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Lecture 3 — Python Strings

Reading​

This material is drawn from Chapter 4 of Practical Programming, 2nd edition.

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 output from your Python programs

Strings — Definition

  • A string is a sequence of 0 or more characters delimited by single quotes or double quotes.
'University of Ghana'
"Legon, Accra"
'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 the Python function print to output the variable and asking the Python interpreter directly 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.\""

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

Practice Problems - Part 1​

We will go over these during training:

  1. Which are valid Python strings:
>>> s1 = '"Hi mom", I said.  "How are you?"'
>>> s2 = '"Hi mom", I said.  '"How are you?"
>>> s3 = '"Hi mom", I said.  '"How are you?"'
>>> s4 = """'Hi mom", I said.  '"How are you?"'"""
>>> s5 = ""I want to be a lion tamer!"'
>>> s6 = "\"Is this a cheese shop?\"\n\t'Yes'\n\t\"We have all kinds!\""

For those that are not valid, what needs to be fixed? For those that are, what is the output when they are passed to the print function?

  1. What is the output?
>>> s = "Cats\tare\n\tgood\tsources\n\t\tof\tinternet\tmemes"
>>> s
>>> print(s)
  1. What is the output?
print('\\'*4)
print('\\\n'*3)
print('Good-bye')
  1. Which of the following are legal? For those that are, show what Python outputs when these are typed directly into the interpreter.
>>> 'abc' 'def'
>>> 'abc' + 'def'
>>> 'abc ' + 'def'
>>> x = 'abc'
>>> y = 'def'
>>> x+y
>>> x y
>>> s1 = 'abc'*4
>>> s1
>>> s2 = 'abc '*4
>>> print(s2)

String Operations — Functions​

  • Python provides many operations for us to use in the form of functions. We have already seen print(), but now we are going to look at other functions that operate on strings.

  • You can compute the length of a string with len().

>>> s = "Hello!"
>>> print(len(s))
  • 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 provided string using its own code, code that is built-in to Python.

  3. len returns the calculated value (in this case, 6) and this value is sent to the print function, which actually generates the output.

  • We will learn more about using functions in Lectures 4 and 5.

Example String Functions​

  • We will look at examples of all of the following during training…

  • 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()

The print Function in More Detail​

  • We already know a bit about how to use print(), but we can learn about more using help()
help(print)
Help on built-in function print in module builtins:

print(...)
    print(value, ..., sep=' ', end='\n', file=sys.stdout, flush=False)

    Prints the values to a stream, or to sys.stdout by default.
    Optional keyword arguments:
    file:  a file-like object (stream); defaults to the current sys.stdout.
    sep:   string inserted between values, default a space.
    end:   string appended after the last value, default a newline.
    flush: whether to forcibly flush the stream.

  • flush is useful when trying to debug. If you are trying to trace your program execution using print, adding flush=True as your final argument will give you more accurate results. We will talk about this more later.

  • For now, we will focus on the sep and end and illustrate with examples.

User Input​

  • Python programs can ask the user for input using the function called 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(input())
print('The square of', x, 'is', x*x)
  • We can also insert the string right into the input function call:
x = input("Enter a new number ")
x = float(x)
print('The square of', x, 'is', x*x)
  • A similar function exists to convert a string to an integer:
x = input("Enter an integer ")
x = int(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 (run from the command line).

    • It will be posted on the course website.

Practice Problems - Part 2​

  1. What is the output for this Python program?
print(len('George'))
print(len(' Tom  '))
s = """Hi
sis!
"""
print(len(s))
  1. Which of the following are legal? For those that are, show what Python outputs when these are typed directly into the interpreter.
>>> 'abc' + str(5)
>>> 'abc' * str(5)
>>> 'abc' + 5
>>> 'abc' * 5
>>> 'abc' + 5.0
>>> 'abc' + float(5.0)
>>> str(3.0) * 3
  1. What is the output of the following when the user types 4 when running the following Python program?
x = input('Enter an integer ==> ')
x = x*2
x = int(x)
x *= 2
print("x is:", x)
  1. What is the output when the user types the value 64 when running the following Python program?
x = int(input('Enter an integer ==> '))
y = x // 10
z = y % 10
print(x, ',', y, z, sep='')

What happens when you do not have the call to the int function?

  1. Write a program that requests an integer from the user as an input and stores in the variable n. The program should then print n 1’s with 0’s inbetween. For example if the user input the value 4 then the output should be
1010101

Summary​

  • Strings represent character sequences — our third Python type

  • String operations include addition (concatenation) and replication

  • 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 \

  • The print() function offers significant flexibility.

  • We can read input using input()

Pratice Problems for review​

  1. Which of the following are valid strings? Upload a text file to Submitty that contains just the variable names that are assigned to strings that are correct. For example if only the first two were correct your file would contain s0 on the first line and s1 on the second. Please use the editor in the Spyder IDE to generate the file. Do not use Microsoft Word, Apple Pages or other document editors. They embed extra characters in the text that will make your answer incorrect.
>>> 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 "Final Five"'s picture?'


>>> s5 = "Have you seen the 'Final Five''s picture?"

  1. Submit a Python file that includes a single line of code that prints 25 '*' characters followed by 25 '+' characters, with no space in between. It must, of course, use the print function. The two characters must appear in your code much less than 25 times - at most three each!

  2. Write a program that assigns the value 4 to the variable x, the value 2 to the variable y and then uses exactly three print function calls to generate the output below (four lines, with the second line blank). The print calls must use the variables x and y rather than the values 4 and 2. The trick is to change the assignment of the sep and end parameters in the call to print. The character 4 is the first character on the 1st, 3rd and 4th lines of output.

4 2

4,2
42