On 05/11/2013 09:58 PM, Daniel Magruder wrote:
Please respond to the list, not the individual. Otherwise you're
robbing yourself and others of the possibility of learning from and
helping multiple people. I don't mind if you ALSO reply to me (which is
what reply-all does by default), but many people do object. In
Thunderbird, you normally just do Reply-list, and it does the right
thing. But with other email programs, you might do reply-all, and
remove whichever recipients you don't need.
Dear Dave,
I can't tell you how much I appreciate your assistance, however as a novice I
doubt my ability to crawl. I sincerely envy your depth of understanding about
this and in trying to learning would really benefit if you could please expound
on a few notions.
Why must I top-level call? What is that actually? Several of the sources I have
seen like learnpython.org suggest that def function(): is sufficient...
A function is defined by a def, but a program that never calls any of
them will do nothing at all (except look for syntax errors and the
like). There are probably tens of thousands of defined functions in the
library, but running python will only execute the ones you call.
Typically at the bottom of every script there needs to be at least one
function call that gets things started. (Unless the script is so
trivial that it's written without using any functions). But in either
case, some top-level code is needed.
That top-level code can be as simple as a single call, or it might
define a few variables, and then make the call(s).
I thought I had to define prime as f(0) so that it would be a float, not an
integer division. Am I wrong? I am so utterly confused.
I'm afraid you're wrong in a couple of points. There's no such function
as f() in the standard library. If you wanted to define a literal
float, you could either use 0.0, or use float(0). But you do not want a
float there, everything can be done with ints.
In many sample codes I have seen num or int seem to be short hand for anything
that is a number, because I though I was dividing and would receive a float
remainder for non-prime numbers, I wan't to use that as a test.
int and float are types (and act like functions), and if you needed to
check the type of an object, you'd use isinstance(myvar, int) or
isinstance(myvar, float). You can't just do a comparison with int. But
dividing two ints will give an int in Python 2.x and will give a float
in Python 3.x. And that's regardless of whether it comes out even or
not. Which version of Python are you using?
Fortunately, there's a much better way of telling if an int can be
divided by another int: The modulo operator, represented by a "%" symbol.
prime%n will give the remainder, and if it's zero, you know that
prime is evenly divisible by n (and therefore isn't prime).
For your suggestion:
def testprime(candidate)
check all ints between 2 and candidate, and return true if any of them
divide the candidate evenly
I don't mean to be so dependent, but how would I do this? Also what is the
difference between return and print?
Otherwise return false
The return statement is the way a function returns a value to its
caller. Let's suppose you wanted a function that returned true if its
int argument was odd. You could do it like this:
def isodd(candidate):
if candidate%2 ==0:
return False
else:
return True
Now, the body of that function could be done in one line, but I wanted
to illustrate the returning of useful values, and also the possibility
that the return might not be at the end, and that there might be more
than one place the function returns.
Why not get the other function to call this one temporarily, and see if
you can make the program find the first 1000 odd numbers instead. Then
you can come back to the testprime() function and see if you can create
one that returns True for primes, and False for anything else. Clearly
that function will need a loop.
def counting_primes()
build a list of primes, where a number is put in the list if testprime()
confirms that it is prime.
Again, how would I do this, so if I had something that told me a number was
prime, how would I take that prime and put it in a list if the function is on
loop?
return the list once it's big enough
You're already doing that in the code you showed. You initialized the
primelist, then started a while loop. The difference I'm suggesting is
that instead of dealing with x in this function, you defer that to the
testprime() function. So your loop would call testprime() and
conditionally append the value prime to the primelist.
def counting_primes():
primelist=[]
prime = 2
while len(primelist) < 1000:
if isodd(prime):
primelist.append(prime)
prime += 1
return primelist
Despite going to many sources I can not seem to find a single source that
adequately explains Python 100% without any technical jargon or assumptions of
prior knowledge. What I mean by this, is that I now have the gist of the utmost
basics, but I would say my understanding is befuddled. Furthermore if I were to
attempt to write code (such as this exercise) I am completely lost. What do you
recommend for a good source to learn Python? I have tried MIT open course ware,
but again even their intro explains the obvious, but not how to use it
collectively.
Sincerely,
Dan
With no experience in programming other languages, you'd need a
different kind of tutorial than I sought when I was learning Python.
And you absolutely need to match your tutorial against the version of
Python you're running on your machine. 2.x and 3.x aren't VERY
different, but there were enough changes that you don't want to be
dealing with that while learning initially.
Anybody else want to recommend a tutorial for someone who has no
programming experience in other languages?
--
DaveA
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