Question

I’m a programming newbie having difficulty with Python multiplication. I have code like this:

def getPriceDiscount():
    price = int(input())
    if price > 3000:
        priceDiscount = price * 0.6
        return priceDiscount
    else:
        return price

But when I execute it and type an input which is a decimal number like 87.94, I get the following error:

ValueError: invalid literal for int() with base 10: '87.94'

Isn’t the int() method able to convert the string ‘87.94’ into a number allowing me to multiply it by 0.6? What should I do to perform that conversion?

I’m using Python 3.2.2.

Answer 1

An int (short for “integer”) is a whole number. A float (short for “floating-point number”) is a number with a decimal point.

int() returns an int created from its input argument. You can use it to convert a string like "15" into the int 15, or a float like 12.059 into the int 12.

float() returns a float created from its input argument. You can use it to convert a string like "10.5" into the float 10.5, or even an int like 12 into the float 12.0.

If you want to force price to be an integer, but you want to accept a floating point number as typed input, you need to make the input a float first, then convert it with int():

price = int(float(input())

Note that if you multiply an int by a float such as your discount factor, the result will be a float.

Also note that my example above doesn’t round the number — it just truncates the stuff after the decimal point. For example, if the input is "0.6" then price will end up being 0. Not what you want? Then you’ll need to use the round() function on the float first.

price = int(round(float(input()))

If you intended to use floating point calculations (which makes sense if we’re talking about a commodity price), then don’t perform the int conversion. Just use float. You may still want to do some rounding. If you want to round to 2 decimal places, you can call round with the second argument as 2:

price = round(float(input()),2)

Finally, you might want to look into Python’s decimal module, since there are limitations when using floating point numbers. See here for more information: http://docs.python.org/tutorial/floatingpoint.html