E77: INTRODUCTION TO COMPUTER PROGRAMMING FOR SCIENTISTS AND ENGINEERS

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E77 INTRODUCTION TO COMPUTER PROGRAMMING FOR
SCIENTISTS AND ENGINEERS

• Lecture Outline
• 1. The switch construct
• 2. Definite loops
• 3. Conditional loops

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Switch construct

• 1. Switch construct
• This is a conditional construct alternative to
  the
• if-elseif-else-end construct. Its general form
  is
• switch (input expression)
• case (value1)
• Execute command group 1
• case (value2)
• Execute command group 2
• otherwise
• Execute command group N
• end

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Switch construct

• Schematically

FALSE
FALSE
VALUEVALUE2
VALUEVALUE1
TRUE
TRUE
COMMANDGROUP 2
COMMANDGROUP 1
COMMANDGROUP N
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Switch construct

• By way of background, recall the general form of
  the
• if-elseif-else-end construct
• if (logical expression 1)
• Execute command group 1
• elseif (logical expression 2)
• Execute command group 2
• else
• Execute command group N
• end

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Switch construct

• Compare the if-elseif-else-end
• to switch-case-otherwise-end
• Both constructs allow branching of the code.
• In both constructs, only one group of commands is
  eventually executed
• In both constructs, the comparisons are made
  sequentially from top to bottom
• Unlike if-elseif-else-end, the decision to
  execute a command group in switch-case-otherwise-e
  nd is based on the value of a single variable

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Definite loops

• 2. Definite loops
• Definite loops allow a group of commands to be
  repeatedly
• executed a fixed and given number of times.
• The general form of the definite loop construct
  in MATLAB
• is
• for loop_variable array
• Execute command group for given value of
  loop_variable
• end
• Here the command group is executed once for each
  column
• of the array array. During the k-th execution,
• loop_variable array(,k)

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Definite loops

• Schematic depiction of a definite loop construct

k 1
variable array( , k )
k k 1
COMMANDGROUP
k gtarraycolumns
FALSE
TRUE
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Definite loops

• Remarks on definite loops
• It is a dangerous practice to change the value of
  the loop variable during the execution of the
  command group.
• MATLAB is not particularly efficient in executing
  definite loops (there are ways to address this
  problem).
• Several MATLAB commands already perform definite
  loops, albeit efficiently (e.g., , find, sum,
  sort).
• Definite loops can be nested.
• It is possible (although not recommended, to
  break a definite loop using the break command).

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Conditional loops

• 3. Conditional loops
• Conditional (or indefinite) loops allow a group
  of commands
• to be repeatedly executed as long as a logical
  expression is
• true.
• The general form of the conditional loop
  construct in
• MATLAB is
• while (logical expression)
• Execute command group
• end
• Conditional loops are useful when the exact
  number of
• repetitive executions is not known in advance

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Conditional loops

• Schematic depiction of a conditional loop
  construct

FALSE
LOGICALEXPRESSION
TRUE
COMMANDGROUP
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Conditional loops

• Remarks on conditional loops
• The logical variable needs to be defined before
  execution of the while statement.
• The logical expression in the while statement is
  reevaluated after each loop and needs to be
  changed at some stage during the execution of the
  command group.
• Care is needed to ensure that the while loop is
  not infinite.
• MATLAB is not particularly efficient in executing
  conditional loops (there are ways to address this
  problem).
• Conditional loops can be nested.

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Summary

• What did we learn today?
• The switch construct is ideal for branching of a
  code based on the value of a single expression.
• Definite loops (for loops) are useful when
  executing the same group of commands a fixed and
  given number of times.
• Conditional loops (while loops) are useful when
  executing the same group of commands repeatedly
  without knowing in advance the total number of
  repetitions.
• Loops are expensive in MATLAB and can be often
  avoided by relying on built-in functions.