python. what is python? a programming language we can use to communicate with the computer and solve...
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Python
What is Python?
• A programming language we can use
to communicate with the computer
and solve problems
• We give the computer instructions
that it will execute
What is Python?
• Python is a high level language that we
(humans) can understand
– Other examples include C++ and Java
• A machine can only understand low level
language
• So we process the high level to low level so
the computer will understand!
Translation• How do we translate high level
languages to low level languages?
–We use interpreters and compilers!
– Interpreters process the program a little
bit at a time and runs it
– Compilers translate everything before
running it
Some Python Vocab
• Program – a file of code that may contain functions
• Script – a short code that we can run in a
command line
• Variable – names we assign the values to, allowing
us to reuse them later on
– For example: x = 1 or msg = “Hello world!”
– Variables can be changed throughout a program
– For example: x = 1, x = x + 1
Python Vocab• Comments – notes ignored by the
computer
– For example: x + y # variables
store user input
• Operators – mathematical symbols
–+, -, *, /, ** (exponents), ==
(equality)
Python Vocab
• Keyword – words with meaning/purpose in
Python
– For example: “and”, “print”, “if”
• Expression – statements that produce values
– 3 + 5, “Hello world!”
• Error – program has a problem in the command
area
• Instance – one run-through of a program
Indentation
• A REQUIREMENT IN PYTHON!
• Indenting specifies the “scope” of
different chunks of your code
– Everything indented after a first,
unindented line “belongs” to that line!
Things to Note• Python is case sensitive
– A function called “first” is different than a function called “FIRST”
or “First” or “fiRSt”
• Python doesn’t like spaces or punctuation marks
– You can’t name your function “spam Five” or “spam.Five”
– You could, however, name your function“spamFive” or “spam_Five”
• Some words in Python can’t be used as names
– Keywords can never be used as function/variable names
– Check the colors of the words! Purple and orange are
KEYWORDS!
LET’S PLAY WITH PYTHON!
Our age(number) function
• We had an argument passed into our
function!
• The argument is known as a parameter
• Example
def add(a, b):
print(“This is a + b: “, a+b)
• a and b are the parameters
Data Types
• Numeric
– Integers (5, 2, -1)
– Floating Point Numbers (0.2, 3.14159, 28.92)
• Non-numeric
– String (text), lists, dictionaries, etc
– Basically anything you can’t add up using a
simple plus sign (+)
Not a String? Not a Problem!
• You can format outputting variables
you’ve already defined
x = 42
print “The value of x is”, x,
“.”
• What does this print out?
Not a String? Not a Problem!
• The output is
– The value of x is 42.
• The bottom will cause an error.
x = 42
print “$” + x
• We can’t combine string and numbers. So
what do we do?
Not a String? Not a Problem!
• We can make our numerical variable
a string!
x = 42
print “$” + str(x)
• This will print out $42
More on Variables• Variables can hold all kind of values, including strings,
numbers, and user input
• To assign a string value to a variable, you have to wrap the
string in quotes
firstName = “Jessi”
lastName = “Cheung”
mathProblem = “5 + 5”
print lastName, “,”, firstName, “;”, mathProblem
• What will this print?
More on Variables
• The output is: Cheung, Jessi; 5 + 5
• Variables can also be assigned new values that
are relative to their old values
total = 10
print “Original total:”, total
total = total + 4
print “New total:”, total
• What does this print?
More on Variables• The output is
Original total: 10
New total: 14
• Remember: a variable has to be defined on a previous line
before it can be used on the right-hand side of an equation
ABC = ABC + 4
print “ABC:”, ABC
• ERROR. There was no mention of the value of “ABC”
before the line trying to redefine it.
Python Arithmetic• Try typing the following code in your program area and see what comes out!
def main():
a = 12
b = 2
c = 16
d = 3
e = 2.5
print “The value of a is”, a
print (a / b) * 5
print a + b * d
print (a + b) * d
print b ** d
print c – e
a = a + b
print “The value of a is”, a
Python Arithmetic
• Is this what you got?
the value of a is 12
30
18
42
8
13.5
the value of a is 14
Exercise time!
• Write a program that takes in a
Celsius temperature (celsius) and
returns the temperature in
Fahrenheit
– Hint: To get Fahrenheit, multiply the
Celsius by (9.0/5.0) and add 32
Taking User Input
• Sometimes, instead of passing in an
argument as a parameter, we can
have the computer ask us what we
want!
