Values, types, variables and print

A program works with pieces of data: a number of plates, a price, a name, an answer. Python calls each piece of data a value. This lesson shows you the four kinds of value you will use most, how to give a value a name, and how to show it on the screen.

Four core types

Every value in Python has a typetypeThe classification of a value, such as an integer, string, or boolean, that determines what operations it supports.In the glossary. The type decides what you can do with the value. You can multiply two numbers, but you cannot multiply two pieces of text.

An integer, or int, is a whole number such as 0, 42 or -7. Python integers can be as large as you need. A number with hundreds of digits works the same way as a small one.

A float, or float, is a number with a decimal point, such as 3.5 or -0.25. Floats are stored with limited precision, so they can be off by a tiny amount. Contest problems usually stick to integers for that reason.

A string, or str, is text inside quotes, such as "sushi" or 'red'. Single and double quotes mean the same thing. Pick one style and keep using it.

A boolean, or bool, is either True or False. Those are the only two boolean values. Both start with a capital letter.

If you are not sure what type a value has, ask Python. The type() function tells you. The program below uses variables, which the next section explains. For now, read count = 42 as "call this value count".

1count = 422price = 3.53label = "42"4ready = True5print(type(count))6print(type(price))7print(type(label))8print(type(ready))

<class 'int'>
<class 'float'>
<class 'str'>
<class 'bool'>

Look at label. Its value "42" is a string, because it is inside quotes. It looks like a number, but Python treats it as text. The next program shows why that matters.

print(7 + 5)print("7" + "5")

12
75

Adding two integers gives 12. Adding two strings joins them, so you get 75. The quotes changed the type, and the type changed the result.

Variables and assignment

A variablevariableA name that refers to a value stored while a program runs.In the glossary is a name that refers to a value. You create one with assignmentassignmentStoring a value under a variable name using the equals sign.In the glossary, which uses the = sign.

plates = 3player = "Jordan"is_open = True

Read plates = 3 as "the name plates now refers to 3". In maths, = says two things are equal. In Python it does something else. It takes the value on the right and attaches the name on the left to it.

Think of the name as a tag. Python creates the value first, then ties the tag to it. Whenever you write plates later, Python follows the tag to the value 3.

A name can use letters, digits and the underscore character. It cannot start with a digit, and it cannot contain spaces. Python also reserves some words for itself, such as if, for and def, so you cannot use those as names.

Names are case-sensitive. count, Count and COUNT are three different names. This causes a common mistake: you create one spelling and then use another.

1count = 52print(Count)

Traceback (most recent call last):
  File "name_error.py", line 2, in <module>
    print(Count)
NameError: name 'Count' is not defined. Did you mean: count?

The program created count but asked for Count. Python has no value tagged Count, so it stops with a NameError. When you see this error, check the spelling and the capital letters first.

Choose names that say what the value means. total_cost tells you more than t or x2. Write names in lowercase, with an underscore between words.

Reassignment

You can assign a new value to a name that already exists. This is called reassignmentreassignmentBinding an existing variable name to a new value.In the glossary. Python moves the tag from the old value to the new one.

1total = 102print(total)3
4total = total + 55print(total)6
7tag = "box"8print(total, tag)

Line 4 is worth a slow read: total = total + 5. Python always works out the right side first. It follows total to 10, adds 5 and gets 15. Only then does it move the tag total to 15.

Step through the program below. Watch the name total move from 10 to 15 while the printed lines build up underneath the code.

(nothing printed yet)

Printing output

The print() function writes a line of text to the screen. In a contest, that text is your answer.

You can give print() several values separated by commas. It prints them in order with one space between each pair. When it finishes, it moves to a new line, so the next print() starts on a line of its own. A print() with nothing inside prints an empty line. A string prints without its quotes: print("Done") shows Done. The quotes only tell Python where the text starts and ends.

1plates = 32price = 53print("Plates:", plates)4print("Total:", plates * price)5print()6print("Done")

Plates: 3
Total: 15

Done

The judge compares your output with the expected output exactly. An extra word, a missing space or a label like Total: counts as a wrong answer, even when the number is right. Print only what the problem asks for.

Reading input into variables

In a contest, your program gets its data from standard input. The input() function reads one line and gives it to you as a string. That is true even when the line holds a number.

To use the line as a number, convert it with int(). The step-through below reads two lines. The first becomes an integer and the second stays a string.

The module on reading input covers this in more detail. For now, you have enough to read a few numbers, store them in variables and print a result. The problems below need exactly that.