Lecture 5 — Python Strings¶

Reading¶

This material is drawn from Chapters 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 message
These require working with strings.
Strings are our third type, after integers and floats.

We’ve already started to use strings in our output, for example,

def area_and_volume(radius, height):
    print "For a cylinder with radius", radius, "and height", height
    print "The surface area is ", area_cylinder(radius,height)
    print "The volume is ", volume_cylinder(radius, height)

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 for the value of the variable and printing 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

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

Exercise Set 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?"

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:
```
>>> 8 * 'Hello'
>>> 'Hello' * 8.1
>>> '123' + 4
```
Think about why

String Operations — Functions¶

You can compute the length of a string with len().
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()
We will look at examples of all of these during lecture.

Exercise Set 2: String Operations¶

What is the output of the following:

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

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

Write a line of code that prints 50 '*' characters.
Write a function that takes a string as an argument and prints the string underlined with = equal to the length of the string. For example, we should have the following output:
```
underline('Tom')
print
underline('Super Bowl')
```
should output
```
Tom
===

Super Bowl
==========
```
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¶

In the Lecture 3 Python program area_volume.py, the last few lines are

def area_and_volume(radius, height):
    print "For a cylinder with radius", radius, "and height", height
    print "The surface area is ", area_cylinder(radius,height)
    print "The volume is ", volume_cylinder(radius, height)

area_and_volume(5,10)

This produces the output

For a cylinder with radius 5 and height 10
The surface area is  471.2385
The volume is  785.3975

Here is better formatting, without the insignificant values

def area_and_volume(radius, height):
    print "For a cylinder with radius %d height %d" %(radius,height)
    print "The surface area is %.2f" %area_cylinder(radius,height)
    print "The volume is %.2f" %volume_cylinder(radius, height)

area_and_volume(5,10)

which produces

For a cylinder with radius 5 height 10
The surface area is 471.24
The volume is 785.40

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 4 and 5.
Review how you can import modules to use. Import and use the math module.
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 quotes

Strings with double quotes

Strings with triple quotes

Strings with a new line in them

Strings with a backslash in it

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 2014'
>>> print 'This is lecture %d of Spring 2014' %5
>>> numlecture = 5
>>> print 'This is lecture %d of Spring 2014' %numlecture
>>> semester = 'Spring'
>>> year = 2014
>>> 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.

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

Note the values are rounded when you use formatting.

Reading input: Try the following and learn the difference

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

And make some mistakes:

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

See if you can spot the mistake before you run it.

Lecture 5 — Python Strings¶

Reading¶

More Than Just Numbers¶

Topics for Today¶

Strings — Definition¶

Combining Single and Double Quotes in a String¶

Multi-Line Strings¶

Escape Characters¶

Exercise Set 1¶

String Operations — Concatenation¶

String Operations — Replication¶

String Operations — Functions¶

Exercise Set 2: String Operations¶

String Output¶

Formatted Output¶

User Input¶

Summary¶

What to work on before next class:¶

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Lecture 5 — Python Strings¶

Reading¶

More Than Just Numbers¶

Topics for Today¶

Strings — Definition¶

Combining Single and Double Quotes in a String¶

Multi-Line Strings¶

Escape Characters¶

Exercise Set 1¶

String Operations — Concatenation¶

String Operations — Replication¶

String Operations — Functions¶

Exercise Set 2: String Operations¶

String Output¶

Formatted Output¶

User Input¶

Summary¶

What to work on before next class:¶

Table Of Contents

Previous topic

Next topic

This Page

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