Help required to read and print lines based on the type of first character

Bruno Desthuilliers bruno.42.desthuilliers at websiteburo.invalid
Thu Mar 5 14:20:23 CET 2009


(answering to the OP)
> En Wed, 04 Mar 2009 07:36:01 -0200, <Abhinayaraj.Raju at emulex.com> escribió:
> 
>> I am a beginner in Python. In fact, beginner to coding/ scripting.
>>
>> Here is a scenario, I need to code. Need your help on this:

Your first task here should be to refine the specs - too much 
ambiguities in it:

>> A script that
>> 1.      Reads from a file (may be a local file say test.txt)

reads what ? The whole content ? A (yet unspecified) portion ? etc...

>> 2.      And, if the line

which line ?

>> begins with a "#", should print the line one 
>> time

"#### yadda" begins with a "#". So according to this rule, it should be 
printed once.

>> 3.      if the line has "##", should print the line 2 times,

"#### yadda" 'has' (contains) "##", so it should be printed twice. But 
it also _begins_ with a '#', so according to rule 2, it should be 
printed once. Is the 'one time' in rule 2 supposed to mean 'at least 
onces', or 'once and only once' ? In this last case, rule 3 contradicts 
rule 2.

Also, "yadday ## woops" 'has' (contains) "##". According to rule 3, it 
should be printed twice. Is that right ?

>> 4.      And, if there are "###" in the beginning of the line, should 
>> print the same line 3times,

"#### yadda" begins with "###", so it should be printed thrice. It also 
contains "##" (cf rule 3) and starts with "#" (cf rule 2). How is this 
rule supposed to be understood ?

>> 5.      And, if the line contains "####" or more "#"'s, then print as 
>> "an invalid line"

"#### yadda" starts with '#', contains '##', and starts with '###'. 
Which rule is supposed to apply here ?

>> 6.      if the line contains no "#" then print "looks like a code line"
>>

The first step in programming is to get accurate, unambigous and 
well-expressed specs. In the above case (I mean, at this level of 
detail), "accurate, unambigous and well-expressed specs" are almost 
pseudocode. Doing a bit of mind-reading (which is certainly *not* the 
right thing to do - in real life, I'd just go back to the customer or 
whoever handed me such specs to sort this out), I came out with the 
following rewrite:

1. open a given file in text mode
2. read it line by line
3. for each line:
3.1. if the line contains/startswith (?) more than three '#':
         print "an invalid line"
3.2. if the line starts with one, two or three '#':
          print as many times the line as there are '#'
3.2 else (imply : the line contains no '#'):
        print "looks like a code line"

So basically, you have your algorithm. Now you just need to find out how 
to do each of these tasks in Python. That is :

a. how to open a file for reading in text mode (NB: 'text mode' may or 
not makes sense, according to the OS)

b. how to  iterate over the lines in this file once it's correctly opened

c. how to test for the presence and position of a given character / 
substring in a string

d. how to "print" something from your program.


Good news: all this is pretty well documented. I'd say that the most 
"tricky" part is c., since there's more than one possible solution, but 
since it's about strings, looking for what features Python strings has 
to offer, and trying them out in the interactive interpreter should 
solve the problem very quickly.


HTH



More information about the Python-list mailing list