Strings: Characters, Substrings, and Manipulation

Objectives

• Access individual characters in a string
• Retrieve a substring from a string
• Search for a substring in a string
• Use string methods to manipulate strings
• Open a text file for output and write strings or numbers to the file
• Open a text file for input and read strings or numbers from the file
• Use library functions to access and navigate a file system

Accessing Characters and Substrings in Strings

• You will learn how to extract portions of a string called substrings.
• This section examines the internal structure of a string.

The Structure of Strings

• An integer cannot be factored into more primitive parts.
• A string is a data structure.
• A data structure consists of smaller pieces of data.
• A string's length is the number of characters it contains, and it can contain 0 or more characters.
• The len function returns the string's length, which is the number of characters it contains.
• len("Hi there!") returns 9.
• len("") returns 0.

The Subscript Operator

• The subscript operator is: <a string>[<an integer expression>]
• Example: Use the subscript operator to examine the first character in the string "Alan Turing"
• Result: 'A'
• Example: Use the subscript operator to examine the fourth character in the string Alan Turing
• Result: 'n'
• Example: Accessing the length of the name string will cause an error.
• The last character of a string can be accessed by name[-1]
• The second to last character can be accessed for example by name[-2]

Subscript Operator Usefulness

• This operator is useful when you want to use the positions as well as the characters in a string.
• A count controlled loop can be used alongside this operator.
• For example, data = "Hi there!" can have its characters printed alongside their index in a loop.

Slicing for Substrings

• Python's subscript operator can be used to obtain a substring through a process called slicing.
• Place a colon (:) in the subscript; an integer value can appear on either side of the colon.
• A string name = myfile.txt can have a substring taken with the subscript operator
• Returns the entire string for name[0:]
• 'm' for name[0:1]
• 'txt' for name[-3:]

Testing for a Substring with the in Operator

• When used with strings, the left operand of in is a target substring and the right operand is the string to be searched.
• True is returned if target string is somewhere in search string, or False otherwise.
• The following code segment traverses a list of filenames and prints just the filenames that have a .txt extension:

fileList = [“myfile.txt”, “myprogram.exe”, “yourfile.txt”]
for fileName in fileList:
    if “.txt” in fileName:
        print(fileName)

String Methods

• Python includes a set of string operations called methods to make tasks like counting the words in a single sentence easy.
• Result of len(listOfWords) from a .split() is the number of words in a string.
• A method behaves like a function, but has a slightly different syntax.
• A method is always called with a given data value, called an object, as so: <an object>.<method name>(<argument-1>,..., <argument-n>)
• Methods can expect arguments and return values
• A method knows about the internal state of the object with which it is called
• All data values are objects in Python.
• View a complete list and documentation of string methods by entering dir(str) at a shell prompt
• Enter help(str.<method-name>) to receive documentation on an individual method

Extracting a filename's extension

• Result of 'myfile.txt'.split('.') is ['myfile', 'txt']
• Result of 'myfile.html'.split('.') is ['myfile', 'html']
• The subscript [-1] extracts the last element.
• This can be used to extract a filename extension.

Text Files

• A text file is a software object that stores data on permanent mediums like a disk or CD
• Advantages of taking input data from a file are:
  • The data set can be much larger
  • The data can be input more quickly and with less chance of error
  • The data can be used repeatedly with the same program or with different programs

Text Files and Their Format

• A text editor such as Notepad or TextEdit permits the creation, viewing, and saving of data in a text file.
• A text file containing six floating-point numbers might look like: 34.6 22.33 66.75 77.12 21.44 44.87
• Data output to or input from a text file must be strings.
• Numbers must be converted to strings before output.

Writing Text to a File

• Data can be output to a text file using a file object.
• Open a file for output with f = open("myfile.txt", 'w').
  • If the file does not exist, it is created.
  • If the file already exists, Python opens it; when data are written to the file and the file is closed, any data previously existing in the file are erased.
• f.write("First line.\nSecond line.\n") writes two lines of text to the file.
• When all outputs are finished, use f.close() to close the file.

Writing Numbers to a File

• The file method write expects a string as an argument.
• Other types of data must first be converted to strings before being written to the output file. This can be done using str.

Reading Text from a File

• Open a file for input with output = open("myfile.txt", 'r')
  • If the path name is not accessible from the current working directory, Python raises an error
• f.read() is one way to read data from a file
  • After input is finished, read returns an empty string
• Alternatively, a file may be read line by line, using f.readline()
  • The next code segment inputs lines of text with readline:
  • >> f = open("myfile.txt”, 'r')
  • >> while True:
  • line = f.readline()
  • if line == “”:
  • break
  • print(line)

Reading Numbers from a File

• String representations of integers and floating-point numbers can be converted to the numbers by using the functions int and float.
• Use the the code:

f = open("integers.txt", 'r')
theSum = 0
for line in f:
    line = line.strip()
    number = int(line)
    theSum += number
print(“The sum is”, theSum)

• The next code segment modifies the previous one to handle integers separated by spaces and/or newlines

f = open("integers.txt", 'r')
theSum = 0
for line in f:
    wordlist = line.split()
    for word in wordlist:
        number = int(word)
        theSum += number
print(“The sum is”, theSum)

File Methods

Chapter Summary

• A string is a sequence of zero or more characters
  • The len function returns the number of characters in its string argument
• A string is an immutable data structure
• The subscript operator [] can be used to access a character at a given position
  • Can also be used for slicing ([<start>:<end>])
• in operator is used to detect the presence or absence of a substring in a string
• Method: operation that is used with an object
• The string type includes many useful methods for use with string objects
• A text file is a software object that allows a program to transfer data to and from permanent storage
• A file object is used to open a connection to a text file for input or output
  • Some useful methods: read, write, readline
• for loop treats an input file as a sequence of lines
  • On each pass through the loop, the loop's variable is bound to a line of text read from the file

Explore how to access individual characters and substrings within strings. Learn to use string methods for manipulation and perform searches. Also, learn about string structure, its length, and the usage of the len function.