Seminar 5:
Data Types
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Learning Objectives
In this lecture, you will learn
● Strings
● Numbers
● List
● Dictionary
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Strings
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•
•
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Sequence of characters
It can contains text, symbols and numbers
Numbers will be treated as text.
Embedded within:
– Single quote (for single line): ‘ ’
– Double quote (for single line): “ “
• Double quote is preferred.
– Triple quote (for multi lines): “””
”””
• Triple quote preserves the original formatting including line
breaks.
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Strings Examples
– Single quote (for single line): ‘ ’
• str1 = ‘this is sentence 1.’
– Double quote (for single line): “ “
• str2 = “this is sentence 2.”
– Triple quote (for multi lines): “””
”””
• str3 = “””this is sentence 3.
the is second line.
this is the third line. “””
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String sequence
• String associates each character with an
index number.
• Index number starts from 0.
• Use square bracket to embed index number.
• To refer to a particular character, refer to it
using the format:
– string_var_name[index]
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String reverse indexing
• Default indexing starts from the left.
• Reverse indexing starts from the right,
using the notation of negative.
• To refer to a particular character, refer to it
using the format:
– string_var_name[-index]
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String slicing
• Slicing extracts a subset of string sequence.
• Syntax as follows:
– string_var_name[start: stop]
– start: starting index of extraction
– stop: stopping index of extraction, excluding last
position
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String slicing
• The default index for start is 0.
• The default index for ending if not specified is
assumed to be till the end of string.
• Slicing also works for step change, syntax as
follows:
– string_var_name[start: stop: step]
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String functions
• Python default
package consists
of rich set of
functions
supporting string.
• We will learn a
few important
ones.
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String functions
• Function like
capitalize returns a
string that capitalize
the string given, in
this case the string
given is text.
• Other types of
function like isXxxx()
returns Boolean
value indicating if the
given string is for
example a kind of
digit or alphabet.
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String formatting
• Strings can be combined using
concatenation:
– Syntax:
• new_string = string1 + string2 + ….
– Example:
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String format
• String formatting using .format() allows for greater flexibility
and better presentation.
• Syntax:
– ”Text {:formatting for variable} other text ”.format(matching
variable/ value)
• In the above syntax, the expression started with a pair of
double quote which contains the desired output string.
• The {} within the output string contains the formatting method,
formatting can indicate whether it is a string (s), integer (d),
float (f).
• It can also contain width (in numbers) and alignment
information. Left align (<), right align (>) and center align (^).
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String format
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See the following example.
The first curly bracket formats the first string variable
text1 to have 10 spaces in total, aligned to center.
• Second formatting doesn’t add additional spaces or
instruction.
• Third one will shift the text3 variable to be right aligned.
• Last variable is an integer variable.
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Formatting numbers
• The number of precision can be specified
in {} with .2 for 2 decimal point.
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Using f”…{var}…” for string formatting
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List
• For the data types we have learnt thus far, a
single variable is used to store single value.
• To store multiple values in one single
variable name, we use data structures.
• List is one form of the data structure.
• Syntax:
– listname = [value1, value2, value3…]
• value can be of number, string, or other
object.
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List
• Defining list with defined values.
• Add in more values by using append()
function from list.
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List
• List is similar to strings.
– Can access individual component using index.
• Eg. employee_names[0]
– List can be concatenated using +
– List can be sliced in a similar manners as string
– len() can be used to get the size of list.
• Eg. len(employee_names)
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List functions
• len(list_name): returns the size of list
• min(list_name): returns the minimum
value in the list
• max(list_name): returns the maximum
value in the list
• sum(list_name): returns the sum of all
numbers in the list
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List iteration
•
for-loop can be applied to string or list variables to
provide access to individual elements.
• This often allows for greater usability to manipulate the
contents.
• The first for-loop iterate the list and store each value of
list into a temporary variable called element.
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Other list functions
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These methods should be applied to list name. It will
change the original list.
–
–
–
–
•
append()
insert()
sort()
reverse()
Example:
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split()
• split() is a string function that is used to
decompose the contents of split.
• A string can be split according to the
delimiter.
• After split, it returns a list contains all the
split values.
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Tuple
• Sequence of data stored in one single variable name.
• Tuple is immutable, value remains the same.
• Data can be different types of Python objects like
numbers, string, or other objects.
• Objects in tuple can be different types.
• Similar to List, it is accessed via index.
• Different from List, it uses brackets instead of square
brackets.
• Syntax:
– tuple_name = (value1, value2, value3…)
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Tuple
• Defining list with defined values.
• When function returns multiple values, it’s
in tuple form.
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Tuple functions
• The immutable tuples do not come with
functions like list that changes the data.
• tuple_name.count(“a”): returns the number of
items whose content matches with “a”.
• tuple_name.index(“a”): returns the index
number of first “a” in tuple_name.
• Other functions which are applicable to tuple:
– enumerate(), len(), max(), min(), sorted(), sum(),
– tuple() # convert a data type to tuple form.
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Dictionary
• A type of data structure that creates key and value
pair.
• Each key can be used to retrieve its corresponding
value.
• Key acts as an index to find its associative value.
• It is similar to how you look up the dictionary where the
word is the key. The definition is the value.
• Dictionary is created using curly bracket { }
• Within the { } are pairs of keys and values. Key and
value are separated by colon, and each pair is
separated by comma.
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Dictionary
• The general structure is as follows:
dict_name = { “key1”: value1, “key2”:
value2}
dict_name
key1
value1
key2
value2
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Dictionary
• Key must be unique. It can be of type
string or integer.
• Value can be string, integer, list and etc.
• It is different from list which goes by
sequential index.
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Dictionary
• Start with declaring an empty
dictionary variable.
• Then add in record one by one.
• To retrieve the value corresponds to
the key, use square bracket.
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Value in dictionary
• See the following example that stores list
of customer details.
• Accessing individual list value is the same
approach as list using square bracket
denoting the index number.
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Useful methods for dict.
• len(dict_name) # returns the number of items
in this dictionary.
• dict_name.clear() # to empty the dictionary
• dict_name.items() # returns pairs of key
value
• dict_name.keys() # returns all keys in the
dictionary
• dict_name.values() # returns all values in the
dictionary
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Iterating dictionary
• Iterating through the variable directly:
for each in dict_name:
# each will store the one key for each iteration
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Iterating dictionary
•
Iterating through items:
for key, value in dict_name.items():
# key and value can be called directly within for loop.
# key and value change for each iteration
• Iterating through values:
for value in dict_name.values():
# this keeps just the value for each iteration
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You have learnt...
1. To use strings and its function.
2. To format the presentation of strings and numbers.
3. About list and use it to store multiple data of the same type.
4. How to use dictionary to store key and value pairs.
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