Longest sequence of equal numbers in python

Question

I tried to generate the longest sequence of equal numbers in python, but it doesn't work

def lista_egale(lst1 = input("numbers go here ")):
    l = 0
    lst1 = []
    maxi = -9999
    prev_one = None
    lmax = -9999
    for current in lst1:
        if prev_one == current:
            l += 1
        else:
            l = 1
        if l > lmax:
            lmax = l
            maxi = current
        prev_one = current
    print("longest sequence is ", lmax, " and ", maxi)

lista_egale()

Input:

1 2 2 2 2 3 4 5 6 2 2 2

Expected Output:

longest sequence is  4  and  2

I tried seeing what it prints if I input ``` 1 2 2 2 2 3 4 5 6 2 2 2 ``` and it should've printed 4 but it doesn't print anything — manowar447233, Oct 12 '21 at 15:28
it doesn't print anything? it should print at least `longest sequence is 1` [example](https://replit.com/@depperm/FaithfulCircularDegrees#main.py) — depperm, Oct 12 '21 at 15:32
it doesn't print anything for me, and even when I tried to use the ```assert``` function it threw an error my way — manowar447233, Oct 12 '21 at 15:34
Are you actually running the function? i.e. calling `lista_egale()` after its declaration? — Sunny Patel, Oct 12 '21 at 15:38
yes, i called the function just below the first print, I will edit the question — manowar447233, Oct 12 '21 at 15:38
What happens when you pass in a list input? i.e. `lista_egale("1 2 2 2 2 3 4 5 6 2 2 2".split(" "))` — Sunny Patel, Oct 12 '21 at 15:41
it outputs this ```longest sequence is -9999 and -9999``` which I assume means that lmax and maxi haven't changed at all, but I don't understand why — manowar447233, Oct 12 '21 at 15:43
Also, argument defaults are executed when the function is defined, not when you call it. It makes little sense to have an interactive default like that. — Barmar, Oct 12 '21 at 15:44
ok, now it works, but I have to ask: what does ```lst1 = list(map(int, lst1))``` do? I'm new to python and a brainlet — manowar447233, Oct 12 '21 at 15:46
@manowar447233 You can read about *map* here: https://docs.python.org/3.9/library/functions.html — , Oct 12 '21 at 15:48

Kenny Ostrom · Answer 1 · 2021-10-12T21:12:52.103

I was going to write up the same concern about your default argument, but that would at least work correctly the first time it is called. This function does not. Everyone jumped on that common problem, and failed to notice the next line. Let's look another look at this abridged version of your code:

irrelevant = input("numbers go here ")

def lista_egale(lst1 = irrelevant):
    # while it is true that your default argument is bad,
    # that doesn't matter because of this next line:
    lst1 = []
    for current in lst1:
        # unreachable code
        pass

To clarify, since your reply indicates this is not clear enough, it doesn't matter what value was passed in to lst1 if you immediately overwrite it with an empty list.

(for others reading this:) Separating out what I labeled "irrelevant" is not quite identical, but I'm trying to point out that the input was overwritten.

I don't think this function should take user input or have a default argument at all. Let it be a function with one job, and just pass it the data to work on. User input can be collected elsewhere.

also note https://stackoverflow.com/questions/1132941/least-astonishment-and-the-mutable-default-argument/11416002 — Kenny Ostrom, Oct 12 '21 at 16:19
I tried to do what you said (i.e. eliminate the argument) and now I messed up everything I am so stupid :( — manowar447233, Oct 12 '21 at 16:48
Require the argument. Eliminate the default. But the real problem was setting lst1 = [], so you lost the input. — Kenny Ostrom, Oct 12 '21 at 19:26

Sunny Patel · Answer 2 · 2021-10-12T15:53:55.343

Based on Barmar's note, and the principle of using only unmutable default values, your code should look something more like this:

def lista_egale(inp1 = None):
    if not inp1:
        inp1 = input("numbers go here ")
    # optionally do some error checking for nonnumerical characters here
    lst1 = [int(i) for i in inp1.split(" ")]

    # rest of your code here

lista_egale()

Basically, input returns a string value, and you need to convert it into a list of integers first before you start working on it.

You can swap out the list comprehension for map(int, inp1.split(" ")) as it will do the same (but you can't iterate through a map more than once unless you wrap it in a list() function first).

Secondly, avoid setting mutable default arguments as (in short) can lead to weird results when rerunning the same function multiple times.

Longest sequence of equal numbers in python

2 Answers2