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Engineering LibreTexts

5.1: Updating Variables

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  • A common pattern in assignment statements is an assignment statement that updates a variable, where the new value of the variable depends on the old.

    x = x + 1

    This means "get the current value of x, add 1, and then update x with the new value."

    If you try to update a variable that doesn't exist, you get an error, because Python evaluates the right side before it assigns a value to x:

    >>> x = x + 1
    NameError: name 'x' is not defined

    Before you can update a variable, you have to initialize it, usually with a simple assignment:

    >>> x = 0
    >>> x = x + 1

    Updating a variable by adding 1 is called an increment; subtracting 1 is called a decrement.

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