Finally, I can get to the topic of this chapter, which is “expressions.” An expression is a combination of one or more values (such as strings, integers, or floats) using operators, which result in a new value. In other words, you can think of expressions as calculations.
Basic calculations combine two values with one operator in between them. Some straightforward operators are:
| operator | description |
|---|---|
+ |
addition |
- |
subtraction |
* |
multiplication |
/ |
division |
// |
integer division |
** |
power |
% |
modulo |
Here are some examples:
print( 15+4 )
print( 15-4 )
print( 15*4 )
print( 15/4 )
print( 15//4 )
print( 15**4 )
print( 15%4 )
I assume you know what each of these operators entails, except perhaps the integer division and modulo operators.
The integer division (also called “floor division”) is simply a division that rounds down to a whole number. If you involve floats in the calculation, the result will still be a float, but rounded down. If you only involve integers in the calculation, the result will be an integer.
The modulo operator (%) takes the remainder of a division. For example: If I divide 14 by 5, the result is 2.8, right? This means I can subtract 5 twice from 14, and still have a positive result, but if I subtract it a third time, the result will become negative. So, after subtracting 5 twice from 14 I have a remainder that is less than 5. This remainder is what the modulo operator produces.
In very simplistic terms: if I have 14 cookies which I have to divide over 5 children, each child gets 2 cookies. And I still have 4 cookies left, because there are more children than I have cookies at that point. Thus, dividing 14 by 5 as an integer division is 2 (cookies per child), while 14 modulo 5 is the remainder 4 (cookies I have left in my hand).
On a side note, I wish to point out that the code shown above consists
of multiple lines. Each line is said to be a “statement,” and it
consists of one command that Python executes (in the code above, a
print() function on every line). Most programming languages make it
mandatory to end each statement with a special character, usually a
semi-colon (\(;\)). Python does not require a semi-colon after each
statement, but each statement must (in general) be on its own line. In
principle, you are allowed to place multiple Python statements on one
line, but then you should put semi-colons between the statements.
However, it is Python practice and convention not to do that, as it
makes code ugly, hard to read, and difficult to maintain. So, please
stick to the convention and give each statement its own line.
You are allowed to combine operators into bigger calculations, just as you can do on the more advanced calculators. To help you out, you are also allowed to used parentheses in your calculations, and you can even nest these parentheses. Python will process the operators in the order prescribed by mathematicians, often referred to as PEMDAS (Parentheses, Exponents, Multiplication and Division, Addition and Subtraction).
Check out the calculation below, and try to predict what it will result in before you copy it to the Python shell and run the code.
print( 5*2-3+4/2 )
There are a couple of things to note about this calculation.
First, the end result is a float (even though it has no decimals, or, if you will, only zero as a decimal). The reason is that a division is part of the calculation, and for Python that means that it should turn this into a floating-point calculation.
Second, just as explained above, spaces are ignored by Python, so the code above is the same as:
print( 5 * 2 - 3 + 4 / 2 )
It is even the same as:
print( 5*2 - 3+4 / 2 )
I have been in long discussions with people who keep arguing that the code above should result in \(6.5\) or \(1.5\), because clearly you have to calculate the \(5*2\) and the \(3+4\) before you do the subtraction and division. That is hogwash. It does not matter how close you place operands together, spaces are ignored. If you really want to calculate the \(3+4\) first, you have to put it between parentheses. You can then still use spaces to improve readability, but they mean nothing to Python.
print( (5*2) - (3+4)/2 )
print( ((5*2)-(3+4)) / 2 )
Now it is time to write your first program. Write a program that displays the number of seconds in a week. You should, of course, not grab your calculator or smartphone to do the calculation and then just print the resulting number, but you should do the calculation in Python code. Since this program needs only one line of code, you could just write it in the Python shell, though you are encouraged to create a program file and use that.
Some of the operators given above can also be used for strings, though not all of them.
In particular, you can use the addition operator (\(+\)) to concatenate two strings, and you can use the multiplication operator (\(*\)) with a number and a string to create a string that contains a repetition of the original string. Check it out:
print( "hello"+"world" )
print( 3*"hello" )
print( "goodbye"*3 )
You cannot add a number to a string, or multiply two strings. Such use of the operators is undefined, and will give error messages. None of the other operators listed for numbers will work on strings either.
Sometimes you need to change the data type of a value into a different data type. You can do that using type casting functions.
I will discuss functions in a lot more detail in a later chapter, but
for now you just need to know that a function has a name, and may have
parameters (values) between parentheses after the name. It will do
something with the parameters, and then may give back a result. For
instance, the print() function displays the parameter values that are
given to it between the parentheses, and gives nothing in return.
The type casting functions take the parameter value between the parentheses and give back a value that is the (almost) the same as the parameter value, but of a different data type. The three main type casting functions are the following:
int() will return the value between the parentheses as an integer
(rounding down if necessary)
float() will return the value between the parentheses as a float
(adding .0 if necessary)
str() will return the value between the parentheses as a string
See the difference between the following two lines of code:
print( 15/4 )
print( int( 15/4 ) )
Or the following two lines of code:
print( 15+4 )
print( float( 15+4 ) )
I stated that you cannot use the addition operator to concatenate a number to a string. However, if you need to do something like that, you can work around the issue by using string type casting:
print( "I own " + str( 15 ) + " apples." )