ] whileTrue: [].
96	Check string prefix Set boolean b to true if string s starts with prefix prefix, false otherwise.	b := s beginsWith: prefix
99	Format date YYYY-MM-DD Assign to the string x the value of the fields (year, month, day) of the date d, in format YYYY-MM-DD.	x := d yyyymmdd.
100	Sort by a comparator Sort elements of array-like collection items, using a comparator c.	\| c \| c := [ :a :b \| a size <= b size ]. " example c for strings " #('a' 'aaa' 'a' 'aaaa') asSortedCollection: c. " => SortedCollection('a' 'a' 'aaa' 'aaaa') "
110	Check if string is blank Set the boolean blank to true if the string s is empty, or null, or contains only whitespace ; false otherwise.	blank := s isAllSeparators.
117	Get list size Set n to the number of elements of the list x.	n := x size.
118	List to set Create the set y from the list x. x may contain duplicates. y is unordered and has no repeated values.	y := x asSet
119	Deduplicate list Remove duplicates from the list x. Explain if the original order is preserved.	x asSet. Does not preserve ordering. Alternative implementation: x intersection: x asSet. Preserves order in some Smalltalks (not in Pharo)
131	Successive conditions Execute f1 if condition c1 is true, or else f2 if condition c2 is true, or else f3 if condition c3 is true. Don't evaluate a condition when a previous condition was true.	true caseOf: { [c1] -> [f1]. [c2] -> [f2]. [c3] -> [f3]}
134	Create a new list Declare and initialize a new list items, containing 3 elements a, b, c.	items := {a . b . c}.
136	Remove all occurrences of a value from a list Remove all occurrences of value x from list items. This will alter the original list or return a new list, depending on which is more idiomatic.	items reject: [: y \| y = x ] returns a new list
137	Check if string contains only digits Set the boolean b to true if the string s contains only characters in the range '0'..'9', false otherwise.	b := s allSatisfy: #isDigit
143	Iterate alternatively over two lists Iterate alternatively over the elements of the lists items1 and items2. For each iteration, print the element. Explain what happens if items1 and items2 have different size.	items1 with: items2 do: [:item1 :item2 \| Transcript showln: item1; showln: item2]. If the collections have different size, this will raise an exception before the iteration starts. Alternative implementation: stream1 := items1 readStream. stream2 := items2 readStream. [stream1 atEnd ifFalse: [Transcript showln: stream1 next]. stream2 atEnd ifFalse: [Transcript showln: stream2 next]] doWhileFalse: [stream1 atEnd or: [stream2 atEnd]].
147	Remove all non-ASCII characters Create string t from string s, keeping only ASCII characters	t := s select: [:character \| character isAscii].
157	Declare constant string Initialize a constant planet with string value "Earth".	planet := 'Earth'. Literal strings in Smalltalk are immutable
158	Random sublist Create a new list y from randomly picking exactly k elements from list x. It is assumed that x has at least k elements. Each element must have same probability to be picked. Each element from x must be picked at most once. Explain if the original ordering is preserved or not.	y := x shuffled first: k. Collections understand the message #take:, which returns k elements. For ordered collections however, this will simply default to taking the first k elements.
163	Print list elements by group of 2 Print all the list elements, two by two, assuming list length is even.	list pairsDo: [:a :b \| Transcript showln: 'a: ' , a , ' b: ' , b].
165	Last element of list Assign to the variable x the last element of the list items.	x := items last. Demo
166	Concatenate two lists Create the list ab containing all the elements of the list a, followed by all the elements of the list b.	ab := a,b
169	String length Assign to the integer n the number of characters of the string s. Make sure that multibyte characters are properly handled. n can be different from the number of bytes of s.	n := s size.
170	Get map size Set n to the number of elements stored in mymap. This is not always equal to the map capacity.	n := mymap size.
172	Insert an entry in a map Insert value v for key k in map m.	m at: k put: v.
179	Get center of a rectangle Return the center c of the rectangle with coördinates(x1,y1,x2,y2)	c := (x1@y1 corner: x1@y2) center.
184	Tomorrow Assign to variable t a string representing the day, month and year of the day after the current date.	t := Date tomorrow asString.
185	Execute function in 30 seconds Schedule the execution of f(42) in 30 seconds.	(f future: 30 "seconds" * 1000) value: 42. - #value executes a BlockClosure - #future: delays the next message send to the object by the specified time in milliseconds
189	Filter and transform list Produce a new list y containing the result of function T applied to all elements e of list x that match the predicate P.	y := x select: [:ea \| ea p] thenCollect: [:ea \| ea t]. Matches the predicate P, then applies the transformation T. Alternative implementation: y := x collect: [:ea \| ea t] thenSelect: [:ea \| ea p]. Applies the transformation T, then matches the predicate P.
201	Euclidean norm Calculate n, the Euclidean norm of data (an array or list of floating point values).	data := #( 5 4 3 2 1 ). n := data squared sum sqrt. No builtin but this is pretty straight forward.
205	Get an environment variable Read an environment variable with the name "FOO" and assign it to the string variable foo. If it does not exist or if the system does not support environment variables, assign a value of "none".	\| foo \| foo := CEnvironment getUserEnvironment at: 'FOO' ifAbsent: [ 'none' ].
256	Count backwards Print the numbers 5, 4, ..., 0 (included), one line per number.	5 to: 0 by: -1 do: [:i \| Transcript showln: i asString].
272	Play FizzBuzz Fizz buzz is a children's counting game, and a trivial programming task used to affirm that a programmer knows the basics of a language: loops, conditions and I/O. The typical fizz buzz game is to count from 1 to 100, saying each number in turn. When the number is divisible by 3, instead say "Fizz". When the number is divisible by 5, instead say "Buzz". When the number is divisible by both 3 and 5, say "FizzBuzz"	1 to: 100 do: [:ea \| (ea isDivisibleBy: 15) ifTrue: [Transcript showln: 'FizzBuzz'] ifFalse: [(ea isDivisibleBy: 5) ifTrue: [Transcript showln: 'Buzz'] ifFalse: [(ea isDivisibleBy: 3) ifTrue: [Transcript showln: 'Fizz'] ifFalse: [Transcript showln: ea]]]].
306	Ensure list capacity Preallocate memory in the list x for a minimum total capacity of 200 elements. This is not possible in all languages. It is only meant as a performance optimization, should not change the length of x, and should not have any effect on correctness.	x := Array new: 200. "#(nil nil nil nil nil ...)"