Strings Ali Taheri Sharif University of Technology Spring 2019 - - PowerPoint PPT Presentation

Fu Fundamentals of Pr Programming (Py Python)

Strings

Ali Taheri Sharif University of Technology

Spring 2019

Some slides have been adapted from “Python Programming: An Introduction to Computer Science”

Outline

• 1. String Data Type
• 2. String Representation
• 3. String Indexing
• 4. Operations on Strings
• 5. String Methods

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ALI TAHERI - FUNDAMENTALS OF PROGRAMMING [PYTHON] Spring 2019

String Data Type

Text is represented in programs by the string data type

A string is a immutable sequence of characters enclosed within quotation marks (") or apostrophes (').

• Example:
• ‘This is a sample sentence.’
• “2x2=4”

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ALI TAHERI - FUNDAMENTALS OF PROGRAMMING [PYTHON] Spring 2019

String Representation

Character Encoding

• A string is stored as a sequence of binary numbers,
• ne number per character
• The mapping used to convert characters to numbers

and vice-versa is called character encoding

• Python 3 uses Unicode

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ALI TAHERI - FUNDAMENTALS OF PROGRAMMING [PYTHON] Spring 2019

>>> ord('a') # Return the Unicode code point for a one-character string. 97 >>> chr(97) # Return a Unicode string of one character with ordinal i 'a'

String Indexing

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ALI TAHERI - FUNDAMENTALS OF PROGRAMMING [PYTHON] Spring 2019

>>> greet = "Hello Bob" >>> greet[0] 'H' >>> print(greet[0], greet[2], greet[4]) H l o >>> x = 8 >>> print(greet[x - 2]) B >>> greet[-1] 'b' >>> greet[-3] 'B'

H e l l

• B
• b

0 1 2 3 4 5 6 7 8

String Indexing

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ALI TAHERI - FUNDAMENTALS OF PROGRAMMING [PYTHON] Spring 2019

H e l l

• B
• b

0 1 2 3 4 5 6 7 8

>>> greet[0:3] 'Hel' >>> greet[5:9] ' Bob' >>> greet[:5] 'Hello' >>> greet[5:] ' Bob' >>> greet[:] 'Hello Bob'

Operations on Strings

Traversal:

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ALI TAHERI - FUNDAMENTALS OF PROGRAMMING [PYTHON] Spring 2019

fruit = 'banana' n = len(fruit) for i in range(n): c = fruit[i] print(c) Output: b a n a n a fruit = 'banana' for c in fruit: print(c) Output: b a n a n a

Operations on Strings

Comparison:

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ALI TAHERI - FUNDAMENTALS OF PROGRAMMING [PYTHON] Spring 2019

Operations on Strings

Strings are immutable

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ALI TAHERI - FUNDAMENTALS OF PROGRAMMING [PYTHON] Spring 2019

Operations on Strings

The in Operator

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ALI TAHERI - FUNDAMENTALS OF PROGRAMMING [PYTHON] Spring 2019

String Methods

• s.capitalize() – Copy of s with only the first

character capitalized

• s.title() – Copy of s; first character of each word

capitalized

• s.center(width) – Center s in a field of given width
• s.count(sub) – Count the number of occurrences of

sub in s

• s.find(sub) – Find the first position where sub
• ccurs in s
• s.join(list) – Concatenate list of strings into one

large string using s as separator.

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ALI TAHERI - FUNDAMENTALS OF PROGRAMMING [PYTHON] Spring 2019

String Methods

• s.ljust(width) – Like center, but s is left-justified
• s.lower() – Copy of s in all lowercase letters
• s.lstrip() – Copy of s with leading whitespace

removed

• s.replace(oldsub, newsub) – Replace occurrences
• f oldsub in s with newsub
• s.rfind(sub) – Like find, but returns the right-most

position

• s.rjust(width) – Like center, but s is right-justified

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ALI TAHERI - FUNDAMENTALS OF PROGRAMMING [PYTHON] Spring 2019

String Methods

• s.rstrip() – Copy of s with trailing whitespace

removed

• s.split() – Split s into a list of substrings
• s.upper() – Copy of s; all characters converted to

uppercase For more, refer to:

https://docs.python.org/3/library/stdtypes.html#string- methods

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ALI TAHERI - FUNDAMENTALS OF PROGRAMMING [PYTHON] Spring 2019