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, Aug 04 '13 at 14:07
FYI you can do a power-of-2 test with `p & (p-1) == 0`, that's somewhat simpler. — arshajii, Aug 04 '13 at 14:13

score 5 · Accepted Answer · edited Aug 04 '13 at 16:30

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 04 '13 at 16:30

msw

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answered Aug 04 '13 at 14:12

Ashwini Chaudhary

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@arshajii it would only work for small integers from -5 to 256, and it's a bad idea to compare integers like that. http://stackoverflow.com/questions/11476190/why-0-6-is-6-false – Ashwini Chaudhary Aug 04 '13 at 14:16
The small integer caching is a Cpython implementation detail and is manifestly confusing to people having trouble with the `is not` and `!=` distinction. – msw Aug 04 '13 at 14:53
@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 '=='. – PaulMcG Aug 04 '13 at 14:54
@PaulMcGuire That's a good point; while in CPython it will always work, it may not work in other implementations. – arshajii Aug 04 '13 at 15:02

Óscar López · Answer 2 · 2013-08-04T14:13:21.093

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 04 '13 at 14:13

answered Aug 04 '13 at 14:07

Óscar López

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python: how do I do simple variable declaration?

2 Answers2