Smalltalk Programming Language Tutorial: Expressions, Messages, and Code Blocks

Posted on May 4, 2024 in Computers

Smalltalk Programming Language Tutorial

Evaluating Expressions

This section explores various expressions in the Smalltalk environment and explains their results.

Unary Messages

  • # ( ‘under’ ‘of’ ‘strings’) size: Returns the number of items (3) in the array.
  • “Today is Thursday ‘asUpperCase’: Converts the string to uppercase (“TODAY IS THURSDAY”).
  • ‘hello Here I am’ reversed: Reverses the order of characters in the string (“yotse iuqa Aloha”).

Binary Messages

  • ‘hello’, ‘Here I am’: Concatenates the strings (“hello Here I am”).
  • # (1 2 3), # (4 5 6): Concatenates the arrays into a single array (1 2 3 4 5 6).
  • 4 = 5: Evaluates the equality of the numbers (false).

Keyword Messages

  • ‘This is a test’ at: 3: Returns the character at position 3 (“t”).
  • ‘Hello’ includes: $ o: Checks if the string includes the character ‘o’ (true).
  • ‘hello’ at: 1 put: $ H: Replaces the character at position 1 with ‘H’ (“Hello”).

Nested Messages

Nested messages involve evaluating expressions within expressions. Parentheses can be used to control the order of evaluation.

  • ‘Hello’ size + 4: Evaluates to 8 (size of ‘Hello’ is 5, plus 4).
  • ‘now’ size + # (1 2 3 4) size: Evaluates to 9 (size of ‘now’ is 3, size of the array is 4, and their sum is 7).

Global Variables

Global variables in Smalltalk start with a capital letter. Examples include:

  • Display: Represents the screen.
  • Transcript: A transcript window.

Comparing Objects

Smalltalk provides operators for comparing objects:

  • 3 < 4: true (3 is less than 4).
  • # (1 2 3 4) = # (1 2 3 4): true (the arrays have the same elements in the same order).
  • ‘hello’ <= ‘goodbye’: false (string comparison is lexicographical).

Code Blocks

Code blocks are anonymous functions that can take arguments and be passed around.

  • [$ a isVowel] value: Checks if the character ‘a’ is a vowel (true).
  • [3 + 4. hello asUpperCase] value: Evaluates to ‘HELLO’ (performs the sum and then converts ‘hello’ to uppercase).

Boolean Expressions

Boolean expressions combine conditions using and:, or:, and not.

  • 5 < 2 or: [$ a isVowel]: true (at least one condition is true).
  • 5 < 2 and: [$ a isVowel]: false (both conditions need to be true for and:).

Object Classes

The class message returns the class of an object.

  • # (Jackie Marisa Francesca Bree) class: Array
  • ‘Vijay Rakesh Charles Daniel Tyler’ class: String
  • 5 class: SmallInteger

Cascading Messages

Cascading allows sending multiple messages to the same object.

  • 3 factorial, factorial, factorial: Calculates the factorial of 3, then the factorial of the result, and so on.

Conditionals

Smalltalk uses ifTrue:ifFalse: for conditional execution.

  • 3 < 4 ifTrue: [block true ‘] ifFalse: [block false ‘]: Executes the “block true” code since 3 is less than 4.

Additional Examples

The document also includes examples of Smalltalk code for various tasks, such as calculating the quotient and product of two numbers, raising a number to a power, solving quadratic equations, checking for prime numbers, determining even or odd numbers, finding multiples of numbers in a list, identifying perfect numbers, and converting the case of characters in a string.