Loops,
Conditionals
Just
imagine you needed a program to do something 20 times. What would you do? You
could copy and paste the code 20 times, and have a virtually unreadable
program. Or, you could tell the computer to repeat a bit of code between point
A and point B, until the time comes that you need it to stop. Such a thing is
called a loop.
The while loop:
The following are examples of a type of
loop, called the while loop:
Code
Example 1 – The while loop
a = 0
while a < 10:
a = a
+ 1
print a
How does this program work? Let's go through it in
English (this is called pseudocode):
Code
Example 2 – Plain-language while loop
'a'
now equals 0
As
long as 'a' is less than 10, do the following:
Make
'a' one larger than what it already is.
print
on-screen what 'a' is now worth.
What does this do? Let's go through what the
computer would be 'thinking' when it is in the while loop:
Code
Example 3 – while loop process
#JUST
GLANCE OVER THIS QUICKLY
#(It
looks fancy, but is really simple.)
Is
'a' less than 10? YES (it's 0)
Make
'a' one larger (now 1)
print
on-screen what 'a' is (1)
Is
'a' less than 10? YES (it's 1)
Make
'a' one larger (now 2)
print
on-screen what 'a' is (2)
Is
'a' less than 10? YES (it's 2)
Make
'a' one larger (now 3)
print
on-screen what 'a' is (3)
Is
'a' less than 10? YES (it's 3)
Make
'a' one larger (now 4)
print
on-screen what 'a' is (4)
Is
'a' less than 10? YES (it's 4)
Make
'a' one larger (now 5)
print
on-screen what 'a' is (5)
Is
'a' less than 10? YES (it's 5)
Make
'a' one larger (now 6)
print
on-screen what 'a' is (6)
Is
'a' less than 10? YES (it's 6)
Make
'a' one larger (now 7)
print
on-screen what 'a' is (7)
Is
'a' less than 10? YES (are you still here?)
Make
'a' one larger (now 8)
print
on-screen what 'a' is (8)
Is
'a' less than 10? YES (it's 8)
Make
'a' one larger (now 9)
print
on-screen what 'a' is (9)
Is
'a' less than 10? YES (it's 9)
Make
'a' one larger (now 10)
print
on-screen what 'a' is (10)
Is
'a' less than 10? NO (it's 10, therefore isn't less than 10)
Don't
do the loop
There's
no code left to do, so the program ends.
So in short, try to think of it that way when you
write while loops. This is how you write them, by the way:
Code
Example 4 – while loop form
while
{condition that the loop continues}:
{what
to do in the loop}
{have
it indented, usually four spaces}
{the
code here is not looped}
{because
it isn't indented}
An example:
Code
Example 5 – while loop example
#EXAMPLE
#Type
this in and see what it does.
x = 10
while x != 0:
print x
x = x
- 1
print "Wow,
we've counted x down, and now it equals", x
print "And
now the loop has ended."
Remember,
to make a program, you open IDLE, click 'File > New Window', type your
program in the new window, then press F5 to run.
Boolean
Expressions (Boolean... what?!?)]
What
do you type in the area marked {conditions that the loop continues}? The answer
is a boolean expression.
What? A forgotten
concept for the non-math people here. Never mind, boolean expression just means
a question that can be answered with a TRUE or FALSE response. For example, if
you wanted to say your age is the same as the person next to you, you would
type:
My age == the age of the person next to me
And the statement would be TRUE. If you were
younger than the person opposite, you'd say:
My age < the age of the person opposite me
And the statement would be TRUE. If, however, you
were to say the following, and the person opposite of you was younger than you:
My age < the age of the person opposite me
The statement
would be FALSE - the truth is that it is the other way around. This is how a
loop thinks - if the expression is true, keep looping. If it is false, don't
loop. With this in mind, lets have a look at the operators (symbols that
represent an action) that are involved in boolean expressions:
Table 1 - Boolean operators
|
Expression
|
Function
|
<
|
less than
|
<=
|
less that
or equal to
|
>
|
greater
than
|
>=
|
greater
than or equal to
|
!=
|
not equal
to
|
<>
|
not equal
to (alternate, != preferred)
|
==
|
equal to
|
Dont get '=' and
'==' mixed up - the '=' operator makes what is on the left equal to what is on
the right. the '==' operator says whether the thing on the left is the same as
what is on the right, and returns true or false.
