Section 2.5: Designing Programs

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• Section 2.5 Designing Programs

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REVIEW Design Recipe

• Figure out precisely what you need to do.
• Tell the computer how to do it.
• Check that the computer does it right.

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Design Recipe Version 1

• Figure out precisely what you need to do.
• 1. Understand the problem
• 2. Function contract
• 3. Write examples (in Scheme notation)
• Tell the computer how to do it.
• 4. Write a skeleton.
• 5. Fill in the function body.
• Check that the computer does it right.
• 6. Testing and debugging.

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Example X

• Purpose
• To determine the left-gtright location of a UFO.
• Contract
• X number -gt number
• Examples
• (X 3) should be 50
• (X 1.5) should be 35
• (X 0) should be 0

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Example X

• Skeleton
• (define (X t)
• )
• Function
• (define (X t)
• ( ( 10 t) 20))
• Testing/Debugging
• Check over yourself, then have DrScheme check
• over, then press Run and type in each example.

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Why do we need a recipe?

• Programming requires creativity, and sometimes
  thats enough.
• But when programs get big and complex, it helps
  to have a structure to follow.
• Analogous to rules of counterpoint, harmony, etc.
  in music. Sonata, rondo, virelai, fugue,
• Helps avoid blank page syndrome.

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What are function contracts?

• A contract does three things
• Specify function name
• Specify how many arguments, and what type(s)
• Specify what type is returned
• !

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What are function contracts?

• A contract does three things
• Specify function name
• Specify how many arguments, and what type(s)
• Specify what type is returned
• NOTE So far we are focusing on numbers. Soon we
  will see other types of data.

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What are function contracts?

• A contract does three things
• Specify function name
• Specify how many arguments, and what type(s)
• Specify what type is returned
• NOTE So far we are focusing on numbers. Soon we
  will see other types of data.
• Cant use any function correctly without knowing
  its contract!

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Examples of contracts

• takes two or more numbers returns a number
• / takes two numbers returns a number
• sqrt takes one number returns a number
• cos takes one number returns a number
• etc. etc.

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Shorter contract notation

• number number -gt number
• / number number -gt number
• sqrt number -gt number
• cos number -gt number
• etc. etc.

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cube Figure out what to do.

• 1. Understand the assignment yourself.
• Take any number and multiply it by itself. Then
  multiply that product by the original number.
• 2. Write a function contract.
• cube takes in a number and returns a number
• 3. Write examples (in Scheme notation).
• (cube 0) should be 0
• (cube 5) should be 125
• (cube -6) should be -216

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cube Tell the computer how to do it.

• 4. Write the function skeleton.
• (define (cube num)
• something involving num )
• 5. Write the function body.
• (define (cube num)
• ( num num num))

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cube Check if its done right.

• 6. Testing and debugging.
• Testing
• Type in (cube 0). Did you get 0?
• Type in (cube 5). Did you get 125?
• Type in (cube -6). Did you get -216?
• Debugging
• If the answers to any of these questions is no,
• look back to find your mistake.

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In conclusion

• The Design Recipe does not replace the need for
  thinking.
• It does guide the thinking process.
• It does have some concrete, automatic steps.
• Step 5 Writing the function body should
  hopefully be easier because of the first 4 steps.
• However, thinking and content-area knowledge are
  also essential, even when using a Design Recipe.

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Next time

• We have now seen Scheme functions that take in
  and return numbers.
• Next time, we will write Scheme functions that
  take in and return other data types, like strings
  and images.