[Tutor] can someone explain the reason for error

[Alan Gauld]

On 07/02/18 11:58, vinod bhaskaran wrote:
> Hi, I am a beginner level programmer and in one assignment the question
> given is:to remove ',' from a list after getting a comma separated input
> from console.

First up I'll say you are doing an awful lot of work
that's not needed. As a result your solution is much
more complex than is necessary.

Remember that a string is just like a list of letters
so you don't need to convert it to a list. Just use
it as is. Also strings have lots of useful methods
that you can use - hint: try reading about them in
the Python documentation, you might find easier ways
to do things.

> I gave the below (most print statements are for reference except the last
> print statement). but i get the attached error. can someone explain why
> this error nd how to rectify?
> 
> inputdata = input ('Enter comma separated data \n')
> type(inputdata)

This line does nothing useful.

> inputlist = list(inputdata)
> print(inputlist)
> a = len(inputdata)
> print(a)

Note that you converted to a list but you are
taking the length of the original string.

> print ('xxxxxxxxxxxxxxxx')
> for b in range(0,a):
>     print(a)
>     a = a - 1

This is a terrible idea. Don't mess with the values
to give to range in a loop. Bad things are likely
to happen.

Also note that Pythons for loops give you each item
in the data, you don;t need to mess about with indexes.
And if you do need indexes enumerate() is usually a
better solution than using range. (range is great
if you just want a list of numbers but its usually
the wrong way to process a collection of any kind)

However in this case you are not using a or b,
just reducing the size of a in each iteration,
while increasing the size of b such that they
will meet in the middle. I'm pretty sure that's
not what you want?

>     print(inputlist)
>     inputlist.remove(',')

This removes the commas starting at the front
of the list. So you remove 1 comma each time
round the loop. But you go round the loop half
as many times as there are characters in the
string. What happens if there are less commas
than that?

You don't show us the error, (if you used an
attachment it will have been stripped by the
server). But if it was a ValueError then I
suspect you hit the above scenario.

> print(inputlist)

Let me suggest a simpler algorithm in pseudo code.
(WE don;t do your homework for you so you will need
to translate the pseudocode into real Python yourself)

inputstr = input()
for char in inputstr:
   if char not a comma
      print char

There is an even shorter way if you use the built
in string methods.

-- 
Alan G
Author of the Learn to Program web site
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