CODEWITHSUNDEEP
pythonalgorithmlogic
rocksNwaves
rocksNwaves

Reputation: 6164

Algorithm logic - counting sundays

I wrote a snippet of code that counts the number of times a Sunday falls on the first of the month from January 1901 to December 2000 (inclusive). I know the correct answer is 171, which is one more than I am getting. This means either there is an error in my algorithm's logic, or I am misunderstanding the problem statement. I was hoping to have someone point out my error either way. Here is the problem statement, and my code:

1 Jan 1900 was a Monday. Thirty days has September, April, June and November. All the rest have thirty-one, Saving February alone, Which has twenty-eight, rain or shine. And on leap years, twenty-nine.

A leap year occurs on any year evenly divisible by 4, but not on a century unless it is divisible by 400.

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

monthdict = {
    1: 31,
    2: 28,
    3: 31, 
    4: 30,
    5: 31,
    6: 30,
    7: 31,
    8: 31,
    9: 30,
    10: 31,
    11: 30,
    12: 31
}

ly_monthdict = {
    1: 31,
    2: 29,
    3: 31, 
    4: 30,
    5: 31,
    6: 30,
    7: 31,
    8: 31,
    9: 30,
    10: 31,
    11: 30,
    12: 31
}

leap_year = False
sundays = 0
first_sunday = 6

for year in range(1901, 2001):
    if year % 4 == 0 and year % 100 != 0:
        leap_year = True
    elif year % 100 == 0 and year % 400 == 0:
        leap_year = True
    else:
        leap_year = False

    if leap_year:
        for month in range(1, 13):
            if first_sunday == 1:
                sundays += 1

            curr_sunday = first_sunday
            while curr_sunday < ly_monthdict[month]:
                curr_sunday += 7

            first_sunday = curr_sunday - ly_monthdict[month]


    else:
        for month in range(1, 13):
            curr_sunday = first_sunday
            while curr_sunday < monthdict[month]:
                curr_sunday += 7

            first_sunday = curr_sunday - monthdict[month]
            if first_sunday == 1:
                sundays += 1

print(sundays, "Sundays occured on the first of the month")

Upvotes: 1

Views: 432

Answers (1)

Sunny Patel
Sunny Patel

Reputation: 8077

Looks silly, but both of your inner for..in blocks aren't the same.

If you move the if first_sunday == 1 check to the start of the loop for non-leap years, you'll get 171 as your answer.

if leap_year:
    for month in range(1, 13):
        if first_sunday == 1:
            sundays += 1
        curr_sunday = first_sunday
        while curr_sunday < ly_monthdict[month]:
            curr_sunday += 7
        first_sunday = curr_sunday - ly_monthdict[month]
else:
    for month in range(1, 13):
        if first_sunday == 1:     #Moved this block up
            sundays += 1
        curr_sunday = first_sunday
        while curr_sunday < monthdict[month]:
            curr_sunday += 7
        first_sunday = curr_sunday - monthdict[month]

Also Kudos to ProjectEuler, I haven't done those problems in over a decade. I suggest you combine both of these loops and switch the reference to which dict you use instead of duplicating the code.

Upvotes: 1

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