diff -r e675f9208b91 -r 4054b1a6392d parsing_data.rst --- a/parsing_data.rst Wed Oct 13 17:32:23 2010 +0530 +++ /dev/null Thu Jan 01 00:00:00 1970 +0000 @@ -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 - -