Classic Basic on Repl.it (in beta)

• Simple syntax based on Classic Basic
• A 50x50 pixel display for graphics and games
• A console for input/output

Start here.

Syntax

Every line starts with a command. Every line starts with a line number followed by a command.

PRINT "Hello world"

Line numbers are optional and allow us to do control flow like subroutines and goto statements (more on this later).

10 PRINT "hello"
GOTO 10

Each basic command has its own syntax.

Commands

REM

REM allows us to add comments to our programs. Comments are for you or other people to read. Computers ignore them.

Example: REM this is a comment

PRINT

PRINT puts variables, strings, or numbers on the console. The console is the input/output area of Basic, where the program shows you useful information and asks you for input.

LET (optional)

LET lets us declare variables. Variables are single letters that hold values. Values could be either strings or numbers. Note that LET is optional and can be left out of an assignement expression.

Example using a number:

10 LET X = 10
20 PRINT X

Example using a string:

10 LET X = "hello"
20 PRINT X

Example without LET:

10 X = "hello"
20 PRINT X

ARRAY

ARRAY also lets us declare an array. Arrays can be thought of as lists of values.

Example:

10 ARRAY a
20 a[0] = "car"
30 a[1] = "bus"
40 a[2] = "bike"
50 print a

If we want to create a multi-dimensional array, which is an array of arrays, we can declare the array with a dimension:

10 ARRAY a, 2

15 REM the first array is ground transportation
20 a[0][0] = "car"
30 a[0][1] = "bus"
40 a[0][2] = "bike"

45 REM the second array aerial transportation
50 a[1][0] = "plane"
60 a[1][1] = "helicopter"
70 a[1][2] = "jetpack"

80 print a

INPUT

INPUT lets you communicate with the program by typing into the console. Whatever you typed will be stored into a variable that you can then use in your program.

Example:

10 INPUT "type your name: "; A
20 PRINT "Hello " + A

END

END ends the program.

Example:

10 END
20 PRINT "We never reach this statement"

GOTO

GOTO tells which line number is executed next. Normally, lines are executed from the lowest to the highest number, but GOTO allows us to "jump" to a specific line.

Example:

10 GOTO 30
20 PRINT "this line never executes"
30 PRINT "Jumped here from 10"

GOTO can be used to create a loop. Loops are when programs repeatedly execute a set of lines.

The following examples prints "hello" for ever:

10 PRINT "hello"
20 GOTO 10

IF...THEN

IF...THEN is like GOTO in that it allows to influence the execution of the program but it's different in that it all happens on the same line. If the mathematical or relational test is true we execute the command that comes after THEN. If it's untrue, we simply proceed to the next line.

10 IF X > Y THEN PRINT "X is larger than Y"

IF...THEN...ELSE

This is the same as IF...THEN except we execute the command following the ELSE command.

10 IF X > 0 THEN PRINT "X is positive" ELSE PRINT "X is negative"

FOR...TO...STEP...NEXT

It's often useful to repeatedly execute a number of lines. This is called "looping," and FOR statements allows to create loops between ranges of numbers.

For example, we can print numbers from 1 to 10:

10 FOR I = 1 TO 10
20 PRINT I
30 NEXT I

Output:

1
2
3
4
5
6
7
8
9
10

But what if we only want to print even numbers between 1 and 10? This is where STEP steps in. STEPS tells the computer how much to add to the variable (in this case I) with each loop.

10 FOR I = 2 TO 30 STEP 2
20 PRINT I
30 NEXT I

Output:

2
4
6
8
10

Remember to call NEXT with the variable name after the last line of the loop.

GOSUB...RETURN

A subroutine is a group of statements that you wish to use repeatedly in a program. They're similar to loops in that they can be executed repeatedly, but you have to use GOSUB to call (move the program to) the subroutine's first line number. After the subroutine finishes executing you can use RETURN to go back to where you used GOSUB.

Example:

10 GOSUB 40
20 PRINT "This is the end of the program"
30 END
40 PRINT "this is the start of the subroutine"
50 PRINT "we can easily call it as many times"
60 PRINT "as we'd like"
70 RETURN

PLOT

PLOT lights a pixel with an x,y coordinates on the display with a certain color.

Example:

10 PLOT 0, 0, "red"

Colors can be any of the these colors.

DISPLAY

DISPLAY changes the display size (rows, columns) and lets us turn off pixel borders.

Example:

DISPLAY 100, 100, FALSE

DISPLAY will affect the values of the constants ROWS and COLUMNS.

Example:

DISPLAY 50, 100, FALSE
PRINT ROWS
PRINT COLUMNS

OUTPUT:

50
100

DRAW

DRAW is like PLOT excepts it lights up multiple pixels on the display. It takes a two-dimensional array of colors.

Example:

10 ARRAY a, 2
20 a[0][0] = "red"
30 a[25][25] = "yellow"
40 DRAW a

TEXT

TEXT draws text on the display at an x,y coordinates. Optional text size and color parameters can be passed in.

