CMU 15-110: Principles of Computing
Strings

String Literals
Some String Constants
Some String Operators
Looping over Strings
Example: isPalindrome
Strings are Immutable
Some String-related Functions
Some String Methods
Basic File IO

String Literals
1. Four kinds of quotes
  
  print('single-quotes') print("double-quotes") print('''triple single-quotes''') print("""triple double-quotes""")
2. Newlines in strings
  
  print('ab\ncd') # \n is a single newline character print(len('ab\ncd')) # 5 print('''ab cd''')
3. String Literals as Multi-line Comments
  
  ''' One way to 'comment out' a large block of code is to place it in triple-quotes. Python will basically ignore that code. '''

Some String Constants
import string print(string.ascii_letters) # abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ print(string.ascii_lowercase) # abcdefghijklmnopqrstuvwxyz print(string.ascii_uppercase) # ABCDEFGHIJKLMNOPQRSTUVWXYZ print(string.digits) # 0123456789 print(string.punctuation) # '!"#$%&\'()*+,-./:;<=>?@[\\]^_`{|}~' print(string.whitespace) # space + tab + linefeed + return + ...

Some String Operators
1. String + and *
  
  print('abc' + 'def') print('abc' * 3) print('abc' + 3) # error
2. The in operator
  
  print('ring' in 'strings') print('wow' in 'amazing!') print('Yes' in 'yes!') print('' in 'No way!')
3. String indexing and slicing
  1. Indexing a single character
    
    s = 'abcdefgh' print(s) print(s[0]) print(s[1]) print(s[len(s)-1]) print(s[len(s)]) # crashes (string index out of range)
  2. Negative indexes
    
    s = 'abcdefgh' print(s) print(s[-1]) print(s[-2]) print(s[-len(s)]) print(s[-len(s)-1]) # crashes (string index out of range)
  3. Slicing a range of characters
    
    s = 'abcdefgh' print(s) print(s[0:4]) print(s[2:4])
  4. Slicing with default parameters
    
    s = 'abcdefgh' print(s) print(s[3:]) print(s[:3]) print(s[:])
Looping over Strings
1. "for" loop with indexes
  
  s = 'abcd' for i in range(len(s)): print(i, s[i])
2. "for" loop without indexes
  
  s = 'abcd' for c in s: print(c)
3. "for" loop with split
  
  names = 'fred,wilma,betty,barney' for name in names.split(','): print(name)
4. "for" loop with splitlines
  
  lines = ''' Q: What do you get when you cross a stream and a brook? A: Wet feet. Q: What do you get when you cross poison ivy with a 4-leaf clover? A: A rash of good luck. ''' for line in lines.splitlines(): if (line.startswith('Q:')): print(line)
Example: isPalindrome
# There are many ways to write isPalindrome. Here are a few: def isPalindrome1(s): for i in range(len(s)): if (s[i] != s[-1-i]): return False return True def reverseString(s): # Note: there are shorter and more efficient ways, but this is very clear result = '' for c in s: result = c + result return result def isPalindrome2(s): return (s == reverseString(s)) def isPalindrome3(s): while (s != ''): if (s[0] != s[-1]): return False s = s[1:-1] return True print(isPalindrome1('abcba'), isPalindrome1('abca')) print(isPalindrome2('abcba'), isPalindrome2('abca')) print(isPalindrome3('abcba'), isPalindrome3('abca'))

Strings are Immutable
1. You cannot change strings! They are immutable.
  
  s = 'abcde' s[2] = 'z' # Error! Cannot assign into s[i]
2. Instead, you must create a new string
  
  s = 'abcde' s = s[:2] + 'z' + s[3:] print(s)
Some String-related Functions
1. input(), str(), and len()
  
  name = input('Enter your name: ') print('Hi, ' + name + '. Your name has ' + str(len(name)) + ' letters!')
2. chr() and ord()
  
  print(ord('A')) # 65 print(chr(65)) # 'A' print(chr(ord('A')+1)) # ?

Some String Methods

Character types

s.isalpha()	True if s only contains letters A-Z,a-z
s.isdigit()	True if s only contains digits 0-9
s.isspace()	True if s only contains whitespace (spaces, tabs, newlines)
s.islower()	True if the letters in s are all lowercase (a-z)
s.isupper()	True if the letters in s are all uppercase (A-Z)

String edits

s.lower()	Returns a copy of s in lowercase
s.upper()	Returns a copy of s in uppercase
s.strip()	Returns a copy of s without leading or trailing whitespace
s.replace(old, new)	Returns a copy of s replacing occurrences of old with new

For example:

print('This is nice. Yes!'.lower()) print('So is this? Sure!!'.upper()) print(' Strip removes leading and trailing whitespace only '.strip()) print('This is nice. Really nice.'.replace('nice', 'sweet'))

Substring search

s.count(t)	Returns number of times t occurs in s
s.startswith(t)	Returns True if s starts with t
s.endswith(t)	Returns True if s ends with t
s.find(t)	Returns the index of where t starts in s, or -1 if not there
s.index(t)	Returns the index of where t starts in s, or crashes if not there

For example:

print('This IS a history test'.count('is')) # 2 print('-------') print('Dogs and cats!'.startswith('Do')) # True print('Dogs and cats!'.startswith("Don't")) # False print('-------') print('Dogs and cats!'.endswith('!')) # True print('Dogs and cats!'.endswith('rats!')) # False print('-------') print('Dogs and cats!'.find('and')) # 5 print('Dogs and cats!'.find('or')) # -1 print('-------') print('Dogs and cats!'.index('and')) # 5 print('Dogs and cats!'.index('or')) # crash!

Basic File IO
# Note: As this requires read-write access to your hard drive, # this will not run in the browser in Brython. def readFile(path): # This makes a very modest attempt to deal with unicode if present with open(path, 'rt', encoding='ascii', errors='surrogateescape') as f: return f.read() def writeFile(path, contents): with open(path, 'wt') as f: f.write(contents) contentsToWrite = 'This is a test!\nIt is only a test!' writeFile('foo.txt', contentsToWrite) contentsRead = readFile('foo.txt') assert(contentsRead == contentsToWrite) print('Open the file foo.txt and verify its contents.')

CMU 15-110: Principles of Computing Strings

CMU 15-110: Principles of Computing
Strings