How to use and operator in python?

Question

I want to make a function that will remove '-' from two sequences of char if both contain it. This is my code.

def normalized(seq1, seq2):
    x = ''
    y = ''
    for a, b in zip(seq1, seq2):
    if a != '-' and b != '-':
        print a,b, 'add'
        x += a
        y += b
    else:
        print a, b, 'remove'
return x,y

x = 'ab--dfd--df'
y = 'rt-bfdsu-vf'

print normalized(x, y)

and this is the result.

a r add
b t add
- - remove
- b remove
d f add
f d add
d s add
**- u remove**
- - remove
d v add
f f add
('abdfddf', 'rtfdsvf')

You can see that - and u should not be removed. What's wrong with my code?

But - b should be removed?

Martijn Pieters
– Martijn Pieters

2013-05-30 13:36:26 +00:00
Commented May 30, 2013 at 13:36 — Martijn Pieters
– Martijn Pieters, Commented May 30, 2013 at 13:36
And your indentation is broken, can you fix it please?

Martijn Pieters
– Martijn Pieters

2013-05-30 13:36:44 +00:00
Commented May 30, 2013 at 13:36 — Martijn Pieters
– Martijn Pieters, Commented May 30, 2013 at 13:36

mgilson · Accepted Answer · 2013-05-30 13:48:04Z

4

You want to use or, not and ...

Another clever way that you could do this is to use operator chaining:

if a == b == '-':
   print a,b, 'remove'
else:
   print a,b, 'add'
   x += a
   y += b

This is extremely succint and clear that you want to remove the dashes only if they appear in both strings.

edited May 30, 2013 at 13:48

answered May 30, 2013 at 13:40

mgilson

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4 Comments

Ashwini Chaudhary Over a year ago

@GrijeshChauhan it is equivalent to (a ==b and b == '-')

tamasgal Over a year ago

It's called operator chaining as mgilson wrote and is a feature of python ;-) you can also do 5 < x < 10 instead of x > 5 and x < 10 etc.

mgilson Over a year ago

It's documented here

Grijesh Chauhan Over a year ago

@AshwiniChaudhary, Septi, mgilsom ..nice feature.. it don't happen in our C :)

Martijn Pieters · Accepted Answer · 2013-05-30 13:47:23Z

3

If you only want to remove if both are -, then test for that:

if not (a == '-' and b == '-'):

which can be shortened to:

if not (a == b == '-'):

or use or instead of and to remove the not:

if a != '-' or b != '-':

but that is not as readable.

Perhaps no more readable, but the list comprehension would be:

def normalized(seq1, seq2):
    return [''.join(v) for v in zip(*[(a, b) 
                for a, b in zip(seq1, seq2) if not (a == b == '-')])]

or using map(), sufficient for Python 2:

def normalized(seq1, seq2):
    return map(''.join, zip(*[(a, b) for a, b in zip(seq1, seq2) if not (a == b == '-')]))

edited May 30, 2013 at 13:47

answered May 30, 2013 at 13:39

Martijn Pieters

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7 Comments

njzk2 Over a year ago

de Morgan's law simplifies this in @mgilson answer.

John La Rooy Over a year ago

or if not a == b == '-':

user2435611 Over a year ago

this one if not (a == b == '-'): works. I just miss the logic here :p Thanks for your reduced code :D

Martijn Pieters Over a year ago

@user2435611: The part in the parenthesis is only True if both a and b are equal and both are equal to -. The not just turns that around to when a is not equal to b, or if they are, at least not equal to -.

user2435611 Over a year ago

Yes, your code is just what I want to do and you reduce it into 2 lines only :D Thanks a lot

|

Ashwini Chaudhary · Accepted Answer · 2013-05-30 13:39:52Z

1

The condition should be not (a =='-' and b == '-'):

def normalized(seq1, seq2):
    x = ''
    y = ''
    for a, b in zip(seq1, seq2):
        if not (a =='-' and b == '-'):  # you need `not` here
            print a,b, 'add'
            x += a
            y += b
        else:
            print a, b, 'remove'
    return x,y

x = 'ab--dfd--df'
y = 'rt-bfdsu-vf'

print normalized(x, y)

answered May 30, 2013 at 13:39

Ashwini Chaudhary

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Comments

njzk2 · Accepted Answer · 2013-05-30 13:41:03Z

0

According to your code, - u shoud be removed.

In fact,

a != '-' and b != '-' is False

the first part is false and the second is true. False and True is False. (Boolean Algebra 101, see http://en.wikipedia.org/wiki/Boolean_algebra#Basic_operations for details)

answered May 30, 2013 at 13:41

njzk2

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Comments

ydaetskcoR · Accepted Answer · 2013-05-30 18:43:40Z

0

You are currently asking for the program to match when a and b are not "-". This means that unless both are not equal to "-" then it will go to your else. The code you want is:

def normalized(seq1, seq2):
    x = ''
    y = ''
    for a, b in zip(seq1, seq2):
        if a == '-' and b == '-':
            print a,b, 'remove'
        else:
            print a, b, 'add'
            x += a
            y += b
    return x,y

x = 'ab--dfd--df'
y = 'rt-bfdsu-vf'

print normalized(x, y)

edited May 30, 2013 at 18:43

answered May 30, 2013 at 13:45

ydaetskcoR

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2 Comments

Martijn Pieters Over a year ago

Why are you adding while printing 'remove'? :-)

ydaetskcoR Over a year ago

Oops, that was a horribly hacked together bit of code but yeah the addition goes on the else. I'll edit that now.

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