Introduction to BASIC
**Foundation of Graphics
Set up the Coordinate System
**
First, set up the coordinate system with a SET WINDOW statement.

The meaning of four parameters of a SET WINDOW statement is as follows.

SET WINDOW left , right , bottom , top

Example.

`SET WINDOW -4,4,0,8`

The range of

[Note]

Ordinary, the drawing pane is square.

If you want to draw correct figures, set the coordinate system so that right - left = top - bottom.

`DRAW AXES`

draws the axes with ticks.

`DRAW GRID`

draws the coordinate grid.

`DRAW axes(`*x*,*y*)

draws the axes with tick intervals

`DRAW grid(`*x*,*y*)

draws the grid with intervals

**Draw curves
**Use PLOT LINES statements to draw curves.

The principle of drawing a curve resembles that of a XY-plotter.

Lines or curves are drawn by a pen.

When the pen moves if it is down, a line or curve is drawn.

When the pen moves if it is up, none are drawn.

PLOT LINES: x, y

moves to the point (x, y) and makes the pen up.

PLOT LINES: x,y;

moves to the point (x, y) and makes the pen down.

PLOT LINES

makes the pen up.

The initial pen state is up.

Example.

(1) A program which draws a graph of a function.

10 DEF f(x)=x^3+2*x+1 20 SET WINDOW -5,5,-5,5 30 DRAW grid 40 FOR x=-5 TO 5 STEP 0.01 50 PLOT LINES: x,f(x); 60 NEXT x 70 END

(2) A program which draws two or more curves.

10 SET WINDOW -5,5,-5,5 20 FOR x=-5 TO 5 STEP 0.01 30 PLOT LINES: x, sin(x); 40 NEXT x 50 PLOT LINES 60 FOR x=-5 TO 5 STEP 0.01 70 PLOT LINES: x,cos(x); 80 NEXT x 90 END

The PLOT LINES in line 50 is necessary for disconnecting the curves.

(3) A curve of parametric equation.

10 option angle degrees 20 SET WINDOW -4,4,-4,4 30 DRAW axes 40 FOR t=0 TO 360 50 PLOT LINES: 3*cos(t),2*sin(t); 60 NEXT t 70 END

(4) A curve defined in the polar coordinate.

10 SET WINDOW -2,2,-2,2 20 DRAW axes 30 FOR t=0 TO 2*pi STEP pi/180 40 LET r=1+cos(t) 50 PLOT LINES: r*cos(t),r*sin(t); 60 NEXT t 70 END

[Supplement]

To convert the rectangular coordinates to the polar coordinates, execute

LET r=SQR(x^2+y^2) LET t=ANGLE(x,y)