General Programming

1
General Programming

• Introduction to Computing Science and Programming
  I

2
General Programming

• Basic set of steps for writing a program
• Define the problem.
• Create an algorithm to solve the problem and
  write it out in pseudocode
• Convert this algorithm into actual code, testing
  small parts of it along the way
• When finished converting the algorithm to code,
  test for errors
• Debug any errors and go back to the previous step

3
General Programming

• When you are finished writing your algorithm,
  look for pieces that you can program and test
  separately.
• To illustrate the point, lets look at the second
  part of Lab 2 and see how we can break it apart

4
Example

• Write a program that displays a menu to the user
  and based on their choice, calculates the area of
  a circle or triangle whos attributes are input
  by the user.
• Algorithm
• write menu to screen
• read option
• if option is triangle
• read base/height from user
• if base and height are gt 0
• calculate and display area
• else
• display error
• else if option is circle
• read radius
• if radius is gt 0
• calculate and display circles area
• else
• display error
• else
• write incorrect option entered

5
Example

• What parts of this algorithm can you code and
  test separately?
• Menu Structure
• Triangle calculation
• Circle calculation
• User input and check for negative values
• Each of these could be coded and tested
  separately before combining.

6
General Programming

• This idea of breaking the problem up may seem
  unimportant when working with the small programs
  we are now, but it is still helpful.
• When you move on to more complex programs the
  idea is essential.
• If you write large amounts of code with having
  made sure small pieces are correct, finding
  errors becomes more difficult.

7
Errors

• You will encounter errors as you write your
  programs.
• There are other ways to categorize them, but we
  will look at three basic types of error.
• Syntax Errors
• Runtime Errors
• Semantic Errors

8
Syntax Errors

• Errors in the syntax of your code.
• Syntax refers to the rules that define proper
  statements.
• Python will detect syntax errors and not allow
  you to run your code until you correct them.

9
Runtime Errors

• Once your code has no syntactic errors, you are
  allowed to execute it
• Runtime errors refer to errors that do not
  present themselves until your code is executed
  (runtime).
• Examples
• Division by zero.
• Improper conversions.
• Your program will be terminated if a runtime
  error occurs during execution.

10
Semantic Errors

• Errors in the semantics of your code.
• Syntax defines how you can create statements.
  Semantics tell you what the meaning of those
  statements are.
• A program has semantic errors if it runs and
  completes, but does not do what it was created to.

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Error Message

• When an error occurs, Python prints some info to
  help you understand what happened. Learning how
  to understand these error message will help you
  solve problems in your programs.
• For Example
• gtgtgt 4/0
• Traceback (most recent call last)
• File "ltpyshell6gt", line 1, in ltmodulegt
• 4/0
• ZeroDivisionError integer division or modulo by
  zero
• The last line tells you what type of error
  occurred.
• The third line prints out the statement from that
  line.
• The second line tells you on what line of code
  the error occurred. This is more helpful when
  executing a program.

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Some Python Errors

• ZeroDivisionError
• NameError
• Try to use a variable that doesnt exist.
• ValueError
• Try to convert an inappropriate string to a
  number

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Correcting Errors

• Strategies for correcting errors (debugging)
• Remove parts of your code to see if they are
  involved in the error.
• Add print statements to check the value of a
  variable that is involved. (remember to remove
  these statements when finished)

14
Coding Style

• Coding style deals with how you format your code.
  Good coding style makes it easy for people to
  understand your code
• Coding style issues.
• Comments
• Proper spacing
• Good variable/function names

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Comments

• A comment is text you put in your code that will
  be ignored. This allows you to give short
  explanations to help people understand your code.
• Use the character and anything written after it
  will be ignored

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Comments

• Dont add comments to explain every line
• add one to x
• x x1
• Often useful to add a comment for chunks of code,
  such as before control structures
• if the user entered good data, add it in
• if value gt 0
• sum sum value
• count count 1
• search for a value that divides num
• while numfactor ! 0
• factor factor 1

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Spacing

• Poor spacing can make lines of code more
  difficult to understand.
• Bad y 100 / x1
• Good y 100/x 1
• Add blank lines in the code to make it easier to
  read.

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Coding Style

• Bad
• a int(raw_input("Enter an integer "))
• b 0
• for i in range(a1)
• if igt0
• if a i0
• b b 1
• print "There are " str(b) " factors.
• Good
• get user input and initialize
• num int(raw_input("Enter an integer "))
• count 0
• check every possible factor (1...num)
• for factor in range(1, num1)
• if factor divides num, it really is a factor
• if numfactor 0
• count count 1