Classic Basic on Replit
- Simple syntax based on Classic Basic
- A 50x50 pixel display for graphics and games
- A console for input/output
Syntax
Every line starts with a command, or 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 assignment 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 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 advises 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" forever:
10 PRINT "hello"
20 GOTO 10
IF...THEN
IF...THEN is like GOTO in that it influences 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 allow us 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 comes 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 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 except 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 coordinate. 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 1/1000th 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 built-in 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 a number is placed between the parenthesis, then the command will return a random number between 1 and that number.
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 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-out 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