Python if-else short-hand

Question

Possible Duplicate:
Ternary conditional operator in Python

I want to do the following in python:

while( i < someW && j < someX){
   int x = A[i] > B[j]? A[i++]:B[j++];
   ....
}

Clearly, when either i or j hits a limit, the code will break out of the loop. I need the values of i and j outside of the loop.

Must I really do

x=0
...
if A[i] > B[j]:
  x = A[i]
  i+=1
else:
  x = B[j]
  j+=1

Or does anyone know of a shorter way?

Besides the above, can I get Python to support something similar to

a,b=5,7
x = a > b ? 10 : 11

@MartijnPieters: the second part certainly is a duplicate, but I'm not sure about the first one. — DSM, Jan 22 '13 at 15:24
@DSM: well, the first part won't be needed once more pythonic structures and loops are discovered by the OP.. Are you going to try and write a full introduction into iterators? — Martijn Pieters, Jan 22 '13 at 15:25
@DSM: I also don't see anyone below addressing that part. ;-) — Martijn Pieters, Jan 22 '13 at 15:26
@Martijn Pieters , while I am thankful for all participations, your comment is rather cheap. If you have an answer for the first part, post it. Ridicule is not reasoning. — learner, Jan 22 '13 at 15:29
@user1612593: I'm sorry, I don't mean to ridicule you. It takes time to get used to a new language and it's idioms. There is too little context here to give a concise and meaningful answer; you mostly do not encounter situations like yours in idiomatic Python. — Martijn Pieters, Jan 22 '13 at 15:31
@user1612593: I know MP (net-know, anyhow! :^), and he wasn't intending ridicule, he was being literal. There's no slick syntax to do exactly what you want in Python, but that's not a problem because with with tools like `zip` and `enumerate` we don't need to. — DSM, Jan 22 '13 at 15:33
I have edited to show that I need j and i outside of the while-loop. — learner, Jan 22 '13 at 15:46
@user1612593: You are basically looping over the sorted concatenation of A and B; use `for x in sorted(A + B):` for that; the [Python sort implementation](http://en.wikipedia.org/wiki/Timsort) can handle a merge like that very efficiently. Or write a generator that takes `A` and `B` as inputs, calls `iter()` on them, and yields the smaller next value of either. — Martijn Pieters, Jan 22 '13 at 16:22
@user1612593: (Very) generic generator to do the same job: https://gist.github.com/4596098 — Martijn Pieters, Jan 22 '13 at 16:36
`X if condition else Y` If condition is true evaluate X else evaluate Y — mrSaraf, Sep 16 '19 at 09:24

Lev Levitsky · Answer 1 · 2013-05-08T19:52:17.563

336

The most readable way is

x = 10 if a > b else 11

but you can use and and or, too:

x = a > b and 10 or 11

The "Zen of Python" says that "readability counts", though, so go for the first way.

Also, the and-or trick will fail if you put a variable instead of 10 and it evaluates to False.

However, if more than the assignment depends on this condition, it will be more readable to write it as you have:

if A[i] > B[j]:
  x = A[i]
  i += 1
else:
  x = A[j]
  j += 1

unless you put i and j in a container. But if you show us why you need it, it may well turn out that you don't.

edited May 08 '13 at 19:52

answered Jan 22 '13 at 15:23

Lev Levitsky

59,844
20
139
166

Thanks. This works for the second part of my question. Up vote! – learner Jan 22 '13 at 15:25
I have edited to show that I need `j` and `i` outside of the while-loop. – learner Jan 22 '13 at 15:42
1

`x = a > b and 10 or 11` also works correctly in a specific set of problems, because anything that evaluates to false instead of `10` would make it fail, for example, a String. – Ihor Husar Nov 09 '15 at 16:52
`[i<2 and i or i-4 for i in range(4)]` returns: `[-4, 1, -2, -1]`. It trolled me for two days and I had to actually read code line by line with my professor to figure out why our algorithm doesn't work. Thanks. – Ch3shire Jan 17 '17 at 10:59
How does this work internally: x = a > b and 10 or 11 – variable Nov 09 '19 at 05:18

score 19 · Answer 2 · answered Jan 22 '13 at 15:22

19

Try this:

x = a > b and 10 or 11

This is a sample of execution:

>>> a,b=5,7
>>> x = a > b and 10 or 11
>>> print x
11

answered Jan 22 '13 at 15:22

DonCallisto

28,203
8
66
94

33

`10 if a > b else 11`. No need to resort to boolean operators. – Martijn Pieters Jan 22 '13 at 15:23
@MartijnPieters: good point, sir. :) – DonCallisto Jan 22 '13 at 15:24
Thanks. This works for the second part of my question. Up vote! – learner Jan 22 '13 at 15:26
How does this work internally: x = a > b and 10 or 11 – variable Nov 09 '19 at 05:19

Python if-else short-hand

2 Answers2

Linked