[Tutor] Making a Primary Number List generator

[Dave Angel]

On 05/13/2013 07:01 PM, Daniel Magruder wrote:
> Dear Dave,
> I am using python 2.
> I am still confused as what return does.   What does it mean if a
 > function returns True to the caller? What is the caller?

Have you ever used (called) a function?  If so, you've written a caller. 
  For example, if you want the absolute value of an integer, you might do:

import math

x = float(raw_input("Type an integer"))
y = math.fabs(x)        #Here I'm calling the function
print "Absolute value is", y

Now, somewhere in the Python library, a function was defined called 
fabs(), and it had a return statement with the value you want.

> Your code worked for returning a list of 1000 items of odd numbers, so I then tried writing a code to replay isodd to give True or False for isprime. However,
>> def isodd(candidate):
>>     if candidate%2 ==0:
>>         return False
>>     else:
>>         return True
> The meaning of this I am still confused about. What does it mean if the value is returned as false if it has 0 remainder, what does it mean if it is returned as true? Is this like a dictionary with keys and values?

I don't have a clue what you're confused about.  Do you not understand 
remainders?  If the remainder is 1 then the number is odd, while if the 
remainder is 0 then the number is even.  That's math, not programming.


> Also, here is my new code, which still does not work. I have no idea why not, and am getting very frustrated by the fact that making a program that determines if a number is prime or not is so hard for me. I would appreciate any assistance, and a verbal walk through of how to logically determine if a number is prime.
>
> def isprime(x):
>      test=2
>      numberinquestion = x
>      while test < numberinquestion:
>          if numberinquestion % test == 0:
>              test += 1
>              return False
>          elif numberinquestion / test == int:

Here you go again;  int is a type, not a value. Anyway, all the testing 
you needed was already done, if there's a nonzero remainder you already 
returned a False.  If there is a remainder, there's nothing more to test 
for this value of 'test'.  So increment and loop around.

>              test += 1
>              return False
>      if test == numberinquestion:

This is also unneeded.  If the while has finished, then you can return True.

>          return True
>
> def counting_primes():
>      prime = 2
>      primelist=[]
>      while len(primelist) < 1000:
>          if isprime(prime):
>              primelist.append(prime)
>          prime += 1
>      return primelist
> Sincerely,
> Dan



-- 
DaveA