Assmnt 1

CRICOS Provider Number 00103D(Vic) 01266K (NSW) 02235J (SA)
ITECH1000/5000 Programming 1
Lecture 2: Some new concepts :
Data types, For Loops and
Functions

Assignment 1
May 28, 20132
Available on Moodle
Due in week 7, Friday April 27th
2012
Start early – written component and programming
component to complete

PASS
Sessions with peers
Start next week, airport lounge
Times on moodle
May 28, 20133

Extension work
Assignment 
Download sample programs from text
(moodle), run the file roller.py
Read text book - graphics
Watch moodle for times for extension
discussion group
(9.30am Friday morning?)
May 28, 20134

CRICOS Provider Number 00103D(Vic) 01266K (NSW) 02235J (SA)May 28, 20135
Last Week We Covered . . .
• Programs provide instructions for
computers
• Follow a strict syntax
• All (or most) programs
• Receive input (eg from keyboard)
• Process that data
• Provide output

• Data is stored in variables
• Variables enable access to values on the activation stack
• The use of variables depends upon where they appear in
the statement.
• Variables have their values set if they appear on the left
side of an assignment statement
• The values of variables are retrieved if they appear on the
right side of an assignment statement or in a print
statement

Last week: Activation stack
• We have said that a computer stores
values in its memory using variables.
• The program uses a structure that looks
like a stack (we call it the activation stack)
• When the program encounters a new
variable it makes space for its value on
top of the stack.

Last week: Expressions
• Made up of operators performing
operations on operands
• Evaluate to a value
• Used on right hand side of
assignment statements (and other
places)
May 28, 20138

Last week: Statements
• Instructions are encoded in
statements
• 3 types of statements:
• Assignment statements
• Input statements
• Output (print) statements
May 28, 20139

Last week: Assignment
statement
• Made up of three parts
• A variable on the left side of ‘=‘
• The operator ‘=‘
• An expression on the right side of ‘=‘
• If a variable appears on the right side of
‘=‘ its value is retrieved and used
• num1 = num1 + 3
• Answer is stored in assigned variable

• We saw how we could use Python
Shell interactively
• However, for programs that we use a
lot it’s best to make a permanent
record
• Make sure you save files with a .py
suffix

Question
• What is the ‘^’ operator?
• It’s the bitwise exclusive or. It
manipulates values at the level of
bits (0’s and 1’s)
• Way beyond the scope of this course
May 28, 201312

Last week: Main function
• All of our Python programs should start with a function
(yep – more on functions later) called ‘main’
• We define this function with the line
‘def main ( ):’ #don’t forget the ‘:’
• The instructions that belong to main are indented (one tab)
• The line ‘main ( )’ tells the program to execute the
instructions belonging to main
• This will all make a lot more sense when we discuss
functions in detail

Program Execution
• Now that we’ve written the instructions we need
to get the computer to perform the actions
• This is called running or executing the program
• The code is executed, one statement at a time,
starting with the statement at the top of the main
function
• When the program has completed the actions
from one statement, it moves onto the next.

Activation stack
num1 6.4
main()
num1 = input (“Enter the first number “)
num2 = input (“Enter the second number “)
result = num1 + num2
print result
Assume the user enters 6.4 and 3.6

Activation Stack
num2 3.6
num1 6.4
main ()
print result

Activation Stack
result 10.0
num2 3.6
num1 6.4
main ()
print result

Activation stack
result 10.0
num2 3.6
num1 6.4
main ()
print result

Today
• Data types
• A looping construct
• called the for loop
• Some built-in functions
• A function is code already written that we can
use to perform an action
• Saves us writing the code ourselves

Data types
• A data type reflect the sorts of data
we are currently working with
• A data type determine the range of
data and the operations that can be
performed on that data

Types of data
• Fundamental data types include:
• Integers (whole numbers) e.g. 5
• Floats (decimal numbers) e.g. 3.8
• Strings (of characters) using quotes
• Many programming languages use
single quotes for a single character
and double quotes for larger strings
• E.g. ‘J’ and “John”

For example
• 3 is an integer and so is 5.
• Knowing this, the program will allow us to do
arithmetic operations on them
• eg ‘+’, ‘-’, ‘*’
• That’s pretty straightforward when considering literals
like 3 and 5
• It becomes a little trickier when working with
variables which can have different types and different
stages of program execution

Strings
• Our third data type is called a string
• A string is textual – not numeric
• It consists of a group of characters contained in
quotes (either single or double)
• Example
• “John”, ‘John’, ‘3’, “4”, “Please enter a number:”

