3. Conditional statements
The IF statement lets a program branch to different statements depending on the outcome of previous computations. An example of the use of the IF statement follows:

PROGRAM decision1
LET x = 0
DO while x < 20
   LET x = x + 1
   IF x <= 10 then
      LET f = 1/x
   ELSE
      LET f = 1/(x*x)
   END IF
   PRINT x,f
LOOP
END

PROGRAM decision2
LET x = 0
DO while x <= 30
   IF x = 0 then
      LET f = 0
   ELSEIF x <= 10 then
      LET f = 1/x
   ELSEIF x <= 20 then
      LET f = 1/(x*x)
   ELSE
      LET f = 1/(x*x*x)
   END IF
   PRINT x,f
   LET x = x + 1
LOOP
END

The decisions of an IF structure are based on (logical or Boolean) expressions which are either true or false. A logical expression is formed by comparing two numerical or two string expressions by a relational operator. These operators are given in Table 1.

4. Subprograms and local and shared variables

Arrays are defined in a DIM statement and the total number of elements of an array is given in parentheses. The array variables a and b in the main program and the array variables r and s in SUB cross are examples of one-dimensional arrays.

The lower and upper limit of each subscript in an array can be specified; the default lower limit is 1. Examples of other limits are DIM r(0 to 2) and DIM s(-3 to 3). A colon may be used instead of TO in the DIM statement, for example, DIM r(0:2) and DIM s(-3:3). The arguments in a DIM statement must be numbers, not variables.

An element of an array is specified by its subscript value. Arrays can be passed to a subroutine or a function, with empty parentheses and commas used to indicate the dimension of the array.

The same name cannot be used for both an array variable and for another type of variable.

The PRINT statement displays output on the screen. Some simple extensions of the PRINT statement include

PRINT "x","y","z"
PRINT x,y,z
PRINT                     ! skip line
PRINT "time ="; t
PRINT tab(7);"time";tab(17);"position";tab(28);"velocity"

Because different computers have different numbers of rows and columns, the statement

ASK SET CURSOR max_row,max_col

Key input
The statement GET KEY k waits until the user presses a key, then converts that key into its corresponding number and then converts this number to the variable k.

PROGRAM space_bar
DO
   GET KEY k
   PRINT k
LOOP until k = 32                 ! pressing space bar exits loop
END

Mouse
The use of the GET MOUSE statement is illustrated in Program mouse:

PROGRAM mouse
DO
   ! return current x and y window coordinates and state of mouse
   GET MOUSE x,y,s
   IF s = 1 then
      PRINT "button down",x,y
   ELSE IF s = 2 then
      PRINT "button clicked",x,y
   ELSE IF s = 3 then
      PRINT "button released",x,y
   END IF
LOOP
END

Files
The following program illustrates how to open a text file, write to the file, close the file, and read the file.

PROGRAM single_column
! save data in a single column
! file$ example of string variable
INPUT prompt "name of file for data? ": file$
! channel number #1 associated with file and can be passed to
! subroutines
! various options may be specified in OPEN statement
! access output: write to file only
! create newold: open file if exists, else create new file
OPEN #1: name file$, access output, create newold
! True BASIC does not overwrite data in a text file
! ERASE #1                          ! erase contents of file
! RESET #1: end   ! allows data to be added to end of file
FOR i = 1 to 4
    LET x = i*i
    PRINT #1: x                   ! print column of data
NEXT i
CLOSE #1
! channel # irrelevant if only one channel open at a time
OPEN #2: name file$, access input
FOR i = 1 to 4
    INPUT #2: y                   ! print column of data
    PRINT y
NEXT i
! files automatically closed when program terminates
CLOSE #2                          ! not necessary but good practice
END

READ/DATA statements
One way to incorporate data into a program from a file. Another way to store information within a program is by using the DATA and READ statements as illustrated in Program example_data.

PROGRAM example_data
DIM x(6)
DATA 4.48,3.06,0.20,2.08,3.88,3.36
FOR i = 1 to 6
    READ x(i)                ! reads input from DATA statement
NEXT i
END

7. Graphics
The platform independent graphics statements of True BASIC are sufficiently powerful to do useful animations and visualizations. It is recommended that a separate plotting program be used for making presentation graphs and fits to data.

