On 09/23/2011 11:28 PM, Wayne Werner wrote:
On Fri, Sep 23, 2011 at 2:25 PM, ADRIAN KELLY <kellyadr...@hotmail.com
<mailto:kellyadr...@hotmail.com>> wrote:
<snip>
can anyone explain the *_tries_* part of this programme to me i
know its meant to count the number of guesses made by the user by
adding 1 but i just cant figure out how it does this..........can
someone explain?? i.e. tries = 1, tries +1 etc.... cant get my
head around it...
The concept that's confusing you here is something called order of
evaluation, or evaluation strategy:
http://en.wikipedia.org/wiki/Evaluation_strategy
The best way to understand these things is to try it out, and the
interactive interpreter is extremely handy.
I presume you understand the concept of assignment, correct? For example:
>>> tries = 1
Now tries contains 1:
>>> tries = 1
>>> tries
1
The variable 'tries' now contains the value 1 (In Python this is not
technically true, but it's useful to describe it that way).
Why?
>>> tries = 4
Now 'tries' contains 4. Of course, just like your standard algebra
variables, you can use your variables in python:
>>> tries * 4
16
>>> tries + tries
8
>>> tries / 1
4
when these expressions are /evaluated/ they produce the mathematical
expression you probably expect. But you can do more than just evaluate
expressions, you can store their results:
>>> lots_of_tries = tries * 100
>>> lots_of_tries
400
So what about the expression that confuses you?
>>> tries = tries + 1
>>> tries
5
Well, the first thing that happens when python sees the equals sign is
that the right hand side of the expression is evaluated, so python takes:
tries = tries + 1
and turns it into
tries = 4 + 1
tries = 5
So each time your program sees 'tries = tries + 1' python evaluates
the right side first, then assigns its value to the variable on the left.
HTH,
Wayne
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