python: how do I do simple variable declaration?

Question

My code is test I'm trying to do to figure out why p & ~(p & (p - 1)) won't test for exponents of 2. anyway, the interpreter doesn't like in = 1 before the while loop for some reason.

code:

def getnumber(str):
    num=0
    for i in str:
        zz = ord(i)-48
        if zz<0 or zz>9:
            num=-1
            break
        else:
            num=(num*10)+zz
    return num

def testexp2(p):
    table = {1:1,2:2,4:3,8:4,16:5,32:6,64:7,128:8,256:9}
    if p & ~(p & (p - 1)):
        print "yes"
    else:
        print "no"


in = 1
while in is not -1:
    in = raw_input("> ")
    in = getnumber(in)
    if in>-1:
        testexp2(in)
    else:
        print "\nDone\n\n"

in is a reserved keyword in python. And you should use while inp ! = -1 not while inp is not -1. — Ashwini Chaudhary
– Ashwini Chaudhary, Commented Aug 4, 2013 at 14:07
FYI you can do a power-of-2 test with p & (p-1) == 0, that's somewhat simpler. — arshajii
– arshajii, Commented Aug 4, 2013 at 14:13

msw · Accepted Answer · 2013-08-04 16:30:58Z

5

Few problems:

in is reserved keyword in python, so you can't use it as a variable name.
while inp is not -1 should be while inp != -1. ( I used inp instead of in)
The getnumber function can be reduced to:

Code:

def getnumber(strs):
    num = int(strs)
    return -1 if num < 0 else num

edited Aug 4, 2013 at 16:30

msw

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answered Aug 4, 2013 at 14:12

Ashwini Chaudhary

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4 Comments

Ashwini Chaudhary Over a year ago

@arshajii it would only work for small integers from -5 to 256, and it's a bad idea to compare integers like that. stackoverflow.com/questions/11476190/why-0-6-is-6-false

msw Over a year ago

The small integer caching is a Cpython implementation detail and is manifestly confusing to people having trouble with the is not and != distinction.

PaulMcG Over a year ago

@arshajii - even worse, it will work sometimes, but not others. It works when an an interning cache finds the same value, so that identity matching with is will work then. But that cache is just an optimization, not a language feature, so you are not guaranteed that it will always be checked when comparing ints. If you are testing for equality in ints, use '=='.

arshajii Over a year ago

@PaulMcGuire That's a good point; while in CPython it will always work, it may not work in other implementations.

Óscar López · Accepted Answer · 2013-08-04 14:13:21Z

3

You can't declare a variable called in, that's a reserved word (or keyword) of the language, it's an operator that tests for membership. Simply rename it to something else in your code:

txt = raw_input("> ")
txt = getnumber(txt)

edited Aug 4, 2013 at 14:13

answered Aug 4, 2013 at 14:07

Óscar López

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