Python Notes - Variables

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The print instruction

Open your Python system. Create a new window. Put the single line:

print "start"
in it, and save as the file

If you want to run the following examples, type them into your file, at the end of what is there already.

We can display calculations, as in:

print 3*8
print 2 + 5.4
which displays 24 and 10.8

Arithmetic operators
** power of
* multiply
/ divide
% modulo
+ add
- subtract

Multiply, modulo and divide are done before add and subtract. Power is done first of all. Examples:

print 2 + 3 ** 2
print 2+3*2
print 11.3 / 2
print 65 / 60
print 65 % 60
 11  8  5.65  1  5

Note that / on whole numbers (integers) gives an integer answer, truncated.
The % operator gives the remainder, imagining that the items were to be divided. Thus, 65 divided by 60 gives 1, with remainder 5. If we have 65 seconds, we can use / and % to break it up into minutes and seconds.

We can use round brackets to enclose calculations which are to be done first, as in:

print 3+2*4
print 3*2+4
print 3*(2+4)

which displays: 8 10 24
print 16/2*2
displays 16, but
print 16/(2*2)
displays 4. The ( ) cause the * to be done first.

We can also use a comma in print, as in:

print 3*2, 4+5

where 6 and 9 are displayed on the same line, separated by a space.

We can also print text in quotes - we can use single quotes, or double quotes. Examples:

print "hello"
print "The answer is ", 3*4, " inches"

which displays:
The answer is 12 inches

Note the comma usage: it causes printing to stay on the same line, as in:
print "hello",
print "world"

which displays
hello world

By the same logic, print by itself causes a new line to be started:

creates a 3-line gap.

In the above , note that when we have a series of instructions, they are performed from top to bottom, down the page, in sequence.


Previously, we used numbers and strings (i.e. text in quotes).

A variable can be thought of as a named box with a value (e.g. a number, or some text) stored in it. One way to put a value in a variable is by assignment, using =

We can put e.g

width = 10
height = 20
Pictorially, 2 storage boxes:

 _                                                                    _ 
|                                                                      |
      | 10    |                                                         
       | 20    |                                                        
|_                                                                    _|

We choose the names (which cannot include spaces, punctuation (such as comma, quote...), and must start with a letter.

The = operates right to left: the value on the right is calculated (evaluated) and then copied into (stored in) the box we mention on the left of the =.

More examples:

n = 23+3.3
m = n+1
x = 1
x = x+1
print n,m,x, x+1
which prints 26.3 27.3 2 3
  • A variable keeps its value until we alter it (e.g by another assignment)
  • the right-hand-side of = can include variables. Their current value is used
  • = does not mean 'equals' - it means 'becomes'. Thus x=x+1 takes the current value of x, adds 1 to it, then stores the result back in x. Its effect is to add 1 to x.
  • print can display the value of variables. It cannot change the value of variables. Thus, we printed x+1, giving 3. But the x still holds 2.
  • we can put extra spaces round = + - * etc if we wish, to improve readability. There are a range of opinions on whether this is worth doing.

Variables Example - area of rectangle

Calculate the area of a rectangle which is 2.3 by 6 metres. We could just put:
print "area is", 3.3 *6 , "sq metres"
but we will use variables, and enhance it later. Solution:
# area program
width = 6
height = 2.3
area = width * height
print "Area is" , area, "sq metres"
The output is:
Area is 13.8 sq metres
Note that area is not in quotes, so its value (not its name ) is displayed

# comments

Just as you might make jottings on your lecture notes, so you can annotate programs with #. Any text after # is ignored, but stays in the program. Do not over-use comments. For example, you might comment sections of programs, or difficult bits.
problem: write a similar program to calculate the volume of a room


Variables can also hold strings, as in:

myName = "Mike"
lastName = "Parr"
fullName = myName + " " + lastName      # holds   Mike Parr

We can use + to join strings end-to-end.

Strings can be quoted with "..." or '...', but the start and finish quotes must match. For example:

print "Mike's room"        # prints:      Mike's room

More operators: += *=

Python has some 'augmented' assignment operators:

x += 4      #means   x = x + 4
y *= 5      #means   y = y * 5


Sometimes, programs make widespread use of 'magic numbers. Perhaps a tax rate is 15.3%, and we have lots of lines such as:

It is clearer to use a name for the number. Such items are called constants - they do not change throughout the run of a program, but we may need to edit them once a year. A convention is to use capitals, as in:

TAX_RATE = 15.3    # put at top of code
#.. lots of code...

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