Identifiers, Strings, Literals
• Strings are surrounded by single or double
quotes (Python does not distinguish)
• Identifiers (variable names) are not
surrounded by quotes
• Literals are specific values such as “John”,
5, 3.8 (compare with variables)

Input with Strings
• name = input(“Enter your name: “)
• If “John” is entered that is fine
• If the quotes are forgotten then Python interprets
the response as an identifier or variable name
that is undefined – an error!
• An alternative is to use raw_input

raw_input( )
name = raw_input(“Enter your name: “)
• Any input is treated as a string
• Entering John or any other string without quotes is no
longer a problem
• However if raw_input() is used to enter a number:
• The number will be of type string
• Arithmetic is not possible without conversion using float()

Concatenation
• Refers to the combining of two or more strings
• E.g. name = “John” + ‘ ‘ + “Smith”
• The string expression on the right hand side is
evaluated (concatenated) and assigned to name
• ‘+’ here is the concatenation operator

Note
• We can only concatenate strings
• We cannot concatenate a string with an integer.
For example
“Jane: Age “ + 35
• Will give an error message
• Trivial question – what will happen if we try to
concatenate two integers?

Joining/Splitting strings
• String Concatenation operator ‘+’
“John” + ‘ ‘ + “Smith”  John Smith
• A library function which is the inverse of
concatenation
string.split(“John Smith”) [“John”, “Smith”]
The string has been separated at the space and
a list of two strings results

String of characters
• “John Smith” is stored as a list of
characters each of which can be accessed
by its index
index 0 1 2 3 4 5 6 7 8 9
Character ‘J’ ‘o’ ‘h’ ‘n’ ‘ ’ ‘S’ ‘m’ ‘i’ ‘t’ ‘h’
• E.g. “John Smith”[0]  ‘J’
• “John Smith”[8]  ‘t’

Substrings
• “John Smith”[0:3]  “Joh”
• The second index is NOT included
•
• “John Smith”[:3]  “Joh”
• If the first index is 0 it can be left out
• “John Smith”[2:6]  “hn S”
• Indexes 2,3,4,5 (6-2=4 characters)

Substrings
• “John Smith”[-1]  ‘h’
• Negative sign means start from the end
(-0 is the same as 0)
• “John Smith”[:-1]  ‘John Smit’
• Select up to but excluding the last
character

Converting type
• Functions: float(), int(), round
• Examples:
• float(3) # gives 3.0
• float(“3.8”) # gives 3.8
• int(3.8) # gives 3
• round(3.8) # gives 4

Converting type
Note that:
• float() adds .0 to an integer
• float() removes the quotes from a numerical
string
• int() cuts off any decimal part of the number
• round() expresses the number to the nearest
integer

Mixed arithmetic
• Any valid numerical expression that
contains at least one float will evaluate as
a float
• Otherwise the result will be of type integer
• It is not possible to do arithmetic with
numeric strings such as “2” + 7

Integer division
• We spent a bit of time last week looking at code using
integers and floats
• One surprise with integers might be integer division
num1 = 11
num2 = 4
result = num1 / num2
• Because num1 and num2 are both integers the result
of the division is also an integer
• Thus result will have the value 2 – not 2.75

Integer division - modulus
• 11 divided by 4 can be expressed
2 remainder 3
• Integer division gives us the whole number (2)
• The modulus operator ‘%’ gives us the remainder (3).
result = 11 % 4
• Will put the value 3 into the variable result
• The modulus operator cannot be used with floats

Floating point division
• A floating point number has a decimal part as well as
a whole number
• Eg 11.4, 454.83737 etc
• The decimal part might be 0. So
• 11 has data type int
• 11.0 has data type float
• If a division has at least one float then the result will
be a float
• result = 11.0 / 4
• Puts the value 2.75 into the variable result

Using our type conversions
• Consider the code
num1 = 11
num2 = 4
result = num1 / num2 #Result assigned ‘2’
• If we want 2.75 we can give one of the variables data
type float
result = float (num1) / num2 #or
result = num1 / float (num2)

Concatenation
• Refers to the combining of two or more strings
• E.g. name = “John” + ‘ ‘ + “Smith”
• The string expression on the right hand side is
evaluated (concatenated) and assigned to name
• ‘+’ here is the concatenation operator

Note
• We can only concatenate strings
• We cannot concatenate a string with an integer.
For example
“Jane: Age “ + 35
• Will give an error message
• Trivial question – what will happen if we try to
concatenate two integers?

