[Tutor] for loops ... (fwd)

[Alan Gauld]

Tadey sent this to me separately, for completeness here is my
reply to him. (Spacing seems to have gone wild due to alphabet
changes, sorry)

Alan G.


> ... the part, where it says j = j + 1 ... looked at first a bit
illogical,
> like 1 = 1 + 1, so 1 = 2 come after,so this equation seems illogical

I think I mention this somewhere in the tutor but maybe not strongly
enough. In python (and many other languages) equality is represented
by a double equals: ==

So

j == j + 1

would, as you expect, be nonsense and always false.

A single equals is what is known as "assignment". Some programmers
like to read it as "becomes", so

j = j + 1

is read as

j becomes j + 1

In fact some languages (Pascal, ADA, Smalltalk) use a different symbol
for assignment and use a single equals for equality - which is frankly
more sensible! So in Pascal it would look like:

j := j + 1

Notice the extra colon before the equals sign.

> means variable j changes from 1 to 2, and then from 2 to 3, every
loop +1,
> and so on to 11 ... is that it  ??

Yes.


> So please, give me some background releted to that kind of
> equations/instructions (j = j + 1), how they "control" loops,

The while loop has the following abstract form:

<Initialise loop control value>
while <test value>:
        < do some processing>
        <change loop control value>

For example:

j = 1
while j < 10:
     print j * 12
     j = j+1


> I don't get it, how it print that high values as a result.
> I thought, that it first "takes" dog value 2, defined in
> tadey(2),then "calculates" current value for this loop,
> so dog = dog - 1, that gives 1, than multiplicate
> with result, which is set 1, which gives the real result ...

That's pretty much right, let's walk through it:

>>> def tadey(glg):
            result = 1
            while glg > 0:
                   result = result * glg
                   glg = glg - 1
            return result

We'll draw a table of the variables:

glg    result
 2            1         while glg > 0, first time
 2            2            result = result * glg
 1            2            glg = glg - 1
 1            2         while glg > 0 , second time
 1            2            result = result * glg
 0            2            glg = glg - 1
 0            2         while glg < 0, third time. Test if false so we
exit loop
 0            2         return result

So the result is 2

Lets do it again for tadey(3):

glg    result
 3        1         while glg > 0, first time
 3        3            result = result * glg
 2        3            glg = glg - 1
 2        3         while glg > 0 , second time
 2        6            result = result * glg
 1        6            glg = glg - 1
 1        6         while glg < 0, third time.
 1        6            result = result * glg
 0        6            glg = glg - 1
 0        6         while glg < 0, 4th time, test fails, loop exits.
 0        6         return result

So the result is 6.

Finally, lets try:

>>> tadey(-1)

glg    result
 -1         1     while glg > 0, first time, test fails, loop is never
entered
 -1         1         return result

Hopefully that makes sense?

Alan G.