From this lesson onwards, examples and code input boxes have buttons labelled Open in console and Visualize. Use them to help debug and explore the code.
In the Hello, World! program we saw that Python was able to repeat a sentence back to us. We have also seen several examples of arithmetic with numbers. Numbers and sentences are fundamentally different objects, and it causes a Python error when you try to mix them in the wrong way:
As you can see, we get an error saying that the two arguments to max are of different types. The error is a good introduction to the rest of the lesson:
"Hello, World!"is a string value, which is shown asstrin Python. A string is any sequence of numbers, letters, and punctuation; we will learn more about them in lesson 7A.35is an integer value, which is shown asintin Python. An integer just means a whole number; for example, 42, -12, and 0 are integers.
Using an object of a bad type is a very common cause of errors in programs. It is like trying to drink a sandwich: you can't do it because you can only drink things of liquid type, and a sandwich is of solid type.
You can determine the type of an object by calling the type function on it.
(The meaning of class is similar to type.) The above example demonstrates that numbers are further divided into two different types, int which we mentioned above, and float, which is used for storing decimal numbers. You should think of floats as inexact or approximate values (we will explain more in lesson 7B). You can usually mix float values with int values, and the result will be another float.
In fact, what Python really does when you mix a float with an int is that it converts the int to a float, and then works with the two floats.
1.2 to 1.5 in the above program, what is first line of output?z was printed as 3.0.
- Computing
x * yis mixing anintand afloat, which Python treats as twofloats, giving backzas afloat. - The mathematical value of
zis 1.5 times 2, which is 3, but stored in inexact decimal form, of typefloat. When Python prints anyfloat, even if its value is a whole number, it is printed ending with.0
y never changed.
It is often necessary to change data from one type to another type. Just as you can convert a sandwich from solid to liquid form by using a blender, you can change data from one type to another type using a typecast function. You write the name of the desired new type in the same way as a function call, for example
x = float("3.4")
print(x-1)changes the string "3.4" to the float 3.4, and then prints out 2.4. Without the typecast, the program would crash, since it cannot subtract a number from a string.
 | Sometimes, Python does let you combine strings and numbers using arithmetic operators. The statement hotshots. Python's rule is that multiplying a string s by an integer n means to put n copies of the string one after another. We'll see later that "addition of two strings" is also well-defined in Python. |
Various typecasts behave differently:
- converting a
floatto anintloses the information after the decimal point, e.g.int(1.234)gives1, andint(-34.7)gives-34. - converting a
strto anintcauses an error if the string is not formatted exactly like an integer, e.g.int("1.234")causes an error. - converting a
strto afloatcauses an error if the string is not a number, e.g.float("sandwich")causes an error.
A common use of typecasting that we will see soon is to convert user input, which is always a string, to numerical form. Here is a quick illustration.
Here is one more exercise to finish the lesson.
| Because there are now lots of editor commands, some have been moved into the menu labelled More actions... |
Once you are done, head over to the next lesson.
