Algorithmic Thinking and the Software Life Cycle

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Algorithmic Thinking and the Software Life Cycle

• What is the job of a programmer?

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Computers Are Just Machines

• With people we rely on that person's experience.
• With a computer we cannot take anything for
  grantedNo
• assumptions,
• inconsistencies
• omissions

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Example of an Omission

• The following instruction would be enough for
  someone with experience
• Buy a loaf of bread from the store.
• Without experience they would become frustrated
  by the following omissions What are the
  directions? How much does it cost? Etc...

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Fewer Omissions

• Expanded instruction
• Tell the person how much bread costs
• Make sure that they have the money.
• Give them the store address
• Send them to purchase the bread

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Even More Detail

• Step 3 above can be expanded as follows
• Make a right turn at the first corner
• Make a left after 2 lights.
• More detail might be necessary.

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Bugs - mistakes

• When something goes wrong because the
  instructions were not accurate enough it is
  referred to as a bug.
• Example of a bug
• When making a purchase Canadian currency was used
  and not accepted at the store. We did not specify
  which currency to use.

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Software Life-Cycle

• Design an Algorithm
• Write a program- in a high-level programming
  language such as C, Pascal, COBOL, Basic Java...
• Compile the program- Translate it into machine
  code.
• Run or execute the program.
• Debug fix errors (bugs)
• Maintenance cycle through these steps

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Factorial Example- Algorithm

• Ask for a number and keep a running total
• Loop for number times
• multiply the running total by a lower number
  each time around
• Print out the final answer

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Factorial Example - C code

• main( )
• int n, count, f1 printf("Enter a number")
• scanf("d",n)
• countn
• while(countgt1)
• ffcount countcount-1
• printf("The factorial is d",f)

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Bug examples in C program

• Countcount1 instead of countcount-1 
• Ff1 instead of ff1
• Countlt1instead of countgt1

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Two famous examples of bugs

• Mars Climate Orbiter was lost in space in 1999.
  In the calculation of its route some programmers
  used the metric system while others used English
  units.
• A patriot missile failed to intercept a scud
  fired at US troops in 1991. The following is from
  an article Specifically, the time in tenths of
  second as measured by the system's internal clock
  was multiplied by 1/10 to produce the time in
  seconds.

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Questions

• What is an algorithm?
• What is the Software Life Cycle?
• What are some examples of bugs