Declarative WHILE: nested example
The previous post
by @yegor256 explained the declarative WHILE nature.
Let’s examine a more complex example when while objects are nested.
Consider the following code:
+alias org.eolang.txt.sprintf
[] > nested-while
memory 0 > x
memory 0 > y
seq > @
while.
x.lt 10
[i]
seq > @
y.write 0
while.
y.lt 2
[i]
seq > @
write.
y
y.plus 1
write.
x
x.plus 1
QQ.io.stdout
sprintf
"x=%d"
x
Here, we have two while loops, one inside the other. Initially, x is set to 0.
The outer loop continues while x is lower than 10. The inner loop executes its body
twice every time it’s dataized.
So what do you think the x will be after dataization of the whole program?
The answer is 15 and here is why.
Every time the inner while is dataized, x is incremented by 3: twice through
while iterations and one more time once the body is returned as a result. So a single
outer while iteration increments x by three. The last true condition of the outer
while would be after 3 iterations when x=3*3=9. The next iteration would set
x to 12, which will stop the outer loop.
After that, due to its declarative nature, the outer while will supply its body object, which
will be dataized one more time as requested by seq object. And the body happens to be the
inner while:
[i]
seq > @
y.write 0
while.
y.lt 2
[i]
seq > @
write.
y
y.plus 1
write.
x
x.plus 1
And we already know that this object would increase x by 3. So the final value of x will
be set to 15.
Sometimes it might be tricky to reason about the result when working with complex objects in EO. But it’s only a matter of practice and getting used to its declarative nature.