Reductions

Unlike other Higher Order Functions which lazily returns a transformed sequence, the word

REDUCE

immediately iterates through a sequence. Because

REDUCE

is not lazy you have to ensure that the target sequence is finite otherwise

REDUCE

will never end:

		0 
		1 2 ... 100 take 
			' max reduce .
		100.0 ok
	

The next thing to remember when using

REDUCE

is that it often requires some existing data ("initial values") on the stack. In the example above, the word

MAX

requires two arguments. Without the existing

0

on the stack, the reduction will fail because of

MAX

. Be reminded also that

REDUCE

requires an XT to perform a calculation. The word

.LIST

immediately prints out every element of a sequence and therefore is not lazy. It can be defined using

REDUCE

:

		: .list ['] . reduce cr ;
	

In the example above,

REDUCE

iterates through the sequence and calls

.

to print out each element. You may notice the word

CR

after

REDUCE

ends.

CR

prints a new line and avoids the confusion between the data list and subsequent items printed by other words.

Quiz

Question 1

Write a word

MINIMUM

to find the minimum of a sequence (assume it is finite) without making any assumptions about the upper bound. Hint: you may consider using the first item (what is it called in the sequence?) as the initial upper bound.

Question 2

Write a word

REVERSE

that reverses any list. Does your word work for empty sequences? Hint: when introducing Sequence, do you still remember the difference between two ways of creating a sequence?

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