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5.1: Floor division and modulus

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  • The floor division operator, //, divides two numbers and rounds down to an integer. For example, suppose the run time of a movie is 105 minutes. You might want to know how long that is in hours. Conventional division returns a floating-point number:

    >>> minutes = 105
    >>> minutes / 60

    But we don’t normally write hours with decimal points. Floor division returns the integer number of hours, rounding down:

    >>> minutes = 105
    >>> hours = minutes // 60
    >>> hours

    To get the remainder, you could subtract off one hour in minutes:

    >>> remainder = minutes - hours * 60
    >>> remainder

    An alternative is to use the modulus operator, %, which divides two numbers and returns the remainder.

    >>> remainder = minutes % 60
    >>> remainder

    The modulus operator is more useful than it seems. For example, you can check whether one number is divisible by another—if x % y is zero, then x is divisible by y.

    Also, you can extract the right-most digit or digits from a number. For example, x % 10 yields the right-most digit of x (in base 10). Similarly x % 100 yields the last two digits.

    If you are using Python 2, division works differently. The division operator, /, performs floor division if both operands are integers, and floating-point division if either operand is a float.

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