SET WINDOW xmin, xmax, ymin, ymax

PLOT POINTS: x,y;

PLOT LINES: x1,y1; x2,y2;

PROGRAM plot_f                     ! plot analytical solution
CALL initial(t,tmax,r,Ts,T0)
CALL set_up_window(t,tmax,Ts,T0)
CALL show_plot(t,tmax,r,Ts,T0)
END

SUB initial(t,tmax,r,Ts,T0)
    LET t = 0
    LET T0 = 83                   ! initial coffee temperature (C)
    LET Ts = 22                   ! room temperature (C)
    INPUT prompt "cooling constant r = ": r
    INPUT prompt "duration = ": tmax   ! time (minutes)
    CLEAR
END SUB

SUB set_up_window(xmin,xmax,ymin,ymax)
    LET mx = 0.01*(xmax - xmin)   ! margin
    LET my = 0.01*(ymax - ymin)
    SET WINDOW xmin - mx,xmax + mx,ymin - my,ymax + my
    ! default background color black on all computers except Macintosh
    PLOT xmin,ymin; xmax,ymin     ! abbreviation for PLOT LINES:
    PLOT xmin,ymin; xmin,ymax
END SUB

SUB show_plot(t,tmax,r,Ts,T0)
    DECLARE DEF f                 ! declare use of external function
    SET COLOR "red"
    DO while t <= tmax
       PLOT t,f(t,r,Ts,T0);       ! abbreviation for PLOT LINES
       LET t = t + 0.1
    LOOP
END SUB

DEF f(t,r,Ts,T0)
    LET f = Ts - (Ts - T0)*exp(-r*t)
END DEF

PLOT x,y; is an abbreviation of PLOT POINT x,y.

Aspect Ratio
When is a circle not a circle? Run the following program and find out.

PROGRAM no_circle
LET r = 1                         ! radius of circle
SET WINDOW -r,r,-r,r
SET COLOR "blue"
BOX CIRCLE -r,r,-r,r
! FLOOD 0,0: start from the point 0,0 and color continuous pixels 
! until boundary of region is reached
FLOOD 0,0
END

PROGRAM circle
LET r = 1                         ! radius of circle
CALL compute_aspect_ratio(r,xwin,ywin)
SET WINDOW -xwin,xwin,-ywin,ywin
SET COLOR "blue"
BOX CIRCLE -r,r,-r,r              ! draw circle
END

SUB compute_aspect_ratio(r,x,y)
    LET m = 0.1*r                 ! margin
    LET size = r + m
    ! px, py: # pixels in horizontal and vertical direction
    ASK PIXELS px,py
    IF px > py then
       LET aspect_ratio = px/py
       LET x = aspect_ratio*size
       LET y = size
    ELSE
       LET aspect_ratio = py/px
       LET x = size
       LET y = aspect_ratio*size
    END IF
END SUB

Multiple windows
So far we have used the entire screen for the default window. The following program illustrates how to use the OPEN statement to use a portion of the screen, make multiple windows, and choose a particular window. Note that each window has an associated channel number and properties such as its own coordinate system and plot color.

PROGRAM multiple_windows
! note passing of channel numbers
CALL initial(#1,#2)               ! initialize windows
CALL show(#1)
CALL show(#2)
END

SUB initial(#1,#2)
    OPEN #1: screen 0,0.5,0,1     ! left-half of screen
    SET WINDOW 0,1,0,1
    SET COLOR "green"
    OPEN #2: screen 0.5,1,0,1     ! right-half of screen
    SET COLOR "red"
    SET WINDOW 0,1,0,1
END SUB

SUB show(#9)
    WINDOW #9                     ! note use of dummy variable
    FOR i = 1 to 100
        PLOT rnd,rnd;
    NEXT i
END SUB

8. String variables
As mentioned, True BASIC recognizes only two types of variables, numeric and strings. A string variable may be any combination of characters. String variables end in a dollar sign ($). A string constant is any list of characters enclosed in quotation marks. An example of an assignment of a string variable is

LET file_name$ = "config.dat"

PROGRAM string
LET a$ = " "
LET b$ = "good"
PRINT b$
LET b$ = b$ & a$ & "morning"     ! & for concatenation
PRINT b$
LET c$ = b$[1:4]                  ! extract substring
PRINT c$
END

The function str$ converts a number to a string. The function val converts a string to a number. An example of the use of str$ is given in the following program.