Example:

10 TEXT 0, 0, "hello world", 25, "red"

Colors can be any of the these colors.

PAUSE

PAUSE pauses the program for a number of milliseconds. Milliseconds are 1/1000 of a second.

Example:

10 PRINT "pause for a second"
20 PAUSE 1000
30 PRINT "end"

CLS

CLS clears the console and the display.

Example:

10 CLS

CLT

CLT clears the console.

Example:

10 CLT

CLC

CLC clears the display.

Example:

10 CLC

SOUND

SOUND lets us play a frequency for a duration in seconds.

Example:

10 SOUND 400, 4

Duration is optional and will default to 1 second.

PLAY

PLAY lets us play a note in an octave for a duration in seconds.

Example:

10 PLAY "C", 4, 5

• Octave is optional and will default to 2
• Duration is optional and will default to 1
• Notes can be one of: C C# D D# E F F# G G# A A# B

Functions

You can think of functions as builtin subroutines you can call. It's important to differentiate between functions and commands. Lines should start with commands, while functions can be used as part of other commands but not on their own.

ABS

ABS returns the absolute value of a number. The sign of the number will always be positive after this function is executed.

10 PRINT ABS(-11)
20 PRINT ABS(11)

Output:

11
11

COS

COS returns the trigonometric cosine of a number.

Example:

10 PRINT COS(1)

Output:

0.5403023058681398

SIN

SIN returns the trigonometric sine of a number.

Example:

10 PRINT SIN(1)

Output:

0.8414709848078965

TAN

TAN returns the trigonometric tangent of a number.

Example:

10 PRINT TAN(1)

Output:

1.5574077246549023

ATAN

ATAN returns the trigonometric arctangent of a number.

Example:

10 PRINT ATAN(1)

Output:

0.7853981633974483

EXP

EXP returns Euler's number (e) raised to the power of a number.

Example:

10 PRINT EXP(2)

Output:

7.38905609893065

INT

INT returns the lowest closest integer of a number.

Example:

10 PRINT INT(2.6)

Output:

2

Alias: FLOOR

ROUND

ROUND rounds a number to the closest integer.

Example:

10 PRINT ROUND(2.6)

Output:

3

LOG

LOG returns the natural logarithm of a number.

Example:

10 PRINT LOG(5)

Output:

1.6094379124341003

SGN

SGN returns the sign of a number. The sign is +1 if the number is positive, 0 if the number is 0, and -1 if the number is negative.

Example:

10 PRINT SGN(-23)

Output:

-1

SQR

SQR returns the square root of a number.

Example:

10 PRINT SQR(25)

Output:

5

VAL

VAL converts a string to a number, and 0 if it cannot be converted.

Example:

10 PRINT VAL("33")

Output:

33

RND

RND returns a random number between 0 and 1.

Example:

10 PRINT RND()

Output:

0.54232

If passed a number between parenthesis then it will return a random bumber between 1 and that number. ht

Example:

10 PRINT RND(10)

Output:

7

ASC

ASC returns the ASCII representation of a letter.

Example:

10 PRINT "s"

Output:

115

LEFT

LEFT returns the first n number of letters from a string.

Example:

10 PRINT LEFT("basic", 2)

Output:

ba

MID

MID returns a substring as defined by a starting and ending position in the string.

Example:

10 PRINT MID("basic", 1, 2)

Output

as

RIGHT

RIGHT returns the last n number of letters from a string.

Example:

10 PRINT RIGHT("basic", 2)

Output:

ic

CHR

CHR returns the ASCII letter from a number.

Example:

10 PRINT CHR(115)

Output:

s

Aliases: STR

LEN

LEN returns the length of a string.

Example:

10 PRINT LEN("basic")

Output:

5

SPC

SPC returns a number of spaces.

Example:

10 PRINT "hello" + SPC(5) + "world"

Output:

hello     world

UPPERCASE

UPPERCASE returns the uppercase string.

Example:

10 PRINT UPPERCASE("basic")

Output:

BASIC

LOWERCASE

LOWERCASE returns the lowercase string.

Example:

10 PRINT LOWERCASE("BASIC")

Output:

basic

COLOR

COLOR returns the color of a pixel at an x, y coordinates.

Example:

10 PLOT 1,1,"red"
20 PRINT COLOR(1, 1)

Output:

red

GETCHAR

GETCHAR returns a single character of user input. The program maintains a first-in-first-put queue of user inputs. If there are no user inputs in the queue, it will return an empty string "".

Example:

10 LET I = GETCHAR()
20 IF I = "" THEN PRINT "no input" else PRINT "input: " + I

GETCLICK

GETCLICK returns an array of x, y coordinates of user mouse clicks. The program maintains a queue of clicks. If there are no user clicks in the queue, it will return an empty string "".

Example:

PRINT GETCLICK()

TIME

TIME returns the current milliseconds elapsed since the UNIX epoch.

Example:

10 PRINT TIME()

Output:

1587345235623