Single ended clear solution for Digit Stack by veky - python coding challenges

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Single ended solution in Clear category for Digit Stack by veky

def digit_stack(commands):
    stack, digitsum = [], 0
    for command in commands:
        if command.startswith("PUSH"):
            stack.append(int(command[-1]))
        elif stack:
            digitsum += stack[-1]
            if command == "POP":
                stack.pop()
    return digitsum

July 2, 2014

Comments:

cle99y on Dec. 10, 2014, 11:48 p.m.
why 'elif stack' and not simply 'else' ? what is he boolean test stack? cheers, PAUL

veky on Dec. 11, 2014, 5:10 a.m.
stack is true iff it contains at least one element - so stack[-1] and stack.pop() don't raise IndexError. I could have gone EAFP way, or even <pre class='brush: python'>with contextlib.suppress(IndexError): </pre> but here I thought it's easier to just check the stack's populatedness, since I way already one if deep. Clear? :-)

cle99y on Dec. 13, 2014, 1:25 p.m.
thanks for the reply, especially as I learned something (did not know that it is possible to do this simple check on non-empty list).

veky on Dec. 13, 2014, 2:54 p.m.
Not only list: any "container" supports such behaviour. Extremely convenient. And numbers too... for them, zero is false, others are true.

PriyaranjanReddyJC on July 19, 2015, 2:21 a.m.
thanks for this brilliant shortcut method. will be using it more frequent : )

veky on July 19, 2015, 3:13 a.m.
Brilliant? This is basic stuff. Even PEP8 speaks about it. ;-) <blockquote> For sequences, (strings, lists, tuples), use the fact that empty sequences are false. </blockquote> <pre class='brush: python'>Yes: if not seq: if seq: </pre>

Ch0bits on Feb. 26, 2015, 10:14 a.m.
Brialliant solution as always.

veky on Feb. 26, 2015, 11:05 a.m.
Thank you, but I'm not quite sure what's brilliant here.

nkapliev on March 2, 2015, 8:06 p.m.
You can push string value. And convert to int only when you need that - in pop or peek operation. Because not every commands sequences will erasing the stack. So you may doing a lot of unnecessary convert operations.

veky on March 2, 2015, 8:20 p.m.
Interesting, but 1) The task speaks about summing digits (so digits are not strings), and pushing digits to the stack (so it's the intention of the task to push ints to the stack). 2) And more interesting, your approach might also do a lot of unnecessary convert operations - and the probability is about the same. The only difference is that for me the "bad command sequence" is repeated PUSH, and for you it's (one PUSH and) repeated PEEK. Since there are less than 20 commands, this is of course only academic - but even if number of commands was unbounded, every digit is just that: a str of len 1, that int converts really quickly. If you had access to the "bare metal", a lookup table (or bit-twiddling, just clearing bits #4 and #5:) might be nice, but in Python, builtin int is probably better than any "cooked" solution. Of course, it's also possible to use the "Perl scalar" approach: store whatever you get, convert when you need, but store and cache all the results (of course, here you need to store only in case of PEEK). Here again the fact that we're much farther from bare metal than the builtins, and the fact that conversion is so trivial, say that we won't really profit from such an approach - though it might be educational, of course. :-)

nkapliev on March 3, 2015, 9:37 a.m.
Interesting, but 1. <blockquote> <ol> <li>The task speaks about summing digits (so digits are not strings), ...</li> </ol> </blockquote> In Output section: <blockquote> Output: The sum of the taken digits as an integer. </blockquote> The task do not speaks about 'summing digits' :p The task speaks about summing digits-as-integer. In my language the difference between digits and numbers is that you cant do math operations with digits. Digits is a symbol, that you can use for represent the numbers. So obviously that it is a little wierd to say smthg like 'summing digits'. You cant apply math sum operation to '3' and '5', because they are symbols, not numbers. So obviously that most suitable type for digit(symbol, that represent number) is a string, not integer. Another words digits are strings. But when you apply math sum you must use digits-as-integer. That about task says ;) Next <blockquote> <ol> <li>... and pushing digits to the stack (so it’s the intention of the task to push ints to the stack).</li> </ol> </blockquote> It task speaks: <blockquote> <ul> <li>"PUSH X" -- add X in the stack, where X is a digit.</li> </ul> </blockquote> There is no intention about push integer. Where did you saw that? Integers is not a digits, digits is not an integers. Digits is a symbols, that represent the numbers. And numbers can be integers. So there is no assumptions in task about how to store values in stack. So you can(must?) store the digit as is.(as symbol) May be in english there is no difference between digit and number? So then I wrong, sorry. <ol> <li>Yes, repeated PEEK is a bad case for my solution. Your right.</li> </ol> And thanks for " “Perl scalar” approach". I didn't know about that approach.