Consider . . .
• A program that
• Prompts the user for a person’s name
• Creates a string greets that person
• Outputs that string
• Does this four times
• For example, if the user enters
“Jane” then “Hello Jane!” should be
output

Demo – sayHello.py
• Note that with this program we repeat the same
code four times
• What if we wanted to repeat it 10 times, 100
times . . .?
• This would become cumbersome very quickly
• Python provides constructs which enable us to
run the same code many times

Structured Programming
• Consists of
• Sequence
• Iteration (looping)
• Decision

#Prompts for a person's name and says greets that person
#Does this four times
def main():
name = input ("Enter a name: ") #prompt for name
result = "Hello " + name + "!" #prepare greeting
print result #output greeting
name = input ("Enter a name: ") #Do it again
result = "Hello " + name + "!"
print result
name = input ("Enter a name: ") #And again
print result
name = input ("Enter a name: ") #And a fourth time
print result
main()
#Notice how similar the four pieces of code are
#Wouldn't it be better if we could simply use the same piece of code 4 times
#Python provides looping structures which allow us to do this
May 28, 2013

Definite loops
• One of the great values of the computer is
that it can repeat actions indefinitely
(without tiring)
• The programming structure that allows
this is the loop
• A definite loop (called a for loop) repeats
loop code a fixed number of times

range
• The number of times the for loop
‘iterates’ is governed by the ‘range’
function
• ‘range(4)’ means 0,1,2,3, so that the
for loop iterates 4 times.

#uses a for loop to repeat greetings four times
def main():
for index in range(4): #do 4 times
name = input ("Enter a name: ")
# name = raw_input ("Enter a name: ")
greeting = "Hello " + name + "!"
print "Person number ", index
print greeting
main()
#The 'for loop' iterates 4 times
#The statements which are part of the loop are defined by the indenting
#Try taking the indenting away and see what happens
#We will also have a look at the variable 'index' to see what values it
contains
May 28, 201348

range
• range(4) takes the values 0, 1, 2, 3
so index is assigned these values in
turn
• adding index to the print statement
shows this

Some more looping examples
(see sum10.py)
• Consider a program that will sum all
of the integers between 1 and 10
• Extend this so that it prompts the
user for a start and finish number. It
then sums the integers between
these values.
• if the use enters 3 and 10 the output
will be 52

Sum10.py
# program to find the total of the integers between 1 and 10
def main():
total = 0
for index in range(10):
total = total + index + 1 # add the number to the total
print "Total: ", total # display the total
main()
May 28, 201351

Algorithm
Need range to start from num1 not
zero HOW?
Range to go up to num2
Loop
add val to total
Print total
May 28, 201352

Algorithm design (not worrying
about syntax)
Need range to start from num1 not
zero HOW?
Range to go up to num2
Input num1 and num2
Initialise total =0 WHY?
Loop from num1 to num2
add val to total
Print total
May 28, 201353

range function
Need to find out if I can start at
something other than zero… how?
Range is a function that has been
previously written as part of the
python library.
Google it: “python range function”
May 28, 201354

Consider
(see sumNumbers.py)
• A program that sums 4 numbers that
are entered by the user.

# program to find the total of all numbers entered by the user
# will cause an error because runningTotal not initialised before use
def main():
# input how many numbers will be entered (numberOfValues)
numberOfValues = input("How many numbers will be entered? ")
for index in range(numberOfValues): # repeat ‘numberOfValues’ times
num = input("Enter a number: ")
runningTotal = runningTotal + num # add the number to the total
# error at this line
print runningTotal,
print "Total: ", runningTotal # display the total
main()

Note
• The character “t” is a control
character which prints out a tab
• A control character is preceded by ‘’
so be careful when using the
backslash in a string
• Another control character is ‘n’
which means new line

Note (2)
• Notice that the print statement in the
inner loop finishes with a comma (,)
• This will tell the program not to print
a new line
• The next output will occur on the same
line

Python standard library
• Can access the library from the Help menu in
either the shell or the editor
• Provides functions additional to the built-in ones
• Has a math section and a string section
• Library functions are accessed via import
statements

Some library functions
>>> import math
>>> math.sqrt(36) displays 6
>>> import string
>>> string.lower(“UPPERCASE”)
>>> string.upper(“lowercase”)

Import statements
• These establish a link with the Python Standard
library
• Any particular import statement is only required
once:
• in any given program
• in an open Python shell
• A new Python shell requires new import
statements if library functions are required

Built–in functions
• A function is a small package of
Python code that performs a certain
task (function)
• Functions can be user-defined
• Coded by the programmer (see week 4)
• Functions can be built-in or library
• Written by someone else for our use
May 28, 201365

Built-in functions
• We have already seen some
examples of a built-in function
• We have used the input function to
retrieve user input from the keyboard
• We used the range function in a for loop
• We will see more examples of built-in
functions today
May 28, 201366

