Transcript Pizza making algorithm

CMT1000: Introduction to
Programming
Ed Currie
Lecture 2A: Pizza
Pizza making algorithm
• Often when devising an algorithm,
we will first express it using
instructions at a high level of
abstraction, that is, in quite general
terms, without great detail.
• How can we do this for a pizza
recipe?
First stab
– 1. Make the base
– 2. Make the topping
– 3. Put the topping on the base
– 4. Cook it
Four smaller problems to solve;
Not sufficiently detailed yet
enable us to make the pizza
to
Stepwise refinement needed
• => an algorithm comprising
instructions sufficiently detailed for
our processor to follow.
• The detailed algorithm is said to be
more concrete, or at a lower level of
abstraction, than the original
attempt.
Refinement
• 1. Make the base => Make the base
according to the recipe on page 15
• 2. Make the topping ?????????
• 3. Put the topping on the base
????????
• 4. Cook it => Bake for 20 minutes
directly on the oven shelf.
It’s like a computer program:
• Input data
• Instructions
• Output
• An algorithm being executed is generally known
as a process, and the thing that is executing it is
called a processor
• Another analogy is that of the text of a play being
the algorithm, and the performance of the play
being the process.
Sub-programs
• Make the base according to the
recipe on page 15...
..is asking the cook to execute another
algorithm, the one on page 15 for
making a pizza base.
• This idea is very important in writing
computer programs.
Question
• What are the advantages of
subalgorithms?
Answer
• The main algorithm will be more
succinct
• The main algorithm will be easier to
understand, as the code for the
subalgorithm is not cluttering it up.
• The subalgorithm can be used over
and over again.
Question
• What would happen if an instruction
in an algorithm told the processor to
execute the algorithm of which the
instruction was a part?
Parameters
• “Make the base…” says follow the base-making
instructions on page 15...
•
...but using the quantities of ingredients (input)
listed in the current recipe,
• causing a double quantity of the base to be made.
• The same instructions can be followed to make
any required quantity of the base
• The quantities of ingredients are parameters to
the sub-algorithm.
Question
• What are the advantages of
parameterising an algorithm?
Answer
• The same algorithm instructions may
be told to operate on different data,
making the algorithm much more
general, versatile and useful.
Re-use of algorithms
• Sometimes we can re-use an
algorithm which we already have
available, as with ‘make the base’.
• This idea of re-use is also extremely
important in computer programming.
• Libraries
• Java API
Pseudocode and structured
programming
• The English language has enormous expressive
power and subtlety..
...but it is often hard to be precise when using
English to describe something accurately.
• Computer programming language instructions
have precisely one meaning.
• We often use pseudocode to express our
algorithms before final translation into
programming languages.
Exercise
• Try to find as many meanings as you
can for the sentence
“The woman made the robot fast”
Sequence
• Consider the instruction “Make the topping”
• We refined this into the instruction sequence:
- Heat the oil in a frying pan
- Add the onion, garlic and chillies and fry for 5
minutes
etc…
• We will now further refine this towards
something resembling a computer program,
using pseudocode notation.
Repetition
• Heat the oil in a frying pan...
• How can we be more precise about this?
• How long should we heat the oil for? Until
it sizzles when we add a piece of onion to
it.
• Express using a pseudocode construct
called a while loop.
Heat the oil in a frying pan
- Put oil in pan
- Light gas
- Place small piece of onion in pan
- WHILE onion doesn’t sizzle
Wait 30 seconds
– Semantics?
Question
– Which of the following are valid
boolean expressions?
– It is raining.
– Is it raining?
– Eat my shorts.
– Doh!
Question
–I am cold and I am wet.
–This statement is false.
–2 = 2
–2 > 8
Selection
–Add the onion
–Add the garlic
–Add the chillies
–Fry for 5 minutes
However, not everyone has a
taste for spicy hot pizza!
- Add
the onion
- Add the garlic
- IF eater likes hot pizza
Add the chillies
- Fry for 5 minutes
Some prefer browner onions
IF eater likes very brown onions
Fry for 10 minutes
ELSE
Fry for 5 minutes
Test and evaluate
• Does the pizza taste as we wanted it
to?
• Is it quick and easy to make?
• Are the ingredients appropriate?
Could one or more have been left
out?
• Does it cost too much to make?