Python newbie

Tino Wildenhain tino at wildenhain.de
Fri Sep 19 09:34:43 CEST 2008


Hi,

Mladen Gogala wrote:
> I am a Python newbie who decided to see what that Python fuss is all about.
> Quite frankly, I am a bit perplexed. After having had few months of
> experience with Perl (started in 1994 with Perl v4, and doing it ever
> since) , here is what perplexes me:
> 
> perl -e '@a=(1,2,3); map { $_*=2 } @a; map { print "$_\n"; } @a;'
> 
> The equivalent in Python looks like this:
> 
> Python 2.5.1 (r251:54863, Jun 15 2008, 18:24:51) 
> [GCC 4.3.0 20080428 (Red Hat 4.3.0-8)] on linux2
> Type "help", "copyright", "credits" or "license" for more information.
>>>> a=[1,2,3]
>>>> map((lambda x: 2*x),a)
> [2, 4, 6]
>>>> map((print),a)
>   File "<stdin>", line 1
>     map((print),a)
>              ^
> SyntaxError: invalid syntax
>>>> for x in a: print x
> ... 
> 1
> 2
> 3
>>>> for x in a: x=2*x
> ... 
>>>> for x in a: print x
> ... 
> 1
> 2
> 3
> 
> There are several questions:
> 
> 1) Why is the array "a" unchanged after undergoing a transformation with 
>    map?

it isn't transformed. You get a new list as result.

> 2) Why is it illegal to pass a built-in function "print" to map?

because its not a function but a statement. If it was a function you
would call it print(x) not print x.

> 3) Why is the array "a" unchanged after undergoing an explicit
>    transformation with the "for" loop?

To understand this, python knows immutable objects and mutable.
Immutables are strings, tuples , ... and integers.
This means the operation above all create a new integer object in memory
and assign it to the name x (where you use = )

> 4) Is there an equivalent to \$a (Perl "reference") which would allow me to 
>    decide when a variable is used by value and when by reference?

No, python always uses by reference.

> PHP also allows changing arrays with "foreach" loop:
> #!/usr/local/bin/php
> <?php
> $a=array(1,2,3);
> foreach($a as &$x) { $x=$x*2; }
> array_walk($a,create_function('$a','print("$a\n"); '));
> ?>
> 
> How can I make sure that 
> for x in a: x=2*x 
> 
> actually changes the elements of the array "a"?

 >>> l=range(10)
 >>> for i in range(len(l)):
... 	l[i]*=2
... 	
 >>> l
[0, 2, 4, 6, 8, 10, 12, 14, 16, 18]


Cheers
Tino
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