[Tutor] Fw: loops

[christopher.henk at allisontransmission.com]

You should probably read some of the links sent to you earlier but here is 
a stab at an explanation.


Richard Hultgren wrote on 12/08/2009 10:36:08 AM:

> ----- Forwarded Message ----
> From: Richard Hultgren <hultgren1946 at yahoo.com>
> To: tutor at python.org
> Sent: Mon, December 7, 2009 2:53:40 PM
> Subject: loops
> I'm quite new but determined.  Can you explain to me, step by step, what 
is going on in the computer in this loop.  I 
> hope I am not being too dumb!

> a = 0
> b = 1
> count = 0
> max_count = 20

The above steps are initialization of your variables.  'a' points to the 
value 0, 'b' points to 1, etc.
Since the values of the variables are used within the loop we need to give 
it somewhere to start, otherwise we will get an error. 

> while count < max_count:

The while loop will continue until 'count < max_count' evaluates to False 
(20 loops, due to line below)
the loop is defined as everything that is indented below this line.

>     count = count + 1

Increase the value of count by one each loop iteration, otherwise the 
statement 'count < max_count' will never change value, and the loop will 
never end.

>     # we need to keep track of a since we change it
>     old_a = a                # especially here
>     old_b = b

Keep the old values of the variables since we want to add them together 
for the new value of b and will change the values in the next steps. 

>     a = old_b
>     b = old_a + old_b

reassign 'a' to the value of 'old_b'.  If we didn't save the value of 'a' 
into 'old_a' above we wouldn't know what it was anymore.
reassign 'b' to the sum of 'old_a' and 'old_b'

>     # Notice that the , at the end of a print statement keeps it
>     # from switching to a new line
>     print old_a,
prints the value of old_a followed by a space.  Without the comma at the 
end the print statement each loop would print on a new line.


The best way to follow what is happening is to trace the variables through 
the program.  you would get a table that loops something like :

max_count       count           a       b       old_a           old_b 
count<max_count OUTPUT
20              0               0       1       no Value        No Value  
T               No output yet 
20              1               1       1       0               1       T  
0 
20              2               1       2       1               1       T  
0 1
20              3               2       3       1               2       T  
0 1 1
20              4               3       5       2               3       T  
0 1 1 2
20              5               5       8       3               5       T  
0 1 1 2 3
20              6               8       13      5               8       T  
0 1 1 2 3 5
.
.
.
20              19              2584    6765    4181            2584    T 
0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597 2584 4181
20              20              2584    6765    4181            2584    F 
0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597 2584 4181 
(You might want to check the last two rows, not guaranteeing the values 
are right)

In the last row count<max_count is False the loop won't run and therefore 
nothing will change. 

Hope this helps some, but would recommend you try following some of the 
tutorials online for beginning programming to help explain some of the 
concepts, feel free to ask here whenever you get confused.

 
Chris
 
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