Lecture 3 — Python Strings¶

Reading¶

This material is drawn from Chapter 3 of Practical Programming and Chapter 8 of Think Python.

More Than Just Numbers¶

• Much of what we do today with computers revolves around text:

  • Web pages
  • Facebook
  • Text messages

  These require working with strings.

• Strings are our third type, after integers and floats.

• We’ve already seen the use of strings in output,

  print "Hello world"
  x = 8
  y = 10
  print "Value of x is", x, "value of y is", y
  

Topics for Today¶

• String basics
• String operations
• Input to and (formatted) output from Python programs

Strings — Definition¶

• A string is a sequence of 0 or more characters delimited by single quotes or double quotes.

  'Rensselaer'
  "Albany, NY"
  '4 8 15 16 23 42'
  ''
  

• We can print strings:

  >>> print "Hello, world!"
  Hello, world!
  

• Strings may be assigned to variables:

  >>> s = 'Hello'
  >>> t = "Good-bye"
  >>> print s
  Hello
  >>> t
  'Good-bye'
  

• Notice that unlike integers and floats there is now a difference between asking Python to print the variable and asking Python for the value of the variable!

Combining Single and Double Quotes in a String¶

• A string that starts with double quotes must end with double quotes, and therefore we can have single quotes inside.

• A string that starts with single quotes must end with single quotes and therefore we can have double quotes inside.

• To illustrate this, we will take a look at

  >>> s = 'He said, "Hello, World!"'
  >>> t = "Many single quotes here '''''''  and here ''' but correct."
  

Multi-Line Strings¶

• Ordinarily, strings do not extend across multiple lines, causing an error if you try.

• But, starting and ending a string """ or ''' tells Python to allow the string to cross multiple lines.

  • Any character other than ''' (or """, if that is how the string started) is allowed inside the string.

• Example,

  >>> s1 = """This
  is a multi-line
  string."""
  >>> s1
  'This\nis a multi-line\nstring.'
  >>> print s1
  This
  is a multi-line
  string.
  >>>
  

• Notice the \n when we ask Python for the value of the string (instead of printing it). This is an escape character, as we will discuss next.

Escape Characters¶

• Inserting a \ in the middle of a string tells Python that the next character will have special meaning (if it is possible for it to have special meaning).

• Most importantly:

  • \n — end the current line of text and start a new one
  • \t — skip to the next “tab stop” in the text. This allows output in columns
  • \' — do not interpret the ' as a string delimiter
  • \" — do not interpret the " as a string delimiter
  • \\ — put a true back-slash character into the string

• We’ll explore the following strings in class

  >>> s0 = "*\t*\n**\t**\n***\t***\n"
  >>> s1 = "I said, \"This is a valid string.\""
  

Exercise Set 1¶

1. Which of the following are valid strings? Fix the mistakes to make them all valid.

   >>> s0 = "Sheldon Cooper's apartment is in Pasedena"
   
   >>> s1 = 'This cheese shop's cheese is all gone"
   
   >>> s2 = """We are
   "The Knights of the Round Table"
   """
   
   >>> s3 = "Toto, I said,\n"We aren't in Kansas, anymore!"
   
   
   >>> s4 = 'Have you seen the "Incredibly Photogenic Guy"'s picture?'
   
   
   >>> s5 = "Have you seen the 'Incredibly Photogenic Guy''s picture?"
   

2. What is the output?

   >>> s = "Cats\tare\n\tgood\tsources\n\t\tof\tinternet\tmemes"
   >>> print s
   

String Operations — Concatenation¶

• Concatenation: Two (or more) strings may be concatenated to form a new string, either with or without the + operator. We’ll look at

  >>> s0 = "Hello"
  >>> s1 = "World"
  >>> s0 + s1
  >>> s0 + ' ' + s1
  >>> 'Good' 'Morning' 'America!'
  >>> 'Good ' 'Morning ' 'America!'
  

• Notice that

  >>> s0 = "Hello"
  >>> s1 = " World"
  >>> s0 s1
  

  is a syntax error but

  >>> "Hello" " World"
  

  is not. Can you think why?

String Operations — Replication¶

• You can replicate strings by multiplying them by an integer:

  >>> s = 'Ha'
  >>> print s * 10
  HaHaHaHaHaHaHaHaHaHa
  

• What do you think multiplying a string by a negative integer or 0 does? Try it.

• Many expressions you might try to write involving strings and either ints or floats are illegal Python, including the following:

  >>> 'Hello' * 8.1
  >>> '123' + 4
  

  Think about why

String Operations — Functions¶

• Python provides many operations for us to use in the form of functions.

  • Our first example functions operate on strings.

• For example, you can compute the length of a string with len().

  >>> s = "Hello!"
  >>> print len(s)
  

  6

• Here is what happens:

  1. Function len is provided with the value of the string associated with variable s
  2. len calculates the number of characters in the string using its own code, code that is built-in to Python.
  3. len returns the calculated value (in this case, 6) for print to use.

• We will learn more about using functions in Lecture 4 and writing our own functions in Lecture 5. Writing good functions is crucial to good programming.

Example String Functions¶

• We will look at examples of all of the following during lecture.
• You can convert an integer or float to a string with str().
• You can convert a string that is in the form of an integer to an integer using int()
• You can convert a string that is in the form of a float to a float using, not surprisingly, float()

Exercise Set 2: String Operations¶

1. What is the output of the following:

   >>> len('George')
   >>> len(' Tom  ')
   >>> s = """Hi
   mom!
   """
   >>> len(s)
   

2. Which of the following are legal? For those that are, show what Python outputs.

   >>> 'abc' + str(5)
   >>> 'abc' * str(5)
   >>> 'abc' + 5
   >>> 'abc' * 5
   >>> 'abc' + 5.0
   >>> 'abc' + float(5.0)
   >>> str(3.0) * 3
   

3. Write a line of code that prints 50 '*' characters.

4. Write a program that starts with a string in a variable name and prints the string underlined with = equal to the length of the string. For example, we should have the following output:

   name = 'Monty Python'
   

   should output

   Monty Python
   ============
   

   Use the len function and string replication.

