Today the coding calendar Advent of Code started.

You should try it too, Advent of Code.

I tried to solve the first challenge. It was pretty straight forward. Except, I was stumped by a simple problem.
Here is my first attempt,

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def part1(lines):
    prev = lines[0]
    more = 0
    for i in range(len(lines) - 1):
        if lines[i+1] > prev:
            more += 1
        prev = lines[i+1]
    print(more)
if __name__ =='__main__':
    with open('./input/day1.txt', 'r') as f:
        lines = [a.strip() for a in f.readlines()]
    part1(lines)

The thing is it gives correct result for the test data. So, I assumed it would work for the input data. But, the program always gave me 1 less than the actual answer. I couldn’t figure out why.
Notice, I use string comparison in the if condition on line 5. I knew using string for comparison of numbers was a bad idea but, the following IDLE experiments convinced me otherwise.

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>>> "123" > "456"
False
>>> "456" > "123"
True

If you didn’t know (I certainly didn’t) you can compare strings with the > and < operators in python. This led me to believe that comparing numbers would work without explicitly using integers.

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>>> "1024">"946"
False

Here is the problem. String comparison compares the first character of the strings and the tries to distinguish which is larger. Here, even though 1024 is greater than 946, 9 is greater than 1! The string comparison operator doesn’t know better. I could’ve casted them to integers or zfilled them to have same number of digits to solve the problem.

And I did, the program now works!

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if __name__ =='__main__':
    with open('./input/day1.txt', 'r') as f:
        lines = [int(a.strip()) for a in f.readlines()]
    part1(lines)

So, the takeaway is, use numbers(the datatype) when dealing with numbers.