Reversing the bits of a 32-bit integer

Question

Here is my code:

noTest=int(input())
reverse=[]
nums=[]
for i in range(noTest):
 nums.append(int(input())) 
for num in nums:
 binary=bin(num)
 binNum=binary[2:]
 rev=binNum[::-1]
 rev=binary[:2]+rev+'0'*(32-len(rev))
 #print int(rev,2)
 reverse.append(int(rev,2))
for r in reverse:
 print(r)

Input:
The first line of input consists number of the test cases. Each test case
contains a single 32 bit integer.

Output:
Print the reverse of integer.

Input:00000000000000000000000000000001 =1
Output:10000000000000000000000000000000 =わ2147483648

• greybeard: I have... to what purpose? Know bit hacks?
• Thrilok: What is the question here?

Answer 1

To expand on Caridorc's answer, you can merge bin and zfill when using str.format. Which gives a much cleaner read IMO:

int('{:032b}'.format(num)[::-1], 2)

Alternately you can use format, which would be:

int(format(num, '032b')[::-1], 2)

• Veedrac: int(format(num, '032b')[::-1], 2)

Answer 2

You can avoid building a list of results to print them after, you can just print them as you calculate them.

Also you assign a lot of variables and:

binNum=binary[2:] 

is not explicit in removing the '0b' that Python prefixes, better is

.replace('0b','')

Also the padding with 0 logic that you write is already implemented in the zfill function, so you can avoid re-writing it yourself.

Finally, you should write a function to read the input to separate concerns and maybe reuse it in a similar problem.

Here are all my suggestions implemented.

def read_numbers(n):
 return [int(input()) for _ in range(n)]
for i in read_numbers( int(input()) ):
 binary_padded = bin(i).replace('0b','').zfill(32)
 print( int( binary_padded[::-1], 2) )

• Veedrac: replace is worse since it does a search and traverses the whole string.
• Caridorc: @Veedrac I think you are right, probably in this case [:2] should be used and maybe just adding a comment to explain the removal of '0b'