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Python strings
Girls’ Programming Network
School of Information Technologies
University of Sydney
Mini-lecture 5
Strings
raw input
Types
Summary
2
Outline
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Strings
2
Reading input with raw input
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Types
4
Summary
Girls’ Programming Network
if statements
Mini-lecture 5
Strings
raw input
Types
Summary
3
Strings hold a sequence of characters
ˆ 'Hello World' is a string
ˆ We need to be able to distinguish the string from the program
ˆ Strings are delimited (a.k.a. separated) using quotes
ˆ You can use either single (') or double (") quotes
ˆ But they’ve got to match
ˆ Syntax highlighting in IDLE turns strings green
Girls’ Programming Network
if statements
Mini-lecture 5
Strings
raw input
Types
Summary
4
What happens when they don’t match?
ˆ Try entering this string "hello world' . . .
ˆ What about 'don't' or "he said "hi""?
ˆ There are two ways to solve these problem:
1 use the opposite quotes: "don't" or 'he said "hi"'
2 escape the quote: 'don\'t' or "he said \"hi\""
Girls’ Programming Network
if statements
Mini-lecture 5
Strings
raw input
Types
Summary
5
A string of digits is not a number!
ˆ Python stores different types of values
ˆ The string type (str) isn’t the same as the number type (int)
ˆ Try this case:
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>>> 2*5
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>>> "2"*"5"
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: can't multiply sequence by non-int of type 'str'
>>>
Girls’ Programming Network
if statements
Mini-lecture 5
Strings
raw input
Types
Summary
6
But some arithmetic operators do work on strings
ˆ Addition of two strings is called concatenation
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>>> 1 + 1
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>>> "1" + '1'
'11'
ˆ Multiplication between strings and integers works too:
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>>> "2"*5
'22222'
>>> 5*"2"
'22222'
ˆ Why does multiplication make sense?
Girls’ Programming Network
if statements
Mini-lecture 5
Strings
raw input
Types
Summary
7
Python provides code for reading from the user
ˆ All you need to do is supply a message:
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>>> name = raw_input('Enter your name? ')
Enter your name? Jemima
>>> name
'Jemima'
>>>
ˆ raw_input is a Python builtin function
ˆ It takes a string argument to prompt the user with
ˆ Then it waits for input (just like the Python prompt does)
ˆ When the user presses Enter the input is returned
ˆ It is returned as a string and put in the name variable
Girls’ Programming Network
if statements
Mini-lecture 5
Strings
raw input
Types
Summary
8
Functions are named pieces of code that do something
ˆ When we refer to the code (followed by parentheses) it runs
ˆ This is called calling or invoking the function
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>>> name = raw_input('Enter your name? ')
Enter your name? Jemima
>>>
ˆ Our example calls the raw_input function
ˆ raw_input goes purple because it is builtin
ˆ When a function needs some information to do its work:
ˆ we put the extra information inside the parentheses
ˆ these are called function arguments
Girls’ Programming Network
if statements
Mini-lecture 5
Strings
raw input
Types
Summary
9
Function calls are substituted by their answers
ˆ When a function call is finished:
ˆ the answer (or value the function was calculating) is returned
ˆ it effectively replaces the function call in an expression
ˆ just like a variable is replaced by its value in expressions
ˆ raw_input returns a string containing the input:
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>>> name = raw_input('Enter your name? ')
Enter your name? Jemima
ˆ now this is the same as:
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>>> name = 'Jemima'
ˆ because raw_input is effectively replaced by the string
'Jemima'
Girls’ Programming Network
if statements
Mini-lecture 5
Strings
raw input
Types
Summary
10
Converting between strings and integers
ˆ You need to explicitly convert between types
ˆ The Python int function converts values to integers:
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>>> int('12345')
12345
>>>
ˆ Notice that there are no longer quotes around 12345
ˆ The Python str function converts values to strings:
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>>> str(12345)
'12345'
>>>
ˆ This time the quotes have been added because it is now a
string
Girls’ Programming Network
if statements
Mini-lecture 5
Strings
raw input
Types
Summary
11
raw input and int
ˆ Often you want to read an integer using raw_input
ˆ But raw_input only gives back strings
ˆ int to the rescue:
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>>> num
Enter a
>>> num
'5 '
>>> num
Enter a
>>> num
5
>>>
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= raw_input('Enter a number? ')
number? 5
= int(raw_input('Enter a number? '))
number? 5
if statements
Mini-lecture 5
Strings
raw input
Types
Summary
12
You should now be able to:
ˆ Create Python strings
ˆ Call Python functions
ˆ Read input from the user with raw_input
ˆ Convert between integers and strings using str or int
Girls’ Programming Network
if statements
Mini-lecture 5