[Tutor] newbie 'while' question

[Danny Yoo]

> Here's what I've got for the second scenario:
>
> s = raw_input('Enter a name: ')
> x = 0
> while x >= 0:
>     print s[x]
>     x += 1
>
> The error is expected because the counter goes on beyond the number of
> characters in the string and runs out of stuff to print.


Hi Matthew,


Ok, so that means that we want:


###
while some_expression_that_is_false_when_x_is_too_large:
    print s[x]
    x += 1
###



The expression that we have, right now:

    x >= 0

is true as long as x is either positive or zero.  ("nonnegative").  The
only problem is that this will always be true: nothing in the loop's body
ever causes x to become negative.  *grin*


'x' is always increasing, hence our program breaks when it tries to print
nonexistant characters.  So we need to choose a different condition,
something that is true only until the index reaches the end of the word.


That is, instead of


        "Matthew Richardson"
        (------------------------------------------>
        0                                        infinity


We want to bound x's range to just:


        "Matthew Richardson"
        (------------------)----------------------->
        0                                        infinity



In mathematical terms, we'd say that we'd like a half-open range,


         0  <=  x   < length of "Matthew Ricahrdson"


And we'll find that the equivalent expression in Python is very similar to
this.  *grin*


I hope this gives some hints on how to correct the while 'condition'
expression.  Good luck!