On 6/29/2016 6:01 AM, Victor Savu wrote:
There are many posts trying to explain the else after for or while.
My take: a while statement *is* a repeated if statement, and that is how
it is implemented.
while condition:
true()
is equivalent to and implemented in machine language without a native
while command as
<label> if condition:
true()
goto label
An else clause is executed exactly when the condition is false.
This understanding is useful in understanding how and why tail recursion
is equivalent to and can be converted to a while loop.
def f(n, accum):
if recurse:
return f(g(n), h(n, accum))
else:
return accum
What does the recursive call do? It assigns new values to the
parameters and goes to the top of the code. Since it is a tail call,
the old parameter bindings are not needed. So the above does the same as
def f(n, accum):
<label> if recurse:
n, accum = g(n), h(n, accum
goto label
else:
return accum
which is the same, by the above, as
def f(n, accum):
while recurse:
n, accum = g(n), h(n, accum
else:
return accum
In other words, convert properly formatted tail recursion (where the
condition is the recurse condition, rather than the terminate condition)
by replacing 'if' with 'while' and the tail recursive call with
unpacking assignment.
It is conventional to drop the else:, but it is implicitly still there.
--
Terry Jan Reedy
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