Project Euler #19: Counting Sundays in the 20th century using Pandas

Question

Project Euler #19 asks:

How many Sundays fell on the first of the month during the twentieth century (1 Jan 1901 to 31 Dec 2000)?

I'm hoping I wasn't too off course from the spirit of the exercise by making use of my favorite libraries: pandas.

import pandas as pd

rng = pd.date_range(start='1/1/1901', end='12/31/2000', freq='D')
count = 0
for date in rng:
    if date.day == 1 and date.weekday() == 6:
        count += 1
count

It's a rather simple problem and a rather simple solution. I just made use of the date_range function that pandas has builtin that works well with datetime objects in python.

While I don't think my usual questions apply. Some specific questions:

• Is it pythonic to run the for loop as such, or would a list comprehension surrounded by a len be more pythonic, e.g. len([x for x in rng if date.day == 1 and date.weekday() == 6])? Or is something entirely else even more pythonic?

• Is there a way to avoid iterating over an object as large as rng is with it's 30,000+ items to avoid memory usage? If so what would be a preferred method (just pseudo-code or however you prefer to explain.)

• As my attention has been brought to how powerful itertools is when improving performance and substituting lists for generators, I'm wondering how I would improve upon the below code with itertools if there is any such a way.

Answer 1

It is good to use a library rather than re-inventing everything yourself. Just be sure to avoid explicit looping in Python:

sum(date.day == 1 and date.weekday() == 6 for date in rng)

The above sums the number of times that date.day == 1 and date.weekday() == 6 automatically, with no loops of counters, It should also be more efficient (sum is implemented in C)

Answer 2

Don't use rng to mean range. I know you were avoiding the builtin function, but it's not clear. It also could be mistaken to mean Random Number Generator (though the usage makes it clear that's not the case). Even if you could use range, I would advise against it. range doesn't really describe what the list contains, just the type of list it is. Instead, name things more for their purpose and contents than meta information about the contents. Looking at the for loop, there's an obvious name to use, dates.

Also ending with just count by itself, not even being printed, is odd. It may come out right in the IDE you use but it often will just pass by unnoticed. Call print and ideally add text around it

print("There are {} first day of the month Sundays in the 20th century.".format(count))

str.format is the accepted way to format variables into strings. It will call the str() method on anything passed to it to coerce them to strings. It's preferable to using "string" + str(var) + "string" because it's clearer and can be shorter even if it might not seem like it from this example, but imagine this:

"My string has two variables, " + str(var) " and the other one is " + str(var)

Versus

"My string has two variables, {} and the other one is {}".format(var, var)

But also str.format has a lot of useful syntax to help format strings better. I wont go into them here as you don't need them for this script, but take a look over here.

You asked about join, but that has a fundamentally different purpose. join takes a collection parameter, like a list, and will make a string by concatenating the list together. It doesn't perform any string conversion and doesn't allow you to insert arbitrary text between values. I definitely wouldn't use it in this case.