PROGRAM read_file
! open n successive files
LET n = 20
FOR i = 1 to n
    LET si$ = str$(i)
    LET file_name$ = "config" & si$ & ".dat"
    OPEN #i: name file_name$, access output, create newold
    PRINT #i: file_name$
    PRINT file_name$
    CLOSE #i
NEXT i
END

9. Advanced topics

Recursion
A simple example of a recursive definition is the factorial function

factorial(n) = n! = n(n-1)(n-2) ... 1

factorial(n) = n factorial(n-1)

PROGRAM n!
DECLARE DEF f
LET n = 11
PRINT "n! ="; f(n)
END PROGRAM

DEF f(n)
    IF n <= 0 then
       LET f = 1
    ELSE
       LET f = n*f(n-1)
    END IF
END DEF

Global variables
So far we have shared information between subprograms by using a parameter list. Another way to share information is by declaring variables to be PUBLIC. An example illustrates its use.

PROGRAM show_public
PUBLIC L,spin(3,3)
CALL initial
FOR y = 1 to L
    FOR x = 1 to L
        PRINT spin(x,y);
    NEXT x
    PRINT
NEXT y
END

SUB initial
    DECLARE PUBLIC L,spin(,)
    LET L = 3
    FOR y = 1 to L
        FOR x = 1 to L
            IF rnd < 0.5 then
               LET spin(x,y) = 1
            ELSE
               LET spin(x,y) = -1
            END IF
        NEXT x
    NEXT y
END SUB

Strong typing
Unlike C, F, and Java, True BASIC does not require variables to be declared before they can be used. The enforced declaration of variables is called strong typing and can be "turned on" in True BASIC by using the OPTION TYPO statement. This statement requires all variables to be declared as either LOCAL, PUBLIC or passed. Public variables are not passed in the arguments of the subroutines.

PROGRAM show_public
OPTION TYPO
PUBLIC L,spin(3,3)
LOCAL x,y
CALL initial
FOR y = 1 to L
    FOR x = 1 to L
        PRINT spin(x,y);
    NEXT x
    PRINT
NEXT y
END

SUB initial
    DECLARE PUBLIC L,spin(,)
    LOCAL x,y
    LET L = 3
    FOR y = 1 to L
        FOR x = 1 to L
            IF rnd < 0.5 then
               LET spin(x,y) = 1
            ELSE
               LET spin(x,y) = -1
            END IF
        NEXT x
    NEXT y
END SUB

Select case
If all the choices in the decision structure are based on the value of a single expression, it is sometimes convenient to use a SELECT CASE structure instead of multiple ELSEIF statements. An example of the use of the SELECT CASE structure in the context of a two-dimensional random walk follows:

LET direction = int(4*rnd) + 1
SELECT CASE direction
CASE 1
     LET x = x + 1
CASE 2
     LET x = x - 1
CASE 3
     LET y = y + 1
CASE 4
     LET y = y - 1
CASE else
     PRINT "error"
END SELECT

Matrix operations
True BASIC has many operations which operate on an entire array at once. MAT READ, MAT INPUT, and MAT PRINT have obvious meanings. Arrays in which all the elements are the same may be established as follows:

DIM usave(0:21)
MAT usave = 1.0

PROGRAM example_matrix
! illustrate simple matrix arithmetic
DIM A(4),B(4),C(4),S(2,2),T(2,2),SI(2,2)
MAT A = 1
MAT B = 2*A
MAT C = A + B
MAT PRINT A,B,C
LET dot_product = dot(A,B)        ! dot product of A and B
MAT C = A*B                       ! matrix product
DATA 1,3,4,8
MAT READ S
MAT PRINT C,S
MAT T = Trn(S)                    ! transposed matrix
MAT SI = Inv(S)                   ! inverse matrix
MAT PRINT T,SI
! check that matrix of most recently inverted matrix is not too small
print det
END