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9.7.3  How do I make a while-loop?

It depends on what you are trying to make.

The most common usage is to use it as a simple for-loop, that is, a loop which loops a certain number of times (for example 10 times) with an index value getting successive values (for example 1, 2, 3, ..., 10).

For this you need to first declare your index identifier with the first value. For example: 

#declare Index = 1;

Now you want to loop 10 times. Remember how the condition worked: The while-loop loops as long as the condition is true. So it should loop as long as our 'Index' identifier is less or equal to 10: 

#while(Index <= 10)

When the 'Index' gets the value 11 the loop ends, as it should.

Now we only have to add 1 to 'Index' at each loop, so we should do it at the end of the loop, thus getting: 

#declare Index = 1;
#while(Index <= 10)

  (some commands here)

  #declare Index = Index + 1;
#end

The incrementation before the #end is important. If we don't do it, 'Index' would always have the value 1 and the loop would go forever since 1 is always less or equal to 10.

What happens here?

  1. First POV-Ray sets the value 1 to 'Index'.
  2. Then it sees the #while statement and evaluates what is between the parentheses: Index <= 10
  3. As 'Index' has the value of 1 and 1 <= 10, the condition evaluates to true.
  4. So, it just continues normally. It executes the commands following the #while statement (denoted in the above example as "(some commands here)").
  5. Then it arrives normally to the last #declare command in the block. This causes the value 2 to be assigned to 'Index'.
  6. Now it arrives the the #end command and so it just jumps to the #while.
  7. After that it executes the steps 2-6 again because also 2 is less or equal to 10.
  8. After this has been done 10 times, the value 11 is assigned to 'Index' in the last command of the block.
  9. Now, when POV-Ray evaluates the condition it sees that it's false (because 11 is not less or equal to 10). This causes POV-Ray to jump to the command after the #end statement.
  10. The net effect of all this is that POV-Ray looped 10 times and the 'Index' variable got successive values from 1 to 10 along the way.

If you read carefully the above description you'll notice that the looping is done in a quite "dumb" way, that is, there's no higher logic hidden inside the loop structure. In fact, POV-Ray doesn't have the slightest idea how many times the loop is executed and what variable is used to count the loops. It just follows orders.

The higher logic in this type of loop is in the combination of commands we wrote. The effect of this combination is that the loop works like a simple for-loop in most programming languages (like BASIC, etc).

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