Python

General syntax questions.

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Join Date: Jun 2009

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Your_mum

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General syntax questions.

Jun 29th, 2009

This will hopefully be my last noobish question post.
Please could somebody explain or link to explanation:
1.yeild
2.return
thanx in advance

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shadwickman shadwickman is offline

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Re: General syntax questions.

Jun 29th, 2009

Yield is for generator functions. Return ends the function and sends back the specified value.
Generators are functions that return iterators, i.e. they 'yield' a value after each iteration of them, before proceeding to the next one. This is generally used for things where you don't need all the results returned at once (which must load them all into memory), but instead only gives back the results one at a time (which can vastly reduce memory usage).

Python documentation on generators,
And on iterators.

Here's their example of a simple generator:

 Help with Code Tags 
 python Syntax (Toggle Plain Text) 
def reverse(data):
    # start at the last index and step back one each time.
    for index in range(len(data)-1, -1, -1):
        yield data[index]
 
for char in reverse('golf'):
    print char
 
"""result:
f
l
o
g
"""
def reverse(data):
    # start at the last index and step back one each time.
    for index in range(len(data)-1, -1, -1):
        yield data[index]

for char in reverse('golf'):
    print char

"""result:
f
l
o
g
"""

What this does is call each iteration of the reverse function each time the "for" loop executes in the statement for char in reverse('golf') . This way, only a single letter is sent back each time, rather than the fully-reversed string.
This isn't a great example of generator usage, but say if you had a dataset with 1000s of numbers and you needed to perform calculations on them, but you'd only be working on one item in the set at a time anyway. Then a generator would be useful as it would save a TON of memory by only returning each item as it's needed.
Hope that helped!

Last edited by shadwickman; Jun 29th, 2009 at 4:12 am. Reason: clarity

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Your_mum

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Re: General syntax questions.

Jun 29th, 2009

Thanks, that really helped me. But what is the point of 'returning' a value instead of just leaving it or printing it.

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shadwickman shadwickman is offline

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Re: General syntax questions.

Jun 29th, 2009

The point is so that you can keep whatever changes you've made to a value. Like:

 Help with Code Tags 
 python Syntax (Toggle Plain Text) 
# double-up a string
def doubleUp(s):
    return s + s
 
a = "Hello world!"
a = doubleUp(a)
b = doubleUp(a)
print a  # should display  Hello World!Hello World!
print b  # should display  Hello World!Hello World!Hello World!Hello World!
# double-up a string
def doubleUp(s):
    return s + s

a = "Hello world!"
a = doubleUp(a)
b = doubleUp(a)
print a  # should display  Hello World!Hello World!
print b  # should display  Hello World!Hello World!Hello World!Hello World!

If I just said s = s + s in the function, 'a' wouldn't be changed by saying doubleUp(a) as 's' is a variable in the local scope of the function. If I did this:

 Help with Code Tags 
 python Syntax (Toggle Plain Text) 
def doubleUp(s):
    s = s + s
 
a = "Hello World!"
doubleUp(a)
print a  # should display  Hello World!
def doubleUp(s):
    s = s + s

a = "Hello World!"
doubleUp(a)
print a  # should display  Hello World!

As you can see in the second example, all we did was change the value of 's' inside the function, but a isn't changed because we did not return the value of 's' and assign it back to 'a'.
In C++ you could pass either by pointer or by value, but Python's way of passing is a bit more complicated...
Hope I clarified that!

Last edited by shadwickman; Jun 29th, 2009 at 4:17 pm.

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