2. Reeborg’s diary

Have Reeborg execute the following program:

print()

You should see a pop-up window appear with the title Reeborg writes:. This window is Reeborg’s diary. Feel free to move it around on your computer screen.

Note

Important: the example on the right does not execute the function move; however, if we do:

print( move() )

so that move is executed, the Python keyword None will be printed. Why that is the case will be explained later.

Now, run the following program:

print(move)

At the time I revised this tutorial, the result in Reeborg’s diary was:

<function move>

(previously, instead of move, _move_ was the name that appeared for reason that you might guess after having read this page.) What if we were to define another variable (name) for the same object (function)?

step = move
print(step)
print(move)

The result is:

<function move>
<function move>

This shows clearly that = simply gives a name to an object, the object in this case being what Python calls <function move> when asked to print it. On the other hand, if we define a completely new function, like:

#  step = move
def step():
    move()

print(step)

the result will be:

<function step>

which is a different object from <function move> even though, if called, step() would have the exact same result as move().

2.1. A function can have an argument

In computer programs, the word argument refers to a variable that determines the result of a function. For example, as we have seen:

print(move)

the variable move is the argument of the function print(). The argument of a function appears between the parentheses which indicate that the function is called.

2.2. Writing text

Writing the name of a function like we have done above by using the print() function is something that is done extremely rarely when writing programs. What is done much, much more often is to write text.

In programming terms, a character is any letter, number or symbol that can be printed and a string of characters, or simply string, is any sequence of character that can be printed. For example, try the following:

print("Hello world!")
print('Hello again.')

Note that the quotes that surround the string have to be the same, either double quotes like ”, or single quotes like ‘. To have a string that contains some quote characters, we can either surround it by quotes of a different type or use the escape character \:

print("Let's go.")
print('Let\'s go.')

We can combine strings using the + symbol:

print("Goodbye! " + "And thanks for all the fish.")

We can also start on a new line using the following escape sequence: \n:

print("Thank you. \nTry again")

Try it!

Make sure you try to run the above code samples or some similar.

2.3. Reeborg knows mathematics

Try this!

Try running the following program and look at the output in Reeborg’s diary.

print( 2 + 3 )  # adding numbers
print( 2 * 3 )  # multiplying numbers
print( 3 - 2 )  # subtracting numbers
print( 6 / 2 )  # dividing numbers
print( 1 + 3 * 2 ) # multiplication is done before addition

# using parentheses to change normal order of operations
print( (1 + 3) * 2 )

print( 2 ** 5 ) # power ... 2**5 = 2 * 2 * 2 * 2* 2

Note

Note that spaces around the operators like + and * are ignored by Python; however, they can make it easier for humans to read the code.

In the above examples, the result was always an integer (or whole number) except when dividing numbers which gave a floating point number: 6 / 2 gives 3.0.

If we want the result of dividing two numbers to be an integer, we use the double division sign instead:

print( 6 // 2 )  # integer division

2.4. Using variables

We have already seen the idea of using different names (variables) as synonyms. Let’s use this idea again as explore mathematical operations some more:

length = 4
width = 6
area = length * width  # area of a rectangle
print(area)            # will output 24

Try it!

Make up your own examples and run them.

Important

The character “2” is not the same as the number 2. Try out the following:

print("2" + 2)

2.5. Multiple arguments

Some functions, like print(), can take many arguments: the various arguments are separated by commas. To illustrate this, try out the following program:

length = 4
width = 6
area = length * width
print("The area of a rectangle of length", length,
      "and width", width, "is", area)