Game 1 Unit 3 of 6 1 hr learning time

A Row of Towers

Wrap the tower in a second loop. An outer FOR steps the column across the screen while the inner FOR raises a tower at each — nested loops turn one tower into a skyline.

50% of Skyline

You can build one tower at one column. To build a row of them, you need a loop around the loop you already have: an outer one that chooses where each tower stands, and the inner one that draws what stands there.

10 PRINT CHR$(147)
20 POKE 53281,0
30 H=8
40 FOR C=2 TO 38 STEP 4
50 FOR R=24 TO 24-H+1 STEP -1
60 POKE 1024+R*40+C,160
70 POKE 55296+R*40+C,12
80 NEXT R
90 NEXT C
A black C64 screen with a row of ten evenly-spaced grey towers across the bottom.
Nested loops: the outer FOR steps the column across in fours, the inner FOR raises a tower at each. One skyline, ten towers.

This is a nested loop — a loop inside a loop. The outer FOR C = 2 TO 38 STEP 4 steps the column across the screen in fours; for every one of those columns, the inner FOR R = 24 TO 24-H+1 runs in full, raising a complete tower before the outer loop moves on. Ten outer steps, eight inner blocks each — eighty blocks, drawn by four lines.

The STEP 4 is doing quiet work: it leaves three empty columns between towers, so the city has streets, not one solid wall. Spacing is composition — change the step to 2 and the towers crowd together; change it to 6 and they stand far apart.

Try this

  • Crowd the city. Change STEP 4 to STEP 2 and watch the streets close up.
  • Taller skyline. Raise H to 12. Every tower grows together, because they all read the same height — which is exactly the thing Unit 5 will set loose.

What's next

The city stands, but the windows are dark. In Unit 4 the colour cell earns its keep — some cells light up against the tower's body, and the skyline comes alive.