module 5 strings and text files

OutlineCharacters and Strings

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Module 5Strings and Text Files

Tamer Abdou, PhD

IND 830 - Python Programming for Data Science

Data Science Laboratory

Ryerson University

Tamer Abdou, PhD Module 5: Control Statements 1/22

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Outline

1 Characters and Strings

2 Text Files


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Accessing Characters and SubstringsStrings and Number SystemsString Methods

Characters and Strings

1 Characters and StringsAccessing Characters and SubstringsStrings and Number SystemsString Methods

2 Text Files


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The Structure of Strings

• A string is a data structure that consists of several smaller pieces of data

• A string is a sequence of zero or more characters

• The positions of those characters are numbered from 0 to the length of the stringminus one

• The len() function can be used to return the number of characters a stringcontains


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The Subscript Operator

<a s t r i n g > [<an i n t e g e r e x p r e s s i o n >]

• The first part is the inspected string

• The second part in brackets indicates the position of a particular character in theinspected string

• The integer expression is also called an index


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Example 5.1 (Script)

1 courseCode = "CIND 830"

2 #Display the length of the string which is '8'

3 print(len(courseCode))

4 #Display the 1st character which is 'C'

5 print(courseCode[0])

6 #Display the 7th character which is '3'

7 print(courseCode[6])

8 #Display the last character which is '0'

9 print(courseCode[len(courseCode)-1])

10 print(courseCode[-1])


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Slicing Strings

• A subscript is a portion of a string that can be extracted

• Python’s subscript operator can be used to obtain a substring through a processcalled ’slicing’

• To extract a substring, a colon ’:’ is used and an integer value can appear oneither side of the colon

• If two integer positions are included in the slice (e.g. [x : y ])• The range extends from x up to y − 1• If x is omitted, all the characters extending to the beginning will be included in

the substring• If y is omitted, all the characters extending to the end will be included in the

substring


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1 progLang = "Python"

2 #Display the first two characters which are "Py"

3 print(progLang[ 0 : 2 ])

4 print(progLang[ : 2 ])

5 print(progLang[ :-4 ])

6 #Display the last four characters which are "thon"


8 print(progLang[ 2 : ])

9 print(progLang[-4 : ])

10 #Display all the characters


12 print(progLang[ : 6 ])

13 print(progLang[ 0 : ])

14 print(progLang[ : ])


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Strings and Number Systems

• Any binary number can be represented by a string of bits

• To determine the integer quantity that a string of bits represents, we multiply thevalue of each bit (0 or 1) by its positional value, then add the results

• 1012 = 1 ∗ 22 + 0 ∗ 21 + 1 ∗ 20

• 10112 = 1 ∗ 23 + 0 ∗ 22 + 1 ∗ 21 + 1 ∗ 20


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1 #Convert 1011 to integer

2 bitString = "1011"

3 result = 0

4 exponent = len(bitString) - 1

5 for bit in bitString:

6 result = result + int(bit) * 2 ** exponent

7 exponent = exponent - 1

8 print("The integer value is", result)


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Example 5.3 (Program Execution)

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Objects

str"101"

str"1"

int5

int-1

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('The integer value is', 5)

#Convert 101 to integerbitString = "101"result = 0exponent = len(bitString) - 1for bit in bitString: result = result + int(bit) * 2 ** exponent exponent = exponent - 1print("The integer value is", result)

12345678


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String Methods

<an object>.<method name>(<argument 1>, . . . , <argument n>)

• A method can be invoked by an object

• The object needs to be placed before the name of the method

• Methods can have arguments and return values. For example:• The split() method creates a list of words based on a specified separator

• The isalpha() returns True if the sentence contains only letters or False otherwise

• The upper() method returns an uppercase version of a given sentence or string

• The count() method returns the total number of non-overlapping occurrences of agiven substring


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1 #Practicing String Methods

2 Sentence = '''String literals are written in a variety of ways:

3 Single, Double, and Triple quotes'''

4

5 print(Sentence.split()) #Split without a delimiter

6 print(len(Sentence.split()))

7

8 print(Sentence.split("\n")) #Split using a delimiter

9 print(len(Sentence.split(":")))

10

11 print(Sentence.upper()) #Uppercase or lowercase

12 print(Sentence.lower())

13

14 print(Sentence.isalpha()) #Letters or Digits?

