Chapter 5: Strings and Text Files

Strings

Data structure: consists of smaller pieces of data
String’s length: no. of characters (including spaces) it contains
>>> len(“”)
0

Subscript operator

<a string>[<an integer operation>]
Index is usually in range (0,len); can be negative

# examine last character in string “name”
name[len(name) - 1]
# OR
name[-1]

Useful when you want to use the positions & characters in a string
E.g. count-controlled loop

data = “Hi there!”
for index in range(len(data)):
print(index, data[index])
OUTPUT:
0 H
1 i
2
3 t
4 h
5 e
6 r
7 e
8 !

Slicing for substrings (:)

[start:end+1] in terms of indices

>>> name = “myfile.txt”
>>> name[0:]                      # entire string
‘myfile.txt’
>>> name[:len(name)]        # entire string (because from index 0 to index len(name)-1)
‘myfile.txt’
>>> name[0:1]                    # first character
‘m’
>>> name[0:2]                    # first 2 characters
‘my’
>>> name[-3:]                    # last 3 characters
‘txt’
Note:
s[2:6]: s[2] to s[5]
s[:6]: first character to s[5]
s[2:]: s[2] to last character
s[::-1]: reverse string (full string but -1 stride)

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
Returns True if target string is somewhere in each string, False otherwise

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

String methods¹²

Behaves like a function, but has a slightly diff syntax
A method is always called with an object (a given data value)
<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
In Python: all data values are objects
Using example: >>> s = “Hi there!”

s.center(width)	Returns a copy of s centred within the given number of columns >>> s.center(11) OUTPUT: ‘ Hi there! ’
s.count(sub [, start [, end]]³)	Returns the no. of non-overlapping occurrences of substring sub in s. Optional arguments start and end are interpreted as in slice notation. >>> s.count(‘e’) OUTPUT: 2
s.endswith(sub)	Returns True if s ends with sub or False otherwise
s.startswith(sub)	Returns True if s starts with sub or False otherwise
s.find(sub [, start [, end]])	Returns lowest index in s where substring sub is found. ⁴Optional arguments start and end are interpreted as in slice notation. >>> s.find(‘the’) OUTPUT: 3
s.isalpha()	Returns True if s contains only letter or False otherwise >>> ‘abc’.isalpha() OUTPUT: True
s.isdigit()	Returns True if s contains only digits or False otherwise >>> “326”.isdigit() OUTPUT: True
s.split([sep])	Returns a list of the words in s, using sep as the delimiter string. If sep not specified, any whitespace string is a separator >>> words = s.split() >>> words [‘Hi’, ‘there!’]
a.join(sequence)	Returns a string that is the concatenation of the strings in the sequence. The separator between elements is s. >>> “”.join(words) ‘Hithere!’ >>> “ ”.join(words) ‘Hi there!’
s.lower() / s.upper()	Returns a copy of s converted to lowercase/uppercase
s.islower() / s.isupper()	Return True is s is lowercase / uppercase or False otherwise
s.replace(old, new [, count])	Returns a copy of s with all occurrences of substring old replaced by new. If the optional argument count is given, only the first count occurrences are replaced. >>> s.replace(‘i’, ‘o’) ‘Ho there!’
s.strip([aString])	Returns a copy of s with leading and trailing whitespace (tabs, space, newlines) removed. If aString is given, remove characters in aString instead >>> “ Hi there! ”.strip() ‘Hi there!’

E.g.

s = "hello s6c students \n"
print(s.split())
print(s.strip()+'end')
print(s)
OUTPUT:
['hello', 's6c', 'students']
hello s6c studentsend
hello s6c students

E.g. counting no. of words and average word length

>>> sentence = input(“Enter a sentence: ”)
Enter a sentence: *This sentence has no long words*.
>>> listOfWords = sentence.split()
>>> print(“There are ”, len(listOfWords), “ words.”)
There are 6 words.
>>> sum = 0
>>> for word in listOfWords:
sum += len(word)
>>> print(“The average word length is”, sum / len(listOfWords))
The average word length is 4.5

E.g. extracting a filename’s extension

>>> “myfile.txt”.split(“.”)
[‘myfile’, ‘txt’]

>>> filename.split(“.”)[-1]     # the subscript [-1] extracts the last element, can be used to write a general expression for obtaining any filename’s extension

A string is an immutable data structure

When we use any method, it does not change the string itself
E.g.

s = “sample”
s.upper()
print(s)
OUTPUT: sample

s = “sample”
s = s.upper()        # assignment
print(s)
OUTPUT: SAMPLE

Note: s = ‘hello’; s[1] = ‘S’ or s[1:5] = ‘test’ gives TypeError: ‘str’ object does not support item assignment
Use

s = “hello 25s6c students”
# change 25s6c to 25S6C
s = s[:8] + ‘S6C’ + s[11:]

Text file = a software object that stores data on permanent medium (e.g. disk or CD)

