[Tutor] Fwd: Re: If loop conditions

[Alan Gauld]

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-------- Forwarded Message --------
Subject: 	Re: [Tutor] If loop conditions
Date: 	Wed, 2 Mar 2016 07:31:57 +0530
From: 	D.V.N.Sarma డి.వి.ఎన్.శర్మ <dvnsarma at gmail.com>
To: 	Alan Gauld <alan.gauld at btinternet.com>



The

newList = []

statement must be above the for loop just after the function definition.
regards,
Sarma.


On Tue, Mar 1, 2016 at 10:22 PM, Alan Gauld <alan.gauld at btinternet.com> wrote:
> On 01/03/16 16:28, Dimitar Ivanov wrote:
>> First time using this mailing list so please excuse me in advance if this
>> mail is not structured properly.
>
> No worries, you've done a pretty good job of telling us
> what we need to know.
>
>> In the following exercise, Google requires you to look up words from a list
>> and count how many of the words are longer or equal to 2 characters where
>> the first and last letter match.
>>
>> I've been trying to assign string slices to variables and then use those
>> variables inside the following if statement:
>
> You probably don't need slices here. They are cool features
> but not always the right thing to use.
>
>> def match_ends(words):
>> +  for word in words:
>> +    length=len(word)
>> +      newList=[]
>> +      firstChar=word[0:]
>> +      lastChar=word[:-1]
>
> You only want the first and last character, not a slice.
> So just use indexes.
>
>       firstChar=word[0]
>       lastChar=word[-1]
>
>> +    if [ length >= 2 and firstChar == lastChar ]:
>
> You don;t need the brackets. And in fact in this case
> you are creating a list containing the bollean result
> of your test. A  non empty list (regardless of the
> value inside) is always going to be "true"...
>
> Just write it as:
>
> if length >= 2 and (firstChar == lastChar):
>
> The () just make it clear that its
>
> a and (b == c)
>
> rather than
>
> (a and b) == c
>
>> +        newList.append(word)
>> +    else:
>> +      break
>
> break exits the loop, you don't want that, instead you want
> to go back to the start of the loop to process the next word.
> The command for that is
>
>         continue
>
>> +  newListCount=len(newList)
>> +  return newListCount
>
>> Eventually I looked up the solution since I tracked down my problem to how
>> I made my if statement and, of course, it turned out to be much simpler
>> than usual.
>>
>> +  if len(word) >= 2 and word[0] == word[-1]:
>>
>> I tried putting brackets around the conditions and that broke the result
>> again so I came to wonder what's the difference between statements written
>> without brackets and those within brackets (such as my original code block)?
>
> It all depends on the type of brackets.
> Using square brackets as you did creates a list with a single value
>
> like: [42]  or in your case, either [True] or [False]
>
> The if statement(not loop) then treats any non empty list as True.
>
> But if you had used () instead of [] then it would probably
> work as you expected.
>
> if (len(word) >= 2 and word[0] == word[-1]):
>
> But you don;t really need them in Python (unlike C or Java)
>
> You can however make the intention clearer for your readers
> by grouping sub tests with params like:
>
> if (length >= 2) and (firstChar == lastChar):
>
> HTH
> --
> Alan G
> Author of the Learn to Program web site
> http://www.alan-g.me.uk/
> http://www.amazon.com/author/alan_gauld
> Follow my photo-blog on Flickr at:
> http://www.flickr.com/photos/alangauldphotos
>
>
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