By the end of this class, you will be able to:
Remember that when you see a line of code with
>>> you should type in the code and run it immediately.
When you see a block of code without
>>> you should type in all the code before running it.
Each variable in Python has a type (also called a class) that affects what you can do with it.
Here are some examples of types; there are many more.
Integer: whole number, e.g.
Float:] decimal approximations to real number, e.g.
Boolean: truth value, e.g.
String: piece of text, e.g.
List: list of other variables, e.g.~
[7, 175.38, "parrot"]
You can use
type() to find out the type of a variable.
>>> approximation = 175.38 >>> type(approximation) >>> secret_password = "dead parrot" >>> type(secret_password)
Different types have different operations that can be performed with them. This week you will look at the integer, float and Boolean types. You will see strings and lists in future weeks.
Integers are whole numbers and are stored exactly.
You can do the usual operations with integers: for multiplication, you use
*; for raising to a power, you use
**; and for the maximum and minimum of a set of numbers, you use
>>> a = 7 >>> b = 2 >>> print(a * b, a + b, a - b, -a, a**b) >>> print(max(a, b), min(a, b))
In general, if you divide two integers you will not get an integer. For example,
7/3 does not give a whole number answer. Python provides an integer division operator
// and an integer remainder operator
>>> print("7/3 is", 7 // 3, "with remainder", 7 % 3)
Exercise 2.1. Write down the answers to the following questions, using Python to help you find the answer.
Floating-point numbers are approximations to real numbers. These are not stored exactly like integer variables. You will notice this if you do high-precision calculations. Python has other variable types that can handle high precision better, but floating-point numbers are generally fine to use as long as you remember they are not exact.
>>> a = 7.2 >>> b = 2.5 >>> print(a * b, a + b, a - b, -a, a**b) >>> print(max(a, b), min(a,b))
You can often convert between between different types. For instance
int() will turn a float into an integer and
float() will turn an integer into a float.
>>> a = 3 >>> b = float(a) >>> c = int(b) >>> print(a, b, c) >>> print(int(5.2), int(5.99))
Exercise 2.2. (i) How can you tell from the output that
b is a float and not an integer? (ii) How does
int() round a number? Does it round down? Round up? Round to the nearest? Forget the decimal part? What do you think
int(-7.8) will be? Check your answer.
Here's a program that converts a temperature in Celsius to a temperature in Fahrenheit, except that I've missed out the way to convert from one to another. Type in the following program and put the correct expression where
temperature_in_C = float(input("Temperature in Celsius: ")) temperature_in_F = correct_expression print("Temperature in Fahrenheit is", temperature_in_F)
Run your program and check that it gives the correct answer of roughly 98.6 °F when you input the temperature of 37 °C.
Exercise 2.3. Write a program that asks for your height in metres and your weight in kilograms and prints out your body mass index (BMI). Check that your program gives a BMI of about 23.7 for a height of 1.78 m and a weight of 75 kg.
We can show the difference between
int being exact and
float being an approximation by calculating 12285 either as an integer or as a float.
>>> print(12 ** 285) >>> print(12.0 ** 285)
The output for the second statement is
3.686847070674066e+307, which is Python's way of writing 3.686847070674066×10307. This is only an approximation to the exact, first answer. Floating-point numbers are stored to about 17 significant figures of decimal.
We understand what it means for a statement to be true or false: ''Paris is the capital of France'' is true; whereas ''7>21'' is false. The terms 'true' and 'false' are known as truth values and you will study these in the logic part of MAS114.
In Python, truth values are written capitalized as
False. A Boolean variable is one whose value is either
False. (They are named after George Boole, one of the founders of modern logic.)
>>> a = 10 > 3 >>> print(a) >>> type(a) >>> print(7 > 21)
The assignment operation above looks a bit strange! It is saying ''let
a be the truth value of the statement
10 < 3'', which is clearly false. Note the difference between
print(7 > 21) and
print("7 > 21").
Boolean expressions are often built up using comparison operators. Here are the standard ones.
||is equal to||This is not the same as
||is not equal to|
||is greater than|
||is greater than or equal to||This is not the same as
||is less than|
||is less than or equal to||This is not the same as
It is important to note that
a = 3 and
a == 3 mean different things and should not be mixed up. The first statement is assigning the value 3 to
a, while the second is checking if
a has the value 3. Type them into the console and compare.
Style convention 1. Comparison operators have a single space either side of them. Type
a <= b and not
In MAS115 you will learn about the logical operations 'and', 'or' and 'not'. You can use these in Python as
>>> x = 25 >>> print((x > 20) and (x < 30))
Now edit the statement
x = 25 to
x = 35, re-run the
Exercise 2.4. Write a short program that inputs an integer and prints either ''It is True that your number is non-zero'' or ''It is False that your number is non-zero'', whichever is correct. (Note that the capitalization will be forced on you.)
Boolean expressions are key in
while loops, as you will now see. Next week you will see their role in conditional statements.
It is often the case that we perform processes repeatedly. For instance, when walking somewhere, you repeatedly put one foot in front of another until you have reached your destination.
Concept 1. A loop in a computer program is a sequence of commands that are repeatedly executed a certain number of times or until some condition is met.
while loops last week without looking at them in depth. In English we have a while-type construction. In the morning you might follow the instructions ''While your bowl has cereal in it, keep lifting out cereal with your spoon and putting it in your mouth.'' Python's
while loop works in a similar way.
The general form of a
while loop is as follows.
while boolean_expression: do_this then_this then_this after_all_that_carry_on_here
If the given Boolean expression is
True, the computer will perform the indented steps in order. Once it has completed the indented steps it will go back and check the Boolean expression again, repeating the steps in order, until it checks and finds that the Boolean expression is false.
Here's an example. Write down exactly what you think the following program will display. Now type it in and run it.
x = 3 while x < 8: print(x) x = x + 1 print("We've finished.")
Exercise 2.5. What will the following program do and why? What happens if you replace
1 < 2 with
1 > 2? Write down your answer and then type in and run the program.
while 1 < 2: print("Spam") print("Spam and eggs") print("Lobster thermidor")
print((3 < 2) or (10 == 5))?
total = 0 i = 10 while i > 5: total = total + i**2 print(i, total) i = i - 2 print("We finished with i =", i)