veky on March 3, 2015, 10:56 a.m.
About 2, Perl scalars were "at the first sight", 5 and "5" at the same time, depending on the operation (2 . 3 was 23, 2 + 3 was 5). Of course, under the hood, the thing I described happened. But on to more interesting things... About 1... hm. While I agree with you in principle (of course digits are usually symbols), the task text unfortunately does say: <pre class='brush: python'>sum all digits which were taken from the stack </pre> And of course it only makes sense if objects on the stack (call them dinosaurs if you want:) can be summed. Of course, you might say "well, if digit doesn't mean what it usually means, what makes you think that summing means what it usually means?". :-D But the result tested is obviously integer, it is 5 and not 23 if digits from stack are 2 and 3 (or "2" and "3"), and the task even specifies the starting value, which is 0 and not "". I really think that the intention of the task is clear, language abuse notwithstanding. :-]

texom512 on Aug. 20, 2015, 12:48 p.m.
One variable per line (PEP8)!

veky on Aug. 20, 2015, 1:23 p.m.
In which universe? Please state the exact wording you think is there.

texom512 on Aug. 20, 2015, 1:48 p.m.
Line 2

veky on Aug. 20, 2015, 2:06 p.m.
Ok, this becomes ridiculous. I'll try just one more time, and after that I'll just consider you an obvious troll. Can you quote the wording from PEP8 that you think applies here? "One variable per line" doesn't appear in the PEP8 text.

texom512 on Aug. 20, 2015, 2:49 p.m.
Sorry I confused with that: <a href="https://www.python.org/dev/peps/pep-0008/">https://www.python.org/dev/peps/pep-0008/</a>#imports "Imports should usually be on separate lines, e.g." Yes: import os import sys No: import sys, os

veky on Aug. 20, 2015, 5:03 p.m.
I'm glad you admitted your mistake. BTW I think you have great potential, but you won't be taken seriously if you continue being so bitter. Not every slight perceived mistake deserves maximal negative vote you can think of. Try to learn something from a solution, before you spit on it. Now can I get more serious reviews on other solutions of mine you've covered in minuses?

vananabu on Aug. 31, 2015, 9:22 a.m.
I like how you mix peek and pop in one condition!

veky on Aug. 31, 2015, 9:44 a.m.
They are the same from the perspective of result, just not from perspective of stack. :-)

vananabu on Aug. 31, 2015, 3:30 p.m.
Yes, I think about it. In both cases we need to check stack and take last element of stack.

veky on Aug. 31, 2015, 5:43 p.m.
Yeah... even if you write it with duplicated code, you'll immediately see that you can factor that line out.

TheOne on Sept. 28, 2015, 5:40 p.m.
I can't understand one thing. What if X in 'PUSH X' is more then 9? E.g. x = 10, you will append 0 but not 10. Won't you please explain?

veky on Sept. 29, 2015, 6:08 a.m.
From the task text: <ul> <li>"PUSH X" -- add X in the stack, where X is a digit.</li> </ul> It's reasonable to assume it is a decimal digit, so it cannot be more than 9. :-)

TheOne on Sept. 30, 2015, 12:53 p.m.
Thanks. Forgot to check original task.

buddyfeagles on Nov. 25, 2015, 6:49 p.m.
Very nice! Love the elif.

veky on Nov. 25, 2015, 6:55 p.m.
Thanks. :-)

Timofei13 on Jan. 18, 2016, 11:52 a.m.
Yes, nice!

veky on Jan. 18, 2016, 10:22 p.m.
Thanks.

Thenbacker on Nov. 13, 2016, 6:45 p.m.
Very nice solution, I know it has been written before in the comments, but I like how you dealt with peek and pop.

veky on Nov. 14, 2016, 8:05 a.m.
Factoring, factoring, factoring! :-)

Amy_Oliver on Aug. 8, 2019, 3:16 a.m.
Always learn lots from your code. Thanks very much.

tssrkt777 on Oct. 21, 2021, 12:36 p.m.
<blockquote> stack.append(int(command[-1])) </blockquote> What if there will be PUSH 15 ? :)

veky on Oct. 22, 2021, 12:47 p.m.
In what world is 15 a digit? :-P

tssrkt777 on Oct. 22, 2021, 2:40 p.m.
What if there will be number, not digit?)

veky on Oct. 22, 2021, 6:10 p.m.
Then it will be a different task. Same as e.g. if there were `ROT 3`. :-D

saitama-1 on Dec. 31, 2022, 7:40 a.m.
I like "elif stack:" part. I always check len(stack) in situations like this

veky on Jan. 4, 2023, 4:14 a.m.
See <a href="https://peps.python.org/pep-0008/#:~:text=use%20the%20fact%20that%20empty%20sequences%20are%20false">PEP8</a>. BTW, len is simply another needless jump in abstraction levels. You don't need to know how many sheep you have if you need to find out whether you have any. ;-)

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