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Vintage & Modern Tech Blog

<5 Min BASIC #11 - Planetary Approach

April 29, 2017

Here's a little space fun, making use of the Sinclair BASIC circle command and a FOR...NEXT loop.

 

 

Restart the ZX Spectrum and enter this listing 
in BASIC:

 

1 REM Under 5 min BASIC#11
2 REM Planetary Approach
3 REM Ste Nichol/Loading Screech
4 REM 29th April 2017
10 BORDER 0: PAPER 0: INK 7: CLS 
20 PRINT AT 2,2;"*"
30 PRINT AT 10,30;"*"
40 FOR r=1 TO 30 STEP 2
50 CIRCLE 128,81,r
60 NEXT r
70 PAUSE 0

 

 

Explanation/Refresher
Lines 1 to 4 are REMarks, comments that do not affect the running of the program.
Line 10 sets up the display colour attributes.
Lines 20 and 30 print a couple of stars.
Line 40 sets up the FOR...NEXT loop
Line 50 draws a circle in the middle of the screen (high-res co-ordinates x=128,y=81) with radius 'r' pixels.
Line 60 increases r by 2 (STEP 2)
Line 70 waits for a keypress before ending the program.

 
Exercises
1. Change the STEP 2 in Line 40 to STEP 4 and run.
2. Change the STEP 2 in Line 40 to STEP 1 and run.
3. Change line 40 to FOR r=30 to 1 STEP -1 and watch
   what happens. Hint - don't ignore the minus.

 

 

Thanks for reading!

Stephen

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