--- a/parsing_data.rst Thu Oct 07 14:33:45 2010 +0530
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@@ -1,287 +0,0 @@
-.. Author : Nishanth
- Internal Reviewer 1 :
- Internal Reviewer 2 :
- External Reviewer :
-
-Hello friends and welcome to the tutorial on Parsing Data
-
-{{{ Show the slide containing title }}}
-
-{{{ Show the slide containing the outline slide }}}
-
-In this tutorial, we shall learn
-
- * What we mean by parsing data
- * the string operations required for parsing data
- * datatype conversion
-
-#[Puneeth]: Changed a few things, here.
-
-#[Puneeth]: I don't like the way the term "parsing data" has been used, all
-through the script. See if that can be changed.
-
- Lets us have a look at the problem
-
-{{{ Show the slide containing problem statement. }}}
-
-There is an input file containing huge no. of records. Each record corresponds
-to a student.
-
-{{{ show the slide explaining record structure }}}
-As you can see, each record consists of fields seperated by a ";". The first
-record is region code, then roll number, then name, marks of second language,
-first language, maths, science and social, total marks, pass/fail indicatd by P
-or F and finally W if with held and empty otherwise.
-
-Our job is to calculate the mean of all the maths marks in the region "B".
-
-#[Nishanth]: Please note that I am not telling anything about AA since they do
- not know about any if/else yet.
-
-#[Puneeth]: Should we talk pass/fail etc? I think we should make the problem
- simple and leave out all the columns after total marks.
-
-Now what is parsing data.
-
-From the input file, we can see that the data we have is in the form of
-text. Parsing this data is all about reading it and converting it into a form
-which can be used for computations -- in our case, sequence of numbers.
-
-#[Puneeth]: should the word tokenizing, be used? Should it be defined before
- using it?
-
-We can clearly see that the problem involves reading files and tokenizing.
-
-#[Puneeth]: the sentence above seems kinda redundant.
-
-Let us learn about tokenizing strings. Let us define a string first. Type
-::
-
- line = "parse this string"
-
-We are now going to split this string on whitespace.
-::
-
- line.split()
-
-As you can see, we get a list of strings. Which means, when ``split`` is called
-without any arguments, it splits on whitespace. In simple words, all the spaces
-are treated as one big space.
-
-``split`` also can split on a string of our choice. This is acheived by passing
-that as an argument. But first lets define a sample record from the file.
-::
-
- record = "A;015163;JOSEPH RAJ S;083;042;47;AA;72;244;;;"
- record.split(';')
-
-We can see that the string is split on ';' and we get each field seperately.
-We can also observe that an empty string appears in the list since there are
-two semi colons without anything in between.
-
-To recap, ``split`` splits on whitespace if called without an argument and
-splits on the given argument if it is called with an argument.
-
-{{{ Pause here and try out the following exercises }}}
-
-%% 1 %% split the variable line using a space as argument. Is it same as
- splitting without an argument ?
-
-{{{ continue from paused state }}}
-
-We see that when we split on space, multiple whitespaces are not clubbed as one
-and there is an empty string everytime there are two consecutive spaces.
-
-Now that we know how to split a string, we can split the record and retrieve
-each field seperately. But there is one problem. The region code "B" and a "B"
-surrounded by whitespace are treated as two different regions. We must find a
-way to remove all the whitespace around a string so that "B" and a "B" with
-white spaces are dealt as same.
-
-This is possible by using the ``strip`` method of strings. Let us define a
-string by typing
-::
-
- unstripped = " B "
- unstripped.strip()
-
-We can see that strip removes all the whitespace around the sentence
-
-{{{ Pause here and try out the following exercises }}}
-
-%% 2 %% What happens to the white space inside the sentence when it is stripped
-
-{{{ continue from paused state }}}
-
-Type
-::
-
- a_str = " white space "
- a_str.strip()
-
-We see that the whitespace inside the sentence is only removed and anything
-inside remains unaffected.
