SmallBasicOverview. RCHS Computer Club

Transcription

1 SmallBasicOverview RCHS Computer Club

2 Constants and Variables 2 kinds of constants Numeric: e.g. 5, 42; but not in scientific notation String: e.g. Hello ; always in double-quotes Variables Give a name to a value Names may contain letters, digits, or underscores Names are case-insensitive: A is the same as a

3 Arrays A variable name may refer to an array, i.e., a collection of values each of which can be selected using an index value Index values may be integers or strings Setting an element value to the empty string ( ) makes it disappear from the array Some library functions are provided for arrays; see the Class Reference document

4 Statements A programis made up of a series of statements Statements end with a newline Statements are normally executed in the order in which they appear in the program Anything following a single quote ( ) is a comment Comments are ignored by SmallBasic, but are important documentation for your program Any program you want to keep or give to someone else needs comments!

5 Types of Statements Assignment Library function call Conditional (If Then Else) Branching (Goto) Repetition (looping) Subroutine definition (Sub, EndSub) Subroutine calls

6 Assignment Statement Gives a value to a variable Has the form variable= expression The expressionmay contain any legal combination of variables, constants and operators The = operator is nota statement of equality (as in Math) so an expressionlike x= x+ 1 makes sense: it means, add 1 to x

7 Library Function Calls The SmallBasicLibrary provides access to a variety of services, such as: Input and output (keyboard, display, mouse) Math functions Graphical functions (shapes, buttons, etc.) The Library consists of a set of objects Objects provide functions and may also control properties (e.g. the colour of the graphics window) Details are provided in the Class Reference document

8 Conditional Statements Test whether something is true or not Three forms: If condition Then statements EndIf If condition Then statements Else statements EndIf If condition1then ElseIfcondition2 Else EndIf Program executes statements following Thenonly if the preceding condition is true ( True is always true!) Multiple ElseIfclauses and a single Elseclause (at the end) may be included in the Ifstatement but are not required Program executes statements following Elseonly if all conditions are false

9 Conditional Statements (cont.) A conditionhas the form: expression operator expression expressionis any expression that could be used in an assignment statement operator is any comparison operator: = (equals), <> (does not equal), < (less than) <= (less than or equal to), >, >= Two or more conditions may be combined into one using the AndandOroperators

10 Conditional Statements (cont.) Examples: Get the absolute value of x: If x < 0 Then x = x EndIf Get the larger of two values: If a > b Then larger = a Else larger = b EndIf

11 Branching (Goto) Changes the order of program statement execution Form is: Gotolabel where labelindicates the desired point in the program A labelnames a point in the program; it consists of the name followed by a colon

12 GotoExample This code displays the integers from 1 to 10: i = 1 starting value start: a label (and a comment!) If i <= 10 Then not done yet TextWindow.WriteLine(i) i = i + 1 add 1 to counter variable Goto start repeat EndIf Note: there are much betterways to control execution order (see later slides)

13 Repetition (Looping) Two ways to repeat a set of statements: For loopcontrol EndFor While condition EndWhile The Forloop is most useful when you can calculate the number of repetitions before the loop starts executing The Whileloop is most useful when the number of repetitions depends on what happens while the loop is executing

14 ForLoops Example (note how the loop controlis written) sum the odd integers between 1 and 10 sum = 0 For i = 1 To 10 Step 2 sum = sum + i EndFor Note: The Steppart is optional, step defaults to 1 Stepcan be negative (e.g. to count down) i gets incremented or decremented at EndFor We can predict the number of iterations in advance

15 ForLoops (cont.) The example on the previous page is equivalent to: sum = 0 i = 1 loop: If i <= 10 Then sum = sum + i i = i + 2 Goto loop EndIf

16 WhileLoops Example (note the conditionis like that in If) n = 0 Ask user for a positive value While n <= 0 TextWindow.Write( Enter a number > 0: ) n = TextWindow.ReadNumber() EndWhile Note: Number of iterations cannot be predicted in advance A Whileloop can do exactly what a Forloop does While True will execute a loop forever

17 Subroutines A subroutine is a named sequence of statements The subroutine can be called, using its name, from multiple places in a program the program can perform the same actions at different points using the single sequence of instructions defined by the subroutine Subroutines can also be used to handle events such as mouse clicks

18 Subroutines (cont.) Example: a subroutine to convert Fahrenheit temperatures to Celcius. Sub makecelcius begin subroutine definition assumes variable fahr contains the Farhenheit temperature. Sets the variable celcius to the Celcius temperature celcius = (fahr 32) * 5 / 9 EndSub end subroutine definition Instructions in a subroutine are executed only when the subroutine is called(see next slide)

19 Subroutine Call Subroutines are called by writing the name of the subroutine followed by () Example using the Sub in the previous slide TextWindow.Write( Min temp in degrees F: ) read min temp fahr = TextWindow.ReadNumber() makecelcius() call the subroutine TextWindow.WriteLine( Celcius= + celcius) TextWindow.Write( Max temp in degrees F: ) read max temp fahr = TextWindow.ReadNumber() makecelcius() call subroutine again TextWindow.WriteLine( Celcius= + celcius) The subroutine code is written only once but is used twice in this example Note: no parameters or local variables for Subs