How to explain the int() function to a beginner

Question

I am tutoring a neighbour's child and we were exploring the int() function before using it with input() - which returns a string. We tried the following:

int(5) int(5.5) int('5') int('5.5') 

The first three returned 5 as expected; the last one threw the error

ValueError: invalid literal for int() with base 10: '5.5'

Given the behaviour of the first three lines how do I explain the error to a 14-year old (background = speaks 4 languages but maths is not so hot)?

UPDATE C# exhibits the same behaviour: Convert.ToInt32("5.5"); throws the error

Input string was not in a correct format.

Answer 1

In a nutshell: because that's what the spec says. That's kind of a useful mindset to get into anyway. ;-)

Now, why does the spec say so? There are only a finite number of types a function can accept as valid input. The int function tries to cover two different kinds of use cases:

1. convert a string representation of an integer into an actual int
2. cast a float value to an int, truncating it*

The third use case, "convert the string representation of a floating point number to an int" is not covered by the spec, because the language designers decided not to cover it. Which seems like a reasonable decision to make, since they needed to draw the line somewhere on what types the function would and wouldn't accept. The string representation of a floating point number should be parsed by float, not int.

* Actually: any object that has an __int__ method, but lets keep it simple.

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As a counter example, in PHP you can try to cast any string to an int, and it will try to give you the best match:

php > echo (int)'3.14'; 3 php > echo (float)'3.14'; 3.14 php > echo (int)'3 little pigs'; 3 php > echo (int)'there are 3 little pigs'; 0 

Which, quite honestly, is rather insane behaviour, especially that last one. Python has a strict(er) type system; if you're trying to parse a string as an int, it must be a perfectly valid representation of an integer number, not merely something that somewhere contains something that can be interpreted as a number.