-
-By now we know enough to seperate fields from the record and to strip out any
-white space. The only road block we now have is conversion of string to float.
-
-The splitting and stripping operations are done on a string and their result is
-also a string. hence the marks that we have are still strings and mathematical
-operations are not possible on them. We must convert them into numbers
-(integers or floats), before we can perform mathematical operations on them.
-
-We shall look at converting strings into floats. We define a float string
-first. Type
-::
-
- mark_str = "1.25"
- mark = int(mark_str)
- type(mark_str)
- type(mark)
-
-We can see that string is converted to float. We can perform mathematical
-operations on them now.
-
-{{{ Pause here and try out the following exercises }}}
-
-%% 3 %% What happens if you do int("1.25")
-
-{{{ continue from paused state }}}
-
-It raises an error since converting a float string into integer directly is
-not possible. It involves an intermediate step of converting to float.
-::
-
- dcml_str = "1.25"
- flt = float(dcml_str)
- flt
- number = int(flt)
- number
-
-Using ``int`` it is also possible to convert float into integers.
-
-Now that we have all the machinery required to parse the file, let us solve the
-problem. We first read the file line by line and parse each record. We see if
-the region code is B and store the marks accordingly.
-::
-
- math_marks_B = [] # an empty list to store the marks
- for line in open("/home/fossee/sslc1.txt"):
- fields = line.split(";")
-
- region_code = fields[0]
- region_code_stripped = region_code.strip()
-
- math_mark_str = fields[5]
- math_mark = float(math_mark_str)
-
- if region_code == "AA":
- math_marks_B.append(math_mark)
-
-
-Now we have all the maths marks of region "B" in the list math_marks_B.
-To get the mean, we just have to sum the marks and divide by the length.
-::
-
- math_marks_mean = sum(math_marks_B) / len(math_marks_B)
- math_marks_mean
-
-{{{ Show summary slide }}}
-
-This brings us to the end of the tutorial.
-we have learnt
-
- * how to tokenize a string using various delimiters
- * how to get rid of extra white space around
- * how to convert from one type to another
- * how to parse input data and perform computations on it
-
-{{{ Show the "sponsored by FOSSEE" slide }}}
-
-#[Nishanth]: Will add this line after all of us fix on one.
-This tutorial was created as a part of FOSSEE project, NME ICT, MHRD India
-
-Hope you have enjoyed and found it useful.
-Thank you
-
-Questions
-=========
-
- 1. How do you split the string "Guido;Rossum;Python" to get the words
-
- Answer: line.split(';')
-
- 2. line.split() and line.split(' ') are same
-
- a. True
- #. False
-
- Answer: False
-
- 3. What is the output of the following code::
-
- line = "Hello;;;World;;"
- sub_strs = line.split()
- print len(sub_strs)
-
- Answer: 5
-
- 4. What is the output of " Hello World ".strip()
-
- a. "Hello World"
- #. "Hello World"
- #. " Hello World"
- #. "Hello World "
-
- Answer: "Hello World"
-
- 5. What does "It is a cold night".strip("It") produce
- Hint: Read the documentation of strip
-
- a. "is a cold night"
- #. " is a cold nigh"
- #. "It is a cold nigh"
- #. "is a cold nigh"
-
- Answer: " is a cold nigh"
-
- 6. What does int("20") produce
-
- a. "20"
- #. 20.0
- #. 20
- #. Error
-
- Answer: 20
-
- 7. What does int("20.0") produce
-
- a. 20
- #. 20.0
- #. Error
- #. "20"
-
- Answer: Error
-
- 8. What is the value of float(3/2)
-
- a. 1.0
- #. 1.5
- #. 1
- #. Error
-
- Answer: 1.0
-
- 9. what doess float("3/2") produce
-
- a. 1.0
- #. 1.5
- #. 1
- #. Error
-
- Answer: Error
-
- 10. See if there is a function available in pylab to calculate the mean
- Hint: Use tab completion
-
-