Why can't I use max with Python to find maximum one-digit integer in array?

Question

I had an interview recently and was asked to solve the following problem using Python:

Write a function:

def solution(A):

such that, given an array A consisting of N integers, it returns the maximum among all one-digit integers.

For example, given array A as follows:

[-6, -91, 1011, -100, 84, -22, 0, 1, 473]

the function should return 1.

Assume that:

N is an integer within the range [1...1,000]
Each element of array A is an integer within the range [-10,000..10,000].
There is at least one element in array A which satisfies the condition in the task statement.

I came up with the following solution which I thought was elegant, but the score I got was only 55%:

def solution(A):
    return max([i for i in A if len(str(i))==1])

Why is this solution incorrect?

Also, any interview question that tells you to call a Python local variable capital A was definitely written by someone who doesn't actually use Python. — Silvio Mayolo
– Silvio Mayolo, Commented Sep 16, 2021 at 1:23
Also converting an integer to a string is not that elegant. You could have used if -9 <= i <= 9. — Selcuk
– Selcuk, Commented Sep 16, 2021 at 1:23
@ZhubeiFederer Not the original commenter, but in my view not only is this terrible style (non-descriptive in nature) but variables beginning with a capital letter are stylistically reserved for class names. — esqew
– esqew, Commented Sep 16, 2021 at 1:36
@ZhubeiFederer - per pep8, local variables should be lower_case_with_underscores and they should be descriptive. A isn't either. — tdelaney
– tdelaney, Commented Sep 16, 2021 at 1:36

tdelaney · Accepted Answer · 2021-09-16 01:50:00Z

4

Here is a solution that ticks more points in my book.

def solution(integers):
    """Return max single digit integer in sequence `integers`"""
    try:
        return max(i for i in integers if -9 <= i <= 9)
    except ValueError:
        return ValueError("Could not find single digit integer in list")

print(solution([-6, -91, 1011, -100, 84, -22, 0, 1, 473]))

It uses a descriptive parameter name, has a comment, does a simple comparison for digit size and uses a generator with the built in function max instead of the needless list. It also reraises the most expected fault with a more topical error message. This function is easier to maintain and use.

answered Sep 16, 2021 at 1:50

tdelaney

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4 Comments

sprogissd Over a year ago

This is the logic I went this, although as Selcuk has pointed out, max(i for i in a if i < 10) is a cleaner approach.

tdelaney Over a year ago

That doesn't work for large negative numbers. Did you mean abs(i)? That was my original solution but I changed to the comparison under the shakey assumption that the comparison is faster.

sprogissd Over a year ago

Why wouldn't it work for large negative numbers? Please provide an example of an array it wouldn't work with?

tdelaney Over a year ago

@sprogissd [-11, 33] would report -11 as a 1 digit number.

Niel Godfrey P. Ponciano · Accepted Answer · 2021-09-16 02:01:19Z

4

Not the most elegant. This assumes that e.g. -3 is single digit.

This algorithm will play by the rules set. Considering the constraints:

The number is -10,000 to 10,000
There is at least one element which satisfies the condition

Then we can just guard all numbers beyond 9 to be the lowest -10,001 so that they are not considered to be max. We don't need to care for numbers lower than -9 as we are sure there is at least 1 element (that is a single digit whether positive or negative, either way greater than those numbers lower than -9) that will satisfy the condition.

print(
    max(
        [-6, -91, 1011, -100, 84, -22, 0, 1, 473, 5, 4, 67, 7, 3, 56],
        key=lambda num: -10001 if num > 9 else num
    )
)
print(
    max(
        [-6, -91, 1011, -100, 84, -22, 10, -5, -8, 67],
        key=lambda num: -10001 if num > 9 else num
    )
)

Output:

7
-5

edited Sep 16, 2021 at 2:01

answered Sep 16, 2021 at 1:35

Niel Godfrey P. Ponciano

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2 Comments

Selcuk Over a year ago

Good thinking. You could also simply use max(i for i in a if i < 10).

Niel Godfrey P. Ponciano Over a year ago

I haven't thought of that! Yes indeed, that is the better (if not best) version that plays by the rules :)

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