using range() in for loops

[John Salerno]

I'm reading Text Processing in Python right now and I came across a
comment that is helping me to see for loops in a new light. I think
because I'm used to the C-style for loop where you create a counter
within the loop declaration, for loops have always seemed to me to be
about doing something a certain number of times, and not about iterating
over an object.

The reason for this distinction comes from the fact that I read a lot
how using range and for is somewhat discouraged, because it doesn't
really use a for loop for it's true purpose. So my question is, is this
just a Python-oriented opinion about for loops, or is it a general idea?

Also, what if you *do* need to just do something a set number of times.
Is this okay, or does it mean you are approaching the problem
incorrectly? Using for and range together seems to be a common idiom,
yet at the same time discouraged, so I'm wondering what is a good balance.

Thanks.

[Paul Rubin]

John Salerno <jo******@NOSPA Mgmail.com> writes:

The reason for this distinction comes from the fact that I read a lot
how using range and for is somewhat discouraged, because it doesn't
really use a for loop for it's true purpose. So my question is, is
this just a Python-oriented opinion about for loops, or is it a
general idea?

Normally you'd use range or xrange. range builds a complete list in
memory so can be expensive if the number is large. xrange just counts
up to that number.

[sushant.sirsikar]

hi John,
Python doesn't provide for loop like C / C++ but using Range() or
Xrange() you can achive all the functionalities of the C for loop.If
you wants distributed for loop You can use Xrange.
John Salerno wrote:

I'm reading Text Processing in Python right now and I came across a
comment that is helping me to see for loops in a new light. I think
because I'm used to the C-style for loop where you create a counter
within the loop declaration, for loops have always seemed to me to be
about doing something a certain number of times, and not about iterating
over an object.

The reason for this distinction comes from the fact that I read a lot
how using range and for is somewhat discouraged, because it doesn't
really use a for loop for it's true purpose. So my question is, is this
just a Python-oriented opinion about for loops, or is it a general idea?

Also, what if you *do* need to just do something a set number of times.
Is this okay, or does it mean you are approaching the problem
incorrectly? Using for and range together seems to be a common idiom,
yet at the same time discouraged, so I'm wondering what is a good balance.

Thanks.

[Ben Sizer]

John Salerno wrote:

The reason for this distinction comes from the fact that I read a lot
how using range and for is somewhat discouraged, because it doesn't
really use a for loop for it's true purpose. So my question is, is this
just a Python-oriented opinion about for loops, or is it a general idea?
The use of range in for loops is not discouraged, just the unnecessary
use of it. In many languages, you were forced to use range - or size,
or length, or count, etc - because the only practical method of
iteration was to use successive indices into the array or other
collection, stopping when the indexes run out. Most loops operate on
some sort of structure in this way.

In Python, you can iterate directly over the structure instead, so you
no longer have to worry about keeping track of the current index,
comparing it to the maximum index, or collecting the right object from
the structure according to its index. Python just gives you the next
object until there are none left. As a result, for most looping tasks,
you have no need to use range or an equivalent.
Also, what if you *do* need to just do something a set number of times.
Is this okay, or does it mean you are approaching the problem
incorrectly?

Do you really need to do something "a number of times"? If so, range
(or xrange) is perfect for the job. More often though, you need to do
something "once for every <whatever>", so put your 'whatevers' in a
list and iterate over that.

--
Ben Sizer

[Ant]

It's not just a Python thing, Java for example generally uses the
idiom:

for (Iterator it = list.iterator() ; it.hasNext(); ) {
Object next = it.next();
//Do stuff to next
}

Horrible compared to the python idiom of course (though the latest
version supports for (x : list){})

Ruby has something similar in:

list.each do |item|
print item
end

Again, not as nice as the python idiom IMHO (Though there may be better
ways - I don't know Ruby very well).

So really, all modern programming languages seem to prefer the
loop-over-iterator idiom in preference to looping over a numerical
range.

[Roel Schroeven]

John Salerno schreef:

I'm reading Text Processing in Python right now and I came across a
comment that is helping me to see for loops in a new light. I think
because I'm used to the C-style for loop where you create a counter
within the loop declaration, for loops have always seemed to me to be
about doing something a certain number of times, and not about iterating
over an object.

The reason for this distinction comes from the fact that I read a lot
how using range and for is somewhat discouraged, because it doesn't
really use a for loop for it's true purpose. So my question is, is this
just a Python-oriented opinion about for loops, or is it a general idea?

Also, what if you *do* need to just do something a set number of times.
Is this okay, or does it mean you are approaching the problem
incorrectly? Using for and range together seems to be a common idiom,
yet at the same time discouraged, so I'm wondering what is a good balance.

I felt more or less the same when I first learned Python; I was also
used to C-style loops, coming from a C/C++ background. In the end
though, it turned out to be a non-issue for me.

In many cases loops really are for iterating over sequences; more so
than I realized when using for loops in C or C++. In these cases,
Python's for statement works better than C-style loops. And if you
really need to do something a certain number of times, there's still
range() or xrange() to do it.

It's quite simple, I think:
- You have a sequence or iterator to loop over? Use for x in sequence.
- You want something done a set number of times? Use for i in range().
- You want to loop over a sequence and also need the index? Use for i, x
in enumerate(seque nce).

--
If I have been able to see further, it was only because I stood
on the shoulders of giants. -- Isaac Newton

Roel Schroeven

[John Salerno]

Roel Schroeven wrote:

In many cases loops really are for iterating over sequences; more so
than I realized when using for loops in C or C++. In these cases,
Python's for statement works better than C-style loops. And if you
really need to do something a certain number of times, there's still
range() or xrange() to do it.

Yeah, I'm starting to see the distinction now. I think one thing that
confused me was that C# had two separate loops for these
functionalities , the for loop and the foreach loop (which is equivalent
to Python's for). But even when just doing something a number of times
(C#'s for, Python's for), it looks much cleaner in Python because you
don't have the long, messy three-part for statement.

[AndyL]

Paul Rubin wrote:

Normally you'd use range or xrange. range builds a complete list in
memory so can be expensive if the number is large. xrange just counts
up to that number.

so when range would be used instead of xrange. if xrange is more
efficient, why range was not reimplemented?

[Georg Brandl]

su************* *@gmail.com wrote:

hi John,
Python doesn't provide for loop like C / C++ but using Range() or
Xrange() you can achive all the functionalities of the C for loop.

Not quite.

Georg

[Georg Brandl]

AndyL wrote:

Paul Rubin wrote:

Normally you'd use range or xrange. range builds a complete list in
memory so can be expensive if the number is large. xrange just counts
up to that number.

so when range would be used instead of xrange. if xrange is more
efficient, why range was not reimplemented?

Because of backwards compatibility. range() returns a list, xrange() an
iterator: list(xrange(... )) will give the same results as range(...).

In for loops, using xrange instead of range makes no difference since the
loop only iterates over the range. But it's a problem when someone just
does

l = range(100)

and assumes that he's got a list, probably doing

l.remove(5)

and so on.

In Python 3000, plans are that range() will be the same as xrange() is now,
and anyone needing a list can call list(range(...) ).

Georg