Matthew Parsons M.Eng
Department of Mathematical and Physical Sciences
Concordia University of Edmonton
Introduction to Computing Science
Numbers and Strings
Programmers While Coding
Objectives
This lecture covers
 Numbers (revisited)
 Integer
 Floating point
 Complex
 Strings
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Defining
Escape sequences
Multi-line strings
String operations
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Numbers
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Numbers in Python are of three types – integers (int),
floating point (float) and complex (complex) numbers.
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Examples:
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integers: 5; -4; 0.
floating point numbers (or floats for short): 3.14; 3.1E-4; 7E10.
complex numbers: (-5+4j); (2.3 - 4.6j); j.
There is no separate 'long int' type. The default integer type
can be any large value.
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Numbers and Strings
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Numbers are fundamental to computing—in fact, crunching
numbers is what computers were invented to do—but there
are many other kinds of data in the world as well, such as
addresses, pictures, and music.
There is also a non-numeric data type that represents text,
such as the words in this sentence or a strand of DNA.
Computers may have been invented to do arithmetic, but
these days, most of them spend a lot of their time processing
text. Computers create text, store it, search it, and move it
from one place to another.
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Strings
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String is a sequence of characters (letters, numbers, and
symbols).
Strings are composed by words. The words can be in English
or any other language that is supported in the Unicode
standard, which means almost any language in the world.
Strings are used in almost every Python program.
By default, all Python strings are in Unicode.
There are no "ASCII-only" strings because Unicode is a
superset of ASCII.
Python strings are immutable, i.e. they cannot be changed.
The len(str) function returns the length of a string.
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Strings
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In Python, we indicate that a value is a string by putting
either single or double quotes around it:
>>> 'This is a string!'
'This is a string!'
>>> "This is also a string!"
'This is also a string!'
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You can specify multi-line strings using triple quotes - ("""
or '''). You can use single quotes and double quotes freely
within the triple quotes. An example is:
'''This is a multi-line string.
This is the second line. "We can quote."
'''
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Logical And Physical Lines
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A physical line is what you see when you write the program.
A logical line is what Python sees as a single statement.
Python implicitly assumes that each physical line
corresponds to a logical line.
An example of a logical line is a statement like
print('Hello World')
– if this was on a line by itself (as you see it in an editor),
then this also corresponds to a physical line.
Implicitly, Python encourages the use of a single statement
per line which makes code more readable.
Writing a logical line spanning many physical lines is
referred to as explicit line joining.
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Strings
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Suppose, you want to have a string which contains a single
quote ('), how will you specify this string? For example, the
string is What's your name?. You cannot specify 'What's your
name?' because Python will be confused as to where the string
starts and ends.
You will have to specify that this single quote does not
indicate the end of the string. This can be done with the help of
what is called an escape sequence. You specify the single
quote as \' - notice the backslash. Now, you can specify the
string as 'What\'s your name?'.
Another way of specifying this specific string would be
"What's your name?" i.e. using double quotes.
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Escape Characters/Sequences
Escape Sequence
Description
\n
ASCII end of line
\\
Backslash
\'
Single quote
\"
\a
\t
\v
Double quote
ASCII Bell
ASCII Horizontal Tab
ASCII Vertical Tab
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Strings
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A two-line string can be coded as
 a triple-quoted string.
 an escape sequence for the newline character - \n to
indicate the start of a new line.
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A single backslash at the end of the line indicates that the
string is continued in the next line, but no newline is added.
"This is the first sentence.\
This is the second sentence."
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The input() function
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The built-in function input reads a single line of text from
the keyboard. It returns whatever the user enters as a string,
even if it looks like a number:
>>> line = input()
CS rocks!
>>> print(line)
CS rocks!
>>> line = input()
123
>>> print(line*2)
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String Operations
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We can join two strings together by putting them side by side:
>>> 'This is a' 'string!'
'This is astring!'
It’s almost always clearer to join strings with +. When + has
two string operands, then it is referred to as the concatenation
operator:
>>> 'This is a' + ' string!'
'This is a string!'
Since the + operator is used for both numeric addition and for
string concatenation, we call this an overloaded operator. It
performs different functions based on the type of operands that
it is applied to.
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String Operations
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Python can repeat a string using the * operator.
If the integer is less than or equals to zero, then this operator
yields the empty string (a string containing no characters).
For each of the following expressions, what value will the
expression give? Verify your answers by typing the
expressions into the Python shell.
a) 'Comp' 'Sci'
b) 'Computing' + ' Science'
c) '1' * 3
d) '111' * 0
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String Operations
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Strings can be subscripted (indexed).
Substrings can be specified with the slice notation: two
indices separated by a colon.
What is the output of the following code?
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
sentence = 'Programming' + ' is fun!'
print(sentence)
print(sentence[0])
print(sentence[5])
print(sentence[0:10])
print(sentence[5:10])
print(sentence[:5])
print(sentence[5:])
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Question 1
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For each of the following phrases, express them as Python
strings using the appropriate type of quotation marks (single,
double or triple) and, if necessary, escape sequences:
a) They’ll hibernate during the winter.
b) "Absolutely not," he said.
c) "He said, ' Absolutely not, "' recalled Mel.
d) left\right
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Question 2
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Rewrite the following string using single or double
quotes instead of triple quotes:
'' 'A
B
C'' '
Use the built-in function len to find the length of the
empty string.
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