Convert integer to binary in python and compare the bits

Question

How to convert a int n into binary and test each bit of the resulting binary number?

I have just got the following after a lot of googling:

def check_bit_positions(n, p1, p2):
    print int(str(n),2)

However i get an error invalid literal for int() with base 2. Let me know how can i get binary form of the input number and test each bit at position p1 and p2

EDIT:

binary = '{0:b}'.format(n)
if list(binary)[p1] == list(binary)[p2]:
     print "true"
 else:
     print "false"

The above code works now, however how can i check for postions p1 and p2 from the end of the list?

What are you trying to do exactly? Test if certain bits are on? — arshajii
– arshajii, Commented Aug 7, 2013 at 19:02
@arshajii Yes, and i want to know how to test at indexes from the start and end of the list. Please see my update. — Tina S
– Tina S, Commented Aug 7, 2013 at 19:05
You can back-index lists with negative indexes. l[-1] is the last element of l, l[-2] is the second to last etc.. — arshajii
– arshajii, Commented Aug 7, 2013 at 19:06
@TinaS you can use negative indexes: binary[-p1] == binary[-p2]. And There's no need to convert a string to list, a string is iterable in python. — Ashwini Chaudhary
– Ashwini Chaudhary, Commented Aug 7, 2013 at 19:06
'binary' is really just a string and can be used without wrapping it in a list. Try binary[p1] == binary[p2] without list-izing it. — tdelaney
– tdelaney, Commented Aug 7, 2013 at 23:02

Rohit Jain · Accepted Answer · 2013-08-07 19:01:35Z

7

Use bin() function:

>>> bin(5)
'0b101'

or str.format:

>>> '{0:04b}'.format(5)
'0101'

edited Aug 7, 2013 at 19:01

answered Aug 7, 2013 at 18:54

Rohit Jain

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6 Comments

Max Over a year ago

If you don't want the leading 0b, then use bin(s)[2:]

Tina S Over a year ago

Thanks @RohitJain, can you even help me on how to check the list from the end, i.e position p from the end of a list. Also i cannot upvote due to low reputation.

Rohit Jain Over a year ago

@TinaS. You can use negative indices - s[-1] == s1[-1]. And you don't need to convert it to list. You can use indices on string too.

Tina S Over a year ago

I have one more generic question, How can i know all the constructs of the language. As in i can see most of the answers here are using very less and optimised code, however i seem to write lengthy code due to my beginner level knowledge. Do you have any useful tips?

Rohit Jain Over a year ago

@TinaS You can start from official Python Tutorial. Also Python Language Reference can be handy.

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gr3co · Accepted Answer · 2013-08-07 19:10:42Z

6

Here's a quick function I wrote to check the nth bit of a number:

def check_nth_bit(num, n):
    return (num>>n)&1

Basically, you bitshift the number n times to the right, which would put the nth digit in the rightmost position, and by bitwise and-ing the new number with 1 (which is all 0's except for in the rightmost position), you can check if that bit is a 1 or a 0. So, you can call this function on num with p1 and p2 and compare the results.

EDIT: This will be p1 and p2 from the end of the number (least-significant bit), not the beginning.

edited Aug 7, 2013 at 19:10

answered Aug 7, 2013 at 18:57

gr3co

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1 Comment

Tina S Over a year ago

Thanks for your answer, but i will go with one of the above mentioned answers as it suits my current needs. However i will use your method as a helper function for future use.

Ashwini Chaudhary · Accepted Answer · 2013-08-07 19:01:07Z

3

You can use format:

>>> format(10, 'b')
'1010'

int is used to convert a number from any base to base 10, and you're trying to use it to convert an integer to binary which is wrong.

>>> int('1010', 2)
10
>>> int('20', 2)
Traceback (most recent call last):
  File "<ipython-input-3-05fc7296a37e>", line 1, in <module>
    int('20', 2)
ValueError: invalid literal for int() with base 2: '20'

edited Aug 7, 2013 at 19:01

answered Aug 7, 2013 at 18:55

Ashwini Chaudhary

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6 Comments

Tina S Over a year ago

Thanks @Ashwini , i combined @Rohit and your answer and able to proceed. Could you please update me on how to test a list element at a index p from end.

Ashwini Chaudhary Over a year ago

@TinaS stackoverflow.com/questions/18111488/…

Tina S Over a year ago

@AshwiniChaudhary You have linked the same question in the comment.

Ashwini Chaudhary Over a year ago

@TinaS I linked my comment not question. I posted that comment on your question before your comment on my answer.

Tina S Over a year ago

@AshwiniChaudhary Thanks for your time and effort. I can accept only one answer though.

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