[Tutor] can someone explain to me how this works

[Kirk Bailey]

I will give it a shot. Try not to laugh too hard if I mung it into kruftyness.

Conrad Koziol wrote:

> Hey thanks for the tips !!!
> 
> def compare(a,b):
>          if a[1]>b[1]: return 0
>          else: return -1
> 
ok. a[1] returns a char from a string, or a number in a list or tuple of 
numbers. So does b[1]. I am ASSUMING they are numbers in a list or tuple. Look 
at the values they would return.

if a[1]=12, and b[1]=4, well, 12>4 is true, so it would return a 1, which is a 
logical TRUE. so the condition after the test would happen, which is 'return 0', 
so your function ends, returning the value 0. now a -1 would be returned if the 
test was NOT true, by executing the other leg in the if/else structure.

First, it's neater to put the conditional after a if statement indented on the 
next line, and I cannot gurantee that placing it on the SAME line as the test 
will work in future versions of python. so let's do this:

def compare(a,b):
	if a[1]>b[1]:
		return 0
	else:
		return -1

This is your function, nice nad neat and quite properly pythonesque. But we can 
make it easier.

def compare(a,b):
	return not a[1]>b[1]

Not inverts the logical vallue of a returned logical value.
Again, a[1]=12 and b[1]=4. with this test, and those values, the return is 0. 
Were the values reversed, this would be a 1 return, which is true, as is a -1 
return value. IF the value HAS to be 0  or -1, we need to define the function 
with customized values. And a simple way to do it is:

def compare(a,b):
	return (a[1]>b[1])-1

Dig; if true, the returned value of a test is 1. If false, it is 0. These are 
plain old fashioned interger values, and you can do basic math on them just 
fine. Subtract 1 from that result, and you get 0 and -1 respectively. Python 
treats 0 as false, and any non 0 value as true, therefore the logic holds and 
you get your desired result faster and simpler.

Any discussion gang?




> i tried this out and it works, but i dont understand the logic behind it
> :(
> can someone help me out
> 
> i also dont get how it compares all the tuples in a list. Dont you need to enter
> in two values not just one list?????
> 
> _______________________________________________
> Tutor maillist  -  Tutor at python.org
> http://mail.python.org/mailman/listinfo/tutor
> 
> 

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Cheers!
         Kirk D Bailey

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