Python list comprehension/ ternary operator: If condition, then append incremented value, else append non-incremented value

Question

I have the following loop in Python that I want to turn into a list comprehension:

index = 0
    for i in range(number):
        vec[i] = index
        if condition:
            index += 1

This should make vec look something like [0, 0, 0, 1, 1, 1, 1, 2, 2, 3, 3, 3, 3, 4, 5, 6, 6, 6]. However, I am struggling to create a list comprehension for this.

My attempts look like this:

y=[]
index = 0
[index+=1 and y.append(index) if condition else y.append(index) for _ in range(number)]

In iPython this just doesn't even execute.

You shouldnt use list comprehensions for side effects to begin with they are for expressing declarative mapping/filtering operations — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Jul 8, 2020 at 23:48

Prune · Accepted Answer · 2020-07-09 00:00:54Z

3

The walrus operator can do the job for you ... but I heartily agree with @juanpa: don't. This is hard to read and hard to debug.

>>> index = 0
>>> [ index := (index+1 if i%3 == 1 else index) for i in range(10) ]
[0, 1, 1, 1, 2, 2, 2, 3, 3, 3]

I've used i%3 == 1 as your "condition".

answered Jul 9, 2020 at 0:00

Prune

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2 Comments

MurderOfCrows Over a year ago

Fascinating, I've never heard of the walrus operator. I'll have to check it out. Are there uses for it which are not difficult to read or debug?

Prune Over a year ago

Remember the first rule of posting: research, research, research. Don't ask me whether there are such uses -- look it up! Yes, there are. The operator is new in Python 3.8, I think, and it has much better uses. The name comes from turning the operator sideways, like a :-) smiley.

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