Control Structures

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Control Structures

• CPS120 Introduction to Computer Science
• Lecture 5

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What Can a Program Do?

• A program can only instruct a computer to
• Read Input
• Sequence
• Calculate
• Store data
• Compare and branch
• Iterate or Loop
• Write Output

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Fundamental Programming Concepts

• Assignment of values to a variable
• Iteration (Looping)
• Over a set of set of statements
• With respect to a logical expressions
  (conditions)
• Delegation of sub-tasks to functions / procedures

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The Structure Theorem

• The Structure Theorem states that any algorithm
  can be built from three basic control structures.
  
• One-after-another (Sequence)
• Decision-making (Selection)
• Making choices between 2 or more alternatives
• Repetition (Iteration)
• Concerned with repetitive tasks (and the
  termination conditions of loops)

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C Control Structures

• "Sequence statements" are imperatives
• "Selection" is the "if then else" statement
• AND, OR, NOT and parentheses ( ) can be used for
  compound conditions
• "Iteration" is satisfied by a number of
  statements
• "while"
• " do "
• "for"
• The case-type statement is satisfied by the
  "switch" statement.
• CASE statements are used for most non-trivial
  selection decisions

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Sequence Control Structures

• Sequence control structures direct the order of
  program instructions.
• The fact that one instruction follows anotherin
  sequenceestablishes the control and order of
  operations.

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Calculate

• A program can instruct a computer to perform
  mathematical operations.

Add 1 to Counter
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Store

• A program will often instruct a computer to store
  intermediate results.

Place 1 in Counter
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Compare and Branch

• A program can instruct a computer to compare two
  items and do something based on a match or
  mismatch which, in turn, redirect the sequence of
  programming instructions.
• There are two forms
• IF-THEN
• IF-THEN-ELSE

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IF-THEN
Test condition p
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IF-THEN-ELSE
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Iterate

• A program loop is a form of iteration. A computer
  can be instructed to repeat instructions under
  certain conditions.

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Iteration Control Structures

• Iteration control structures are looping
  mechanisms.
• Loops repeat an activity until stopped. The
  location of the stopping mechanism determines how
  the loop will work
• Leading decisions
• Trailing decisions

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Leading Decisions

• If the stop is at the beginning of the iteration,
  then the control is called a leading decision.
• The command DO WHILE performs the iteration and
  places the stop at the beginning.

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DO WHILE Loop
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Trailing Decisions

• If the stop is at the end of the iteration, the
  control mechanism is called a trailing decision.
• The command DO UNTIL performs the iteration and
  puts the stop at the end of the loop.

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DO UNTIL Loop
Loop statement a