15 print(Sentence.isdigit())

16

17 print(Sentence.count("le")) #Number of occurrences of "le"


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Example 5.4 (Program Execution)(drag lower right corner to resize code editor)Write code in Python 3.6

Print output (drag lower right corner to resize)

['String', 'literals', 'are', 'written', 'in', 'a', 'variety', 'of', 'ways:', 'Single,', 'Double,', 'and', 'Triple', 'quotes'] 14['String literals are written in a variety of ways:', 'Single, Double, and Triple quotes']2STRING LITERALS ARE WRITTEN IN A VARIETY OF WAYS:SINGLE, DOUBLE, AND TRIPLE QUOTESstring literals are written in a variety of ways:single, double, and triple quotesFalseFalse3

#Practicing String MethodsSentence = '''String literals are written in a variety of ways:Single, Double, and Triple quotes'''

print(Sentence.split()) #Split without a delimiterprint(len(Sentence.split()))

print(Sentence.split("\n")) #Split using a delimiterprint(len(Sentence.split(":")))

print(Sentence.upper()) #Uppercase or lowercaseprint(Sentence.lower())

print(Sentence.isalpha()) #Letters or Digits?print(Sentence.isdigit())

print(Sentence.count("le")) #Number of occurrences of "le"

123456789

1011121314151617

Objects

str"String literals are written in a variety of ways:Single, Double, and Triple quotes"

Frames

Global frame

Sentence


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Reading and Writing FilesAccessing and Manipulating Files

Text Files

1 Characters and Strings

2 Text FilesReading and Writing FilesAccessing and Manipulating Files


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Reading/Writing Text from/to a File

• Data can be input or output to a text file using a file object

• The open() function opens a connection to a specific file with an adjusted mode

parameter

• The mode parameter ’r’ is for input/reading files and ’w’ for output/writing files

• String data can be read using the read() method, and written using the write()

method

• Any opened file should be closed after finishing writing the data in it, and thatcan be achieved using the close() method

• Failure to close an opened file may result in data loss


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1 #Text file Operations and Methods

2

3 textFile = open("myfile.txt", "w") #Opening a file for writing

4 textFile.write("First line.\nSecond line.\n")

5 textFile.close()

6

7 textFile = open("myfile.txt", "a") #Opening a file for appending

8 text = ["Third line.\n"]

9 textFile.writelines(text)

10 textFile.close()

11

12 textFile = open("myfile.txt", "r") #Opening a file for reading

13 contentsFile = textFile.read() #Read and Store

14 print(contentsFile) #Display the contents

15 textFile.close()


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Reading/Writing Numbers from/to a File

• All file input and output operations return data as strings

• Functions int() and float() are used to convert data items to integers andfloating-point numbers respectively

• A simple for-loop can be used to access one line of text at a time

• A programmer can use the split() method to obtain a list of substrings thenconvert them into either integers or floating-point numbers


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Accessing Files on Disks

• When interacting with files and before attempting to open them, it is preferableto check the existence of those files with the given path names

• The file system function os.path.exists() can be used to check the existenceof a file

• To return or change the path of the current working directory, the getcwd() orchdir(path) function can be used

• The mkdir() function is used to create folders, and the rmdir() function is usedto remove folders


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1 #Accessing and Maniplulating Files

2

3 import os #import the OS library

4

5 currentDir = os.getcwd() #acquiring the current working directory

6

7 listOfFiles = os.listdir(currentDir) #Store the file names as a list

8

9 for name in listOfFiles:

10 if ".py" in name:

11 print(name)


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Example 5.6 (Control Flow)

Accessing and Maniplulating Files

module

Shebang line: not specifiedEncoding: not specified

os import the OS library

currentDir = os.getcwd() acquiring the current working directory

listOfFiles = os.listdir(currentDir) Store the file names as a list

for

name in listOfFiles

Y".py" in name

print(name)


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References

• Lambert, K. A. (2010). Strings and text files. In Fundamentals of Python: Fromfirst programs through data structures (pp. 121-157). Boston, MA: CengageLearning

• Python Software Foundation. (2020). Python 3.7.8 documentation. Retrievedfrom https://docs.python.org/3.7/index.html


https://docs.python.org/3.7/index.html

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module 5 strings and text files

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