Functions
• When we start writing programs to solve more
complicated problems, we’ll find that we need to
write large amounts of code
• Some programs have millions of lines of code
• Putting all of this code into the main function
would make it impossible to work with
• Functions are a way to split code up into pieces
that can be dealt with individually

Functions
• Can be built-in
• Examples are float (), input () etc
• Included in an external library
• string and math libraries for example
• Can be user-defined
• The programmer writes them
• We will be covering built-in functions today

Advantages of Functions
• Code can be written, tested and debugged
in smaller pieces
• Allows a divide and conquer strategy for
dealing with your code
• Functions can be reused in different
situations meaning you don’t have to
rewrite code
• input ( ) – is a built-in function - think about
how many times we’ve already used and
reused that function

Splitting strings
• An library function which is the inverse of
concatenation
“John” + ‘ ‘ + “Smith”  John Smith
string.split(“John Smith”) [“John”, “Smith”]
• The string has been separated at the space and a
list of two strings results

The typical function
• Functions consist of code that carries out a
specific task (or ‘function’)
• Many will require arguments
• Many will return (output) data as the
prime reason for their existence, others
will produce what are sometimes called
side effects
• Eg input function

Familiar functions
• Built-in functions
• float(7)  7.0
• float(“7.8”)  7.8
• int(7.8)  7
• round(7.8)  8.0
• len(“a string”)  8
• Argument types are int, str, float, float
and str respectively
• Return types for these functions are
• float – two occasions
• int – three occasions

Familiar functions
• Library functions
• math.sqrt(144)  12.0
• string.split(“the end”)  [“the”, ”end”]
• math.sqrt() - the argument can be of type int or
float, a float is returned
• string.split() – the argument is a string and a list
of strings is returned
• Remember these libraries need to be imported

# sqrtExample.py
# a program to demonstrate the library function 'sqrt'
import math # library that holds the function 'sqrt'
def main ( ):
num = input ("Please enter a number: ")
numRoot = math.sqrt (num); # Use the library name
followed by '.'
print "The root of ", num, " is ", numRoot
main ( )
Argument (input)
to the function
math.sqrt()

Arguments
(see sqrtExample.py)
• In the example on the previous slide,
‘num’ is an argument of the function
‘sqrt’
• An argument is an expression which
must be evaluated.
• Eg. math.sqrt (num * 2)

Using functions
• When a function is used with a program, e.g.
math.sqrt(), we say the function is called
• The function from which the call is made (in this
case main ( )) is the calling function
• The argument of the function call becomes
essential input for the function
• When a function is called, control of execution
passes to the function

Using functions
• When a function has completed its
processing (terminates), control
returns to the calling function
• the calling function can make use of
the value returned by the called
function

Reuse of Code
• The functions that have been referred to
(and many others) are used time and time
again
• This is a significant example of reuse of
code, saving countless hours of work
• Modifying existing programs for slightly
different purposes is another important
example of reuse of code (see labs)

The black box
• Note that when we use these built-in
and library functions, we do not need
to know how they work
• We only need to know how to use them
• We only need to know the inputs and
outputs. The processing is already
done.

Functions
• Often require arguments
• Expressions or values which provide
input for the function
• Normally provide a return value
• A value, calculated by the function, that
we can use in our code
May 28, 201380

Example
num = input (“Enter a number”)
“Enter a number” is an argument
The value entered at the keyboard by the
user becomes the return value of the
function
The return value is used in the assignment
expression
May 28, 201381

Today we have covered
May 28, 201382
• Data Types – strings, integers, floats
• For loops
• Functions

Review
May 28, 201383
• Data types – strings, integers, floats
• Input() if you enter a string need to
enclose in “ “
• raw_input() no need for “ “
• Functions take arguments as input

Review
• Integer division gives an integer result (ignoring
the remainder)
• Operator % gives the remainder
• Built-in functions include: type(), float(), int(),
round(), str(), len()
• Library functions extend the range of functions
substantially – they require one-off import
statements

Definite loops
•A definite loop (called a for loop) repeats
loop code a fixed number of times
for index in range(x)
stmt
stmt

Review
May 28, 201386
• Functions enable us to design good programs and to re-use code
• Functions take input through arguments and return a result to the
calling code (usually)
def main() : #define the function main
stmt
stmt
main() # call the function main

Remember
• Giving full attention to labs is essential –
practice, experiment and have fun!
• Ensure you have each lab completed
before the next one
• The theory test in week 5 is based on
experience gained from the first four labs
(and lectures)

Assmnt 1

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