Taking User Inputname = requestString("Enter your name:")
print name
first pops up a dialog box (where you can enter a name, say
‘John Doe’):
then outputs
John Doe
Taking User Input
• Let’s try it with numbers!
def requestNumber():
num = input(“Enter a number:”)
print “Your number is:”, num
print “Your number squared:”,
num*num
Taking User Input
• What if you tried inserting a string into…
def requestNumber():
num = input(“Enter a number:”)
print “Your number is:”, num
• If you type hello, there will be an error. If you
type “hello”, it will work
• This is where raw_input comes into play!
Taking User Input
• raw_input will take exactly what you
type and make it into a string
def requestName():
name = raw_input(“Enter your
name:”)
print name, “is awesome!”
• Try typing in a number!
Let’s Write a Program!
• Let’s write a program that will calculate the area and
the circumference of a circle!
• Open a new window (File/New Window)!
• At the top of your (blank) file, write the following:
# file name: circle.py
# author: Jessi Cheung
# description: a program to calculate the area and
the circumference of a circle
• Save the program as circle.py
Let’s Write a Program
• Let’s define this program as main.
def main():
• Your turn!
– Use input to ask the user for the
radius!
Let’s Write a Program
• We now have (besides our heading)
def main():
radius = input(“What is the radius? “)
• Now let’s start the calculations!
– Circumference of a circle: Pi (3.14) times (radius
times two)
– Area of a circle: Pi (3.14) times (radius squared)
Let’s Write a Program!
• We now have (besides our heading)
def main():
radius = input(“What is the radius?
“)
circumference = 3.14 * (2 * radius)
area = 3.14 * (radius * radius)
Let’s Write a Program!
• Now for the finishing touches!
• Let’s print out the output so we can
see it!
Let’s Write a Program
def main():
radius = input(“What is the radius? “)
circumference = 3.14 * (2 * radius)
area = 3.14 * (radius * radius)
print “The radius of our circle is”, radius
print “The circumference of our circle is”,
circumference
print “The area of our circle is”, area
Let’s RUN the Program!
• Once you save your program, press
F5 on your keyboard
• Nothing happens?!
Let’s RUN the Program!
• You must call your program!
• Call using main()
Let’s RUN the Program
• Another way you could run the
program…
For Loops
• Also known as the “definite loop” – we know
exactly how many times the loop will happen!
• Allows you to specify a list of items (numbers,
words, letters, etc.) and specify actions to be
performed on each one
• The official syntax for the for loop is:
for <var> in <sequence>:
<body>
Help the Kittens!
• You are working at an animal shelter, and you’re asked
to take a group of kittens and bathe, dry, and feed each
one individually
Use a Loop!
• Using a for-loop type notation, your
instructions would look like this:
Kittens = [kitty #1, kitty #2, kitty
#3, ...]
for kitty in Kittens:
bathe kitty
dry kitty
feed kitty
Basic Loop
• See what happens when you put in
this:
phrase = “Hello world!”
for letter in phrase:
print “the next letter is:”, letter
Basic Loop• The output!
the next letter is: H
the next letter is: e
the next letter is: l
the next letter is: l
the next letter is: o
the next letter is:
the next letter is: w
the next letter is: o
the next letter is: r
the next letter is: l
the next letter is: d
the next letter is: !
What Just Happened?
• Python went through the string one
character at a time, treating the
string like a sequence
• That means that the string can be
split into its components (the
characters)
Accumulator Variables
• When you’re using a for loop,
sometimes you might want to keep a
running total of numbers you’re
calculating, or recombine bits of a
string
Accumulator Variables
• Steps:
1. Define a variable for the first time before the loop starts
2. Redefine it as itself plus some operation in the body of
the for loop
total = 0
for num in [1,2,4,10,20]:
total = total + num
print “Total:”, total
• This will give the output
Total: 37
Accumulator Variables
• What is the point of accumulator
variables?
– Counting
– Keeping score
– Debugging
Conditional Statements
• Equals: ==
• Does not equal: !=
• Try this:
x = 1
if (x != 2):
print “Artemis rocks”
Want to learn more?
• Go to:
wiki.python.org/moin/BeginnersGuide