String Output¶

• We already know a bit about how to use print, but here are a few things to remember.
  • A space is added between each value that is output in a print statement
  • Each print statement starts a new line of output... unless the previous print statement ended with a ,
• But, let’s look at some nicer ways to create output...

Formatted Output¶

• Let us look at two different ways to output results

  length = 6.823
  width = 2.512
  height = 5.32
  area = 2* (length*width + length*height + width*height)
  volume = length*width*height
  
  print "Rectangular prism with length:", length, ", width:", width, ", height:", height
  print "Area:", area, "Volume", volume
  

• The program above produces the output

  Rectangular prism with length: 6.823 , width: 2.512 , height: 5.32
  Area: 133.603152 Volume 91.18148032
  

• Here is better formatting for the print statements, without the insignificant values and extra spaces

  print "Rectangular prism with length: %.3f, width: %.3f, height: %.3f" %(length, width, height)
  print "Area: %.3f, Volume: %.3f" %(area, volume)
  

  which produces

  Rectangular prism with length: 6.823, width: 2.512, height: 5.320
  Area: 133.603, Volume: 91.181
  

• We will discuss the significance of

  • %d
  • %.2f
  • %(radius,height)

User Input¶

• Python programs can ask the user for input using the function call raw_input.

• This waits for the user to type a line of input, which Python reads as a string.

• This string can be converted to an integer or a float (as long as it is properly an int/float).

• Here is a toy example

  print "Enter a number"
  x = float(raw_input())
  print "The square of %.1f is %.1f" %(x,x*x)
  

• We can also insert the string right into the raw_input function call:

  x = float(raw_input("Enter a new number "))
  print "The square of %.1f is %.1f" %(x,x*x)
  

• We will use this idea to modify our area and volume calculation so that the user of the program types in the numbers.

  • The result is more useful and feels more like a real program (albeit one run from the command line).
  • It will be posted on the course website.

Summary¶

• Strings represent character sequences — our third Python type

• String operations include addition (concatenate) and replication

• We can concatenate by ’+’ or by using formatted strings:

  >>> 'a' + 'b'
  >>> '%d eggs and %s spam' %(2,'no')
  

• Functions on strings may be used to determine length and to convert back and forth to integers and floats.

• Escape sequences change the meaning of special Python characters or make certain characters have special meaning.

• Some special characters of note: \n for new line, \t for tab. They are each preceded by \

• Output may be nicely formatted using %

• We can read input using raw_input()

What to work on before next class:¶

• Go through all the exercises in the course notes 2 and 3.
• There are many ways to write strings, so you should spend some time on learning how strings work before using them in programs.
• Creating strings: Create strings in many different ways. Write a program that creates different strings and prints them

• Strings with single, double, triple quotes
• Strings with a new line or backslash in them
• Strings with three lines and three columns separated by tabs

>>> x = 'abc'
>>> y = "def"
>>> z = """ghi"""
>>> w = "def\\abc"
>>> x = 'abc\ndef'
>>> y = 'abc\tdef\nghi\tklm'

Now try:

>>> y
>>> print y

What is the difference? Can you guess what the string will look like from the way it is written? Remember the quotes must match.

• String functions: Practice all the functions to make sure you understand how they work.

• Concatenation (what is the output?)

  >>> 'abc' 'def'
  >>> 'abc' + 'def'
  >>> 'abc ' + 'def'
  >>> x = 'abc'
  >>> y = 'def'
  >>> x+y
  >>> x y
  

• Replication

  >>> 'abc'*4
  >>> 'abc '*4
  

• String length len()

  >>> x = 'abc'
  >>> len(x)
  

• String conversion

  >>> x = str(3)
  >>> y = int('3')
  >>> z = float('3')
  >>> z = float('3.2')
  >>> w = 4.5
  >>> str(4.5)
  

  Make some mistakes and see what happens:

  >>> x = int('abc')
  >>> y = '3'
  >>> y+5
  >>> z = 'abc"
  >>> x = int('3.2')
  

  You see int() works differently for strings. The string must contain an integer for it to work.

• Printing strings: now understand how to print strings

  >>> x = 'abc'
  >>> print x
  >>> x = "'abc'"
  >>> print x
  >>> x = """ "abc" """
  >>> print x
  >>> print 'This is lecture %d of Spring 2016' %5
  >>> numlecture = 5
  >>> print 'This is lecture %d of Spring 2016' %numlecture
  >>> semester = 'Spring'
  >>> year = 2016
  >>> print 'This is lecture %d of %s %d' %numlecture
  >>> print 'This is lecture %d of %s %d' %(numlecture, semester, year)
  

  Compare with:

  >>> print 'This is lecture', numlecture, 'of', semester, year
  

  Every time you put a comma, a new space is added.

  >>> pi = 3.14159
  >>> print 'value of pi is', pi
  >>> print 'value of pi with 4 decimal points is %.4f' %pi
  >>> print 'value of pi with 2 decimal points is %.2f' %pi
  

  Note the values are rounded when you use formatting.

• Reading input: Try the following and learn the difference

  >>> x = raw_input()
  >>> x = raw_input('Enter a value ==> ')
  >>> x = int(raw_input('Enter a value ==> '))