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Visual Cue Narration
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Containing title, name of the production team along with the logo of MHRD

Hello friends and welcome to the tutorial on "Parsing Data".
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Learning objectives

At the end of this tutorial, you will be able to,
  1. Split a string using a delimiter.
  2. Remove the whitespace around the string.
  3. Convert the datatypes of variables from one type to other.

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Pre-requisite slide

Before beginning this tutorial,we would suggest you to complete the tutorial on "Getting started with Lists".
Open the terminal
Invoke the ipython interpreter by typing ipython on your terminal.
Open the file sslc.txt and show Let us start this tutorial with the help of an an exercise.

There is an input file containing huge no. of records. Each record corresponds to a student.

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'Data set'

As you can see, each record consists of fields separated by a semicolon. The first record is region code, then roll number,name, marks of second language,first language, maths, science and social and total marks.

Our job is to calculate the arithmetic mean of all the maths marks in the region "B".

Open the file sslc.txt and show Now let us understand, what is meant by '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,it will be a sequence of numbers.

We can clearly see that the problem involves reading files and tokenizing.

Switch to the terminal
line = "parse this           string"
Let us learn about tokenizing strings. We shall define a string first. Type
line.split() We are now going to split this string on whitespace.
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.
record = "A;015163;JOSEPH RAJ S;083;042;47;0;72;244"
The function split can also split on a string of our choice. This is achieved by passing that as an argument. But first lets define a sample record from the file.
We can see that the string is split on the semi-colon and we get each field separately. We can also observe that an empty string appears in the list since there are two semi colons without anything in between.

In short, split splits on whitespace if called without an argument and splits on the given argument if it is called with an argument.

Pause the video here, try out the following exercise and resume the video

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Assignment 1

Split the variable line using a space as argument. Is it same as

splitting without an argument ?

Continue from paused state Switch to the terminal
Switch to terminal for the solution
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Solution 1

We see that when we split on space, multiple whitespaces are not clubbed as one and there is an empty string every time there are two consecutive spaces.
Switch to the terminal Now that we know how to split a string, we can split the record and retrieve each field separately. But there is one problem. The region code "B" and a "B" surrounded by whitespace are treated as two different regions. Hence, we must find a way to remove all the whitespace around a string so that the region code "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 "
We can see that strip removes all the whitespace around the sentence.

Pause the video here, try out the following exercise and resume the video

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Assignment 2

What happens to the white space inside the sentence when it is stripped
Continue from paused state Switch to the terminal
a_str = "         white      space            "
Switch to the terminal for solution
We see that, the whitespace inside the sentence is only removed and the rest remains unaffected.
By now we know enough to separate 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.

mark_str = "1.25"
mark = int(float(mark_str))
We shall now look at converting strings into floats. We define a float string first. Type
We can see that string is converted to float. We can perform mathematical operations on them now.

Pause the video here, try out the following exercise and resume the video

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Assignment 3

What happens if you do int("1.25")
Continue from paused state Switch to the terminal
Switch to the terminal for solution
dcml_str = "1.25"
flt = float(dcml_str)
number = int(flt)
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 then 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/sslc.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 == "B":
math_marks_mean = sum(math_marks_B) / len(math_marks_B)
Now we have all the math 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.
Hence we get our final output. This is how we split and read such a huge data and perform computations on it.
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Summary slide

This brings us to the end of the tutorial. In this tutorial, we have learnt to,
  1. Tokenize a string using various delimiters like semi-colons.
  2. Split a data separated by delimiters by using the function split().
  3. Get rid of extra white spaces around using the strip() function.
  4. Convert datatypes of numbers from one type to another.
  5. Parse input data and perform computations on it.

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Self assessment questions slide

Here are some self assessment questions for you to solve
  1. How do you split the string "Guido;Rossum;Python" to get the words.
  2. How will you remove the extra whitespace in this sentence " Hello World "
  3. What does int("20.0") produce
    • 20
    • 20.0
    • Error
    • "20"

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Solution of self assessment questions on slide

And the answers,
  1. We can split the string the semi-colons by passing it as an argument to the split function as line.split(';')
  2. " Hello World ".strip() will remove the extra whitespaces around the string.
  3. int("20.0") will give an error, because converting a float string, 20.0, directly into integer is not possible.

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Acknowledgment slide

Hope you have enjoyed this tutorial and found it useful. Thank you.

Contributors and Content Editors

Chandrika, Pravin1389