Advantages of taking input data from a file compared to keyboard input from human user
Dataset can be much larger
Data can be be input much more quickly and with less chance of error
Data can be used repeatedly with the same program or with different program
Can create, view, and save data in a text file using text editor (e.g. Notepad or TextEdit)
All data output to or input from a text must be strings

open(pathname, mode)	Opens a file at the given pathname and returns a file object. Mode can be ‘r’, ‘w’, ‘rw’ (read/write), or ‘a’ (append)⁵.
f.close()	Closes an output file. Not needed for input file E.g. f = open(“myfile.txt”, ‘w’) f.write(“First line.\nSecond line.\n”) f.close() [Alternative (not recommended): A levels requires a comment that file is closed] to automatically close files: with open(“myfile.txt”, ‘w’) as f: f.write(“First line.\nSecond line.\n”) `# the file will close automatically.`
f.write(aString)	Outputs aString to a file
f.read()	Inputs contents of a file and returns them as a single string. Returns ‘ ’ if end of file is reached
f.readline()	Inputs a line of text and returns it as a string, including the newline. Returns ‘ ’ if end of file is reached

Writing Text to a File

Data can be output to a text file using a file object
To open a file for output:

f = open(“myfile.txt”, ‘w’)

If file does not exist, it is created in the same folder of the Python scripts
If file already exists, Python opens it
When data are written to the file and file is closed, data previously existing in the file are erased

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 output file (e.g. using str)

import random
f = open(“integers.txt”, ‘w’)
for count in range(500):
number = random.randint(1,500) # randint is inclusive of both end points (?)
f.write(str(number) + “\\n”)
f.close()

Reading Text from a File

Open a file for input in a manner similar to opening a file for output
If pathname is not accessible from current working directory, Python raises an error

for loop method

f = open(“myfile.txt”, ‘r’)
for line in f:                           # read each line
line  = line.strip()      # remove \n
print(line)

read method

f = open(“myfile.txt”, ‘r’)
>>> text = f.read()
>>> text
First line.\\nSecond line.
>>> print(text)
First line.
Second line.
>>> lines = text.split(“\\n”)            # split string by \\n
['First line.', 'Second line.']
>>> for line in lines:
line = line.strip()
print(line)
f.close()

readline method

f = open(“myfile.txt”, ‘w’)
f.write(“First line.\\nSecond line.\\n”)
f.write(‘234’) # convert integer / float to string before writing to text file
f.close()
f = open(“myfile.txt”, ‘r’)
line = f.readline().strip()                  # read only one line, and remove \n
while line != ‘’:
print(line)                             # print current line
line = f.readline().strip()       # read next line (?), and remove \n

readlines method

f = open(“myfile.txt”, ‘r’)
lines = f.readlines()        # create a list of lines
for line in lines:
line = line.strip() # remove \n
print(line)

Reading numbers from a file

Text file “Integers1.txt”
10
20
40
60

Text file “Integers2.txt”
10 20
40 60

f = open(“Integers1.txt”, ‘r’)
sum = 0
for line in f:
line  = line.strip()
number = int(line)
sum += number
print(“The sum is ”, sum)

f = open(“Integers2.txt”, ‘r’)
sum = 0
for line in f:
wordlist  = line.split()
First line → ['10', '20']
Second line → ['40', '60']
for word in wordlist:
number = int(word)
sum += number
print(“The sum is ”, sum)

with open(“Integers1.txt”, ‘r’) as f:
sum = 0
for line in f:
line = line.strip()
number = int(line)
sum += number
# file is closed automatically (MUST WRITE for “with open() as f”)
print(“The sum is ”, sum)

with open(“Integers2.txt”, ‘r’) as f:
sum = 0
for line in f:
wordlist = line.split()
for word in wordlist:
number = int(word)
sum += number
# file is closed automatically (MUST WRITE for “with open() as f”)
print(“The sum is ”, sum)

OUTPUT for both: The sum is 130

CSV file reading and writing

CSV file = text file in which data is separated by commas

# to read operation on CSV file
import csv
f = open(“myfile.csv”, ‘w’, newline = ‘ ’)
obj = csv.writer(f)
obj.writerow([“Mobile”, “Redmi”, 1500])
obj.writerow([“TV”, “Samsung”, 2500])
f.close()

# to write operation on CSV file
import csv
f = open(“myfile.csv”, ‘r’, newline = ‘ ’)
obj = csv.reader(f)
for row in obj:
print row
OUTPUT:
[‘Mobile’, ‘Redmi’, 1500]
[‘TV’, ‘Samsung’, 2500]

Comments from the Word document

Comment by ANDREA TAN KAI XUAN HCI: how to use in pseudocodes ↩
Comment by ANDREA TAN KAI XUAN HCI: cannot ↩
Comment by ANDREA TAN KAI XUAN HCI: index of start and end+1? ↩
Comment by ANDREA TAN KAI XUAN HCI: what if not found ↩
Comment by ANDREA TAN KAI XUAN HCI: whats these for ↩

Quartz 5

Explorer

06 Chapter 5 - Strings and Text Files

Chapter 5: Strings and Text Files