Incrementing different variables via the ternary operator in a for loop

Question

I'm trying to add to a different variable based on a test, within my for loop:

for i in range(alist.__len__()):
        if alist[i] is not blist[i]:
            ascore +=1 if alist[i] > blist[i] else bscore+=1
    print(ascore,bscore)

This code does not work. What I understand is happening is that the if condition is not applying to the whole assignment (we increment ascore if condition), it is instead applying to my value 1 (we increment ascore by 1 if condition). I would prefer functionality similar to the first. Anything I can do here? I understand just breaking it down into if elsif could easily solve this particular problem, but I ma more interested in the way the ternary operator (one line conditionals) work in python. Thankyou!

Assignments aren't expressions in Python (unlike C/C++) so you can't use the ternary operator here (which only works with expressions). — Michael Butscher, Oct 08 '17 at 01:28

score 7 · Answer 1 · answered Oct 08 '17 at 01:35

No. Unfortunately, you cannot use the ternary operator. As the name implies, it's an operator, so both the left hand and right hand sides must be expressions. However, unlike many other languages, Python assignments are statements, thus they cannot be used in the place of an expression.

The solution, as you correctly noted, is to use a normal conditional statement:

for i in range(len(list)):
        if alist[i] is not blist[i]:
            if alist[i] > blist[i]:
                ascore +=1 
            else:
                bscore +=1
    print(ascore, bscore)

UniBreakfast · Answer 2 · 2017-10-29T13:22:28.703

1

If you do not insist on using augmented assignmeng, you can do it like this:

ascore, bscore = (ascore + 1, bscore) if alist[i] > blist[i] else (ascore, bscore + 1)

Thanks to ShadowRanger for pointing my mistake (I missed the parentheses).

edited Oct 29 '17 at 13:22

answered Oct 24 '17 at 12:26

UniBreakfast

55
1
7

1

Your operator precedence is wrong. You just tried to assign a three-`tuple` to two elements; guaranteed `ValueError`. You wanted: `ascore, bscore = (ascore+1, bscore) if alist[i] > blist[i] else (ascore, bscore+1)`, but you effectively wrote: `ascore, bscore = ascore+1, (bscore if alist[i] > blist[i] else ascore), bscore+1` – ShadowRanger Oct 27 '17 at 00:06

Yaroslav Surzhikov · Answer 3 · 2017-10-08T02:18:47.173

If you need to increment only two variables - you could do it via dictionary, with the following code:

scores = {
    True: 0,   # ascore
    False: 0   # bscore
}

for i in range(len(alist)):
    if alist[i] is not blist[i]:
        scores[alist[i] > blist[i]] += 1

print(scores)

Or same, but more clear:

scores = {
    'ascore': 0,
    'bscore': 0
}

for i in range(len(alist)):
    if alist[i] is not blist[i]:
        scores['ascore' if alist[i] > blist[i] else 'bscore'] += 1

print(scores)

Joshua Like · Answer 4 · 2017-10-27T00:00:35.870

0

you should also make sure you use != and normal if statements

for i in range(len(alist)):
    if alist[i]!=blist[i]:
        if alist[i]>blist[i]:
            ascore+=1
        else:
            bscore+=1
    print(ascore,bscore)

edited Oct 27 '17 at 00:00

answered Oct 08 '17 at 01:58

Joshua Like

31
9

`is` will return True if two variables point to the same object, `==` if the objects referred to by the variables are equal. This operators have different purposes – Yaroslav Surzhikov Oct 08 '17 at 02:06

score 0 · Answer 5 · answered May 04 '23 at 12:55

Following is a simple way to increment one of the selected using ternary operator.

Assign the variable to increment using the ternary operator to the sore variable, and increment the score variable.

score = ascore  if alist[i] > blist[i] else bscore 
score +=1

Incrementing different variables via the ternary operator in a for loop

5 Answers5