Chapter 2 - Control Structures

1
Algorithms
2
Introduction

• Before writing a program
• Have a thorough understanding of the problem
• Carefully plan an approach for solving it
• While writing a program
• Know your building blocks (e.g. data types,
  data structures, control structures, selection
  structures, etc)
• Use good programming principles for efficiency
  and correct results

3
Pseudocode

• What is pseudocode?
• Artificial, informal language that helps us
  develop algorithms
• Similar to everyday English
• Not actually executed on computers
• Helps us think out a program before writing it
• Converts into a corresponding program
• Consists only of executable statements

4
Formulating Algorithms with Top-Down, Stepwise
Refinement

• First, define the problem in words
• For palindromes, the problem becomes
• What are the steps necessary to test the string?

5
Formulating Algorithms with Top-Down, Stepwise
Refinement, Step 2

• Top-down, stepwise refinement
• Begin with a pseudocode representation of the
  problem
• Determine whether the string is a palindrome
• Divide the problem into smaller tasks and list
  them in order Initialize variables and
  arrays
• Input the string
• Test the string
• Return the result
• Many programs have four phases
• Initialization declare initialize all program
  variables
• Input input data values and adjust any program
  variables
• Processing manipulate the data
• Termination calculate and print the final
  results (output)

6
Formulating Algorithms with Top-Down, Stepwise
Refinement, Step 3

• Refine the initialization phase from Initialize
  variables to
• set any counters to zero determine the data
  type length of the array and declare it
• Refine test the string to
• Check first and last elements
• While the first and last elements are the same,
  continue
• If they differ, exit and the string is not a
  palindrome
• Otherwise, test until all pairs are matched

7
Formulating Algorithms with Top-Down, Stepwise
Refinement, Step 4

• Refine Print the result to
• If the string is a palindrome Print The
  string is a palindrome.
• else Print The string is not a
  palindrome.