Additional Control Structures

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Additional Control Structures

• Chapter 9

2
New and Improved . . .
3
Branching Statements

• Recall the current branching capability provided
  by the if ( ) statement
• Only branches twoways
• What if we desiremultiway branching?

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Multiway Branching

• C provides the switch statement

switch (choice) case 1 do_option_one()
breakcase 2 case 3 do_2_3_a ()
do_2_3_b () breakdefault do_something_else
()
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Switch Statement

• Value of the switch expression matched with one
  of the labels attached to a branch
• The statement(s) with the match get executed

switch (choice) case 1 do_option_one()
breakcase 2 case 3 do_2_3_a ()
do_2_3_b () breakdefault do_something_else
()
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Switch Statement

• Switch expression gt the expression in
  parentheses whose value determines which switch
  label is selected
• cannot be floating point
• usually is int or char
• Identifiers following case must be constants

switch (choice) case 1 do_option_one()
breakcase 2 case 3 do_2_3_a ()
do_2_3_b () breakdefault do_something_else
()
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Switch Statement

• The break causes control to be shifted to first
  statement after the switch statement
• the default statement is executed if the value of
  the switch expression is NOT found among switch
  labels

switch (choice) case 1 do_option_one()
breakcase 2 case 3 do_2_3_a ()
do_2_3_b () breakdefault
do_something_else () // next statement
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A Different Looping Statement

• Recall that the while ( ) statement always
  checked the condition BEFORE the loop
• In certain situations wewish to check the
  condition at the END ofthe loop

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The Do-While Statement

• Condition tested at end/bottom of loop
• Guarantees loop body executes at least once

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The do-while Illustrated
Note alwaysat least oneiteration
Loop Iteration
Loop Entry
Loop Exit
Loop Trest
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The Do-While Statement

• Can be used for a counting loop

What gets printed??
How do you change it to go 10 times?
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The Do While Statement

• This logic sometimes more suitable for some
  algorithms
• Example trap for valid input

do cout ltlt Enter value (1 - 5) -gt cin
gtgt value if (value lt 1 value gt 5)
cout ltlt Invalid input\a ltlt endl while
(value lt 1 value gt 5)
What makes this easier than the while ( ) loop
for this task?
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Do-While Loop vs. While Loop

• POST-TEST loop (exit-condition)
• The looping condition is tested after executing
  the loop body.
• Loop body is always executed at least once.

• PRE-TEST loop (entry-condition)
• The looping condition is tested before executing
  the loop body.
• Loop body may not be executed at all.

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A Special Count Controlled Loop

• The for ( ) statement
• Initialization initializes the LCV
• Condition usually a comparison, acts like a
  while ( )
• Incrementing statement LCV is incremented (or
  decremented)

for ( initialization test expression update
) 0 or more statements to repeat
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The for ( ) Loop

• Example -- what gets printed?

How did it Happen?

• x gets initialized
• value of x inspected
• x gets incremented

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The for ( ) Loop

• All three of the portions inside the parentheses
  can be multiple statements separated by commas
• Any or all of the three portions inside the
  parenthesis may be missing
• accomplish those tasks some other way
• the two semicolons MUST be there

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The break Statement

• We saw it in the switch statement
• Causes immediate exit from innermost block
• switch, while, do-while, for
• Can be used to break out of purposely designed
  infinite loop
• not a good idea a lazy shortcut
• Use only as last resort to avoid baffling
  combinations of multiple Boolean flags and nested
  ifs

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The continue Statement

• Valid only in loops
• Terminates current loop iteration
• NOT entire loop
• Causes branch to bottom of loop
• skips rest of loop statements
• Loop then prepares for next iteration
• for () would increment lcv
• all loops would check condition

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Guidelines for Choosing a Looping Statement

• Simple count-controlled
• use for () loop
• Event controlled, body always executed at least
  once
• use do-while
• Event controlled and nothing known about first
  execution
• use while, possibly for
• When in doubt gt use while

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Testing and Debugging

• For do-while loops, make sure to try data sets to
  make loop go exactly one time
• For data-dependant loop where expressions based
  on values other than constants
• make sure to test for proper number of iterations
• Make sure switch statements have every branch
  tested
• including default

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Testing and Debugging

• Remember to use break at end of case alternatives
  in switch statements
• otherwise next case is also executed!
• Case labels in switch statement must be values or
  named constants -- no variables
• Both switch expression case constant cannot be
  floating point
• Provide default for switch when possibility of
  case values not being matched

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Testing and Debugging

• Make sure all needed switch cases are present
• Choose looping structure carefully
• for( ) loop heading must have two semicolons
  -- even if portions omitted
• Break statement can exit only one level of
  nesting
• innermost switch or loop where break is located