Decisions, Loops, and the Frame

Two ideas carry every program ever written: doing something many times (a loop) and choosing between paths (a decision). You've likely met both elsewhere, so this unit moves quickly — the point isn't that loops and If are new, it's seeing them in Blitz, drawing real graphics, and meeting the one command that turns a loop into a game loop: VWait, which paces your program to the screen's heartbeat.

What you'll see by the end

Ten bars, drawn by a loop that ran ten times. The sixth is a different colour because, on that one pass, an If decided to change it. Loop plus decision: nearly every pattern you'll ever draw is some mix of those two.

The whole program

; A loop draws ten bars; a decision picks out the middle one.
BLITZ
  BitMap 0,320,256,3
  Cls 0
  For x = 0 To 9
    c = 2
    If x = 5 Then c = 5
    Boxf x*30+10, 90, x*30+30, 160, c
  Next
  VWait 100
  Slice 0,44,3
  Show 0
  MouseWait

Inside the Blitz-mode setup you know, the work is the loop:

For x = 0 To 9
  c = 2
  If x = 5 Then c = 5
  Boxf x*30+10, 90, x*30+30, 160, c
Next

• For x = 0 To 9 … Next runs the lines between them ten times, with x counting 0, 1, 2 … 9. Each pass draws one bar, and x is used to place it — x*30+10 spaces the bars across the screen. A counter you can read is what makes a loop draw ten different bars instead of one bar ten times.
• c = 2 sets the colour for this bar to 2 (pink).
• If x = 5 Then c = 5 is the decision: on the pass where x is 5, change the colour to 5. Every other pass leaves it pink; that one pass makes it purple.
• Boxf …, c draws the bar in whatever c now holds.

So the loop repeats, and the If makes one pass behave differently. That's the whole engine of variety in a program.

Blitz's loops and decisions

Blitz gives you the full set, and they read much like any BASIC:

• For … Next — count a known number of times (as above).
• While … Wend — repeat while something stays true.
• Repeat … Until — repeat until something becomes true.
• If … Then …, with Else and EndIf for longer branches.

You'll lean on While … Wend in a moment, because it's the natural shape for a game loop: keep going while the game is running.

The frame: VWait

Here's the Blitz-specific piece. The Amiga's display redraws 50 times a second, top to bottom. VWait pauses your program until the next redraw begins. That matters for two reasons: it's the metronome that makes motion smooth (one step per redraw, in time with the screen), and it's the heartbeat of a game loop:

While Joyb(0) = 0    ; keep looping until the fire button
  VWait              ; wait for the next frame
  ; ... move things, draw things ...
Wend

Wait for the frame, do a frame's work, repeat — fifty times a second. You used VWait 100 earlier just to pause; inside a loop, a bare VWait is what paces a game. The next unit puts real motion inside this shape; here, just recognise it — the While/VWait/Wend skeleton is what every Blitz game beats to.

Type it and compile it

Type the program into Ted and press right-Amiga + X. (Choose the recompile option if Blitz asks about memory.) The ten bars appear with the middle one picked out. Click the mouse to return to the editor.

Try this: change the decision

Change If x = 5 Then c = 5 to If x = 5 Then c = 5 Else c = 3. Now the loop sets every bar that isn't the middle to colour 3 instead of 2 — Else gives the "all the other times" path. The middle stays purple; the rest change together.

Try this: every other bar

Replace the If line with a decision based on whether x is even:

If x/2*2 = x Then c = 5

x/2*2 throws away any remainder and multiplies back, so it equals x only for even numbers. Run it, and every other bar turns purple — a striped pattern, from one small condition inside the loop. (This even-or-odd trick is a staple you'll see again.)

If it doesn't work

• Blitz complains about Next. Every For needs a matching Next. In Blitz, Next on its own is fine — it doesn't need the variable name after it.
• All ten bars are the same colour. The If isn't changing c, or c isn't being passed to Boxf. Check the decision sets c, and that the Boxf ends with c.
• The bars are on top of each other. The loop isn't using x to place them. Each bar's position must be worked out from x — that's what spreads them across.

What you've learnt

The control flow every game is built from. For … Next repeats a known number of times with a counter you can use; While … Wend and Repeat … Until repeat on a condition; If … Then (with Else, EndIf) branches. And the Blitz heartbeat: VWait waits for the screen's next redraw, pacing a loop to the Amiga's 50-frames-a-second — the While/VWait/Wend shape that every game loop runs on.

What's next

You have the loop, the decision, and the frame. Time to put them to work at speed. Next — Many Things at Once — you'll move a crowd of objects inside the frame loop and watch compiled Blitz do, smoothly, what would leave an interpreted BASIC gasping.