Conditional
Statements:
OK!
We've (hopefully) covered 'while' loops. Now let's look at something a little
different - conditionals.
Conditionals
are where a section of code is only run if certain conditions are met. This is
similar to the 'while' loop you just wrote, which only runs when x doesn't
equal 0. However, Conditionals are only run once. The most common conditional
in any program language, is the 'if' statement. Here is how it works:
Code
Example 6 - if statement and example
'''
if
{conditions to be met}:
{do
this}
{and
this}
{and
this}
{but
this happens regardless}
{because
it isn't indented}
'''
#EXAMPLE
1
y = 1
if y == 1:
print "y
still equals 1, I was just checking"
#EXAMPLE
2
print "We
will show the even numbers up to 20"
n = 1
while n <= 20:
n = n
+ 1
if n
% 2 == 0:
print n
print "there,
done."
Example 2 there
looks tricky. But all we have done is run an 'if' statement every time the
'while' loop runs. Remember that the % just means the remainder from a
division - just checking that there is nothing left over if the number is
divided by two - showing it is even. If it is even, it prints what 'n' is.
'else'
and 'elif' - When it Ain't True
There are many ways you can use
the 'if' statement, to deal with situations where your boolean expression ends
up FALSE. They are 'else' and 'elif'.
'else' simply tells the computer what to do if the
conditions of 'if' aren't met. For example, read the following:
Code
Example 7 - the else statement
a = 1
if a > 5:
print "This
shouldn't happen."
else:
print "This
should happen."
'a' is not greater than five, therefore what is
under 'else' is done.
'elif' is just a shortened way of saying 'else if'.
When the 'if' statement fails to be true, 'elif' will do what is under it IF
the conditions are met. For example:
Code Example 8 - The elif statement
z = 4
if z > 70:
print "Something
is very wrong"
elif z < 7:
print "This
is normal"
The 'if' statement, along with 'else' and 'elif'
follow this form:
Code Example 9 - the complete if syntax
if
{conditions}:
{run
this code}
elif
{conditions}:
{run
this code}
elif
{conditions}:
{run
this code}
else:
{run
this code}
#You
can have as many or as few elif statements as you need
#anywhere
from zero to the sky.
#You
can have at most one else statement
#and
only after all other ifs and elifs.
One of the most important points
to remember is that you MUST have a colon (:) at the end of every line with an
'if', 'elif', 'else' or 'while' in it. I forgot that, and as a result a stack
of people got stumped at this lesson (sorry ;)).
Indentation:
One
other point is that the code to be executed if the conditions are met, MUST BE
INDENTED. That means that if you want to loop the next five lines with a
'while' loop, you must put a set number of spaces at the beginning of each of
the next five lines. This is good programming practice in any language, but
python requires that you do it. Here is an example of both of the above points:
Code
Example 10 - Indentation
a = 10
while a > 0:
print a
if a > 5:
print "Big
number!"
elif a
% 2 != 0:
print "This
is an odd number"
print "It
isn't greater than five, either"
else:
print "this
number isn't greater than 5"
print "nor
is it odd"
print "feeling
special?"
a = a
- 1
print "we
just made 'a' one less than what it was!"
print "and
unless a is not greater than 0, we'll do the loop again."
print "well,
it seems as if 'a' is now no bigger than 0!"
print "the
loop is now over, and without furthur ado, so is this program!"
Notice the three levels of indents there:
Each line in the first level starts with no spaces. It is the main program, and
will always execute. Each line in the second level starts with four spaces.
When there is an 'if' or loop on the first level, everything on the second
level after that will be looped/'ifed', until a new line starts back on the
first level again. Each line in the third level starts with eight spaces. When
there is an 'if' or loop on the second level, everything on the third level
after that will be looped/'ifed', until a new line starts back on the second
level again. This goes on infinitely, until the person writing the program has
an internal brain explosion, and cannot understand anything he/she has written.
There is another loop, called the 'for' loop, but we will cover that in a later
lesson, after we have learnt about lists.
