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Changes Made

• Fixed smart quotes
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ITM 352Flow-Control LoopsLecture 8
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Iteration

• Summary of loops
• while-do
• for
• do-while
• while loop examples
• for loop examples

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Iteration

• Traditional method of repeating statements (not
  using recursion)

loop control ---------------
---------------
repeat these statements zero or more
times, controlled by loop control
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Iteration (cont.)

• Three different forms of iteration
• while
• for
• do-while

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While loops
contains variables that are changed in loop
while (condition) statement
repeat until condition becomes false
Keep in mind that statement can be a compound or
block statement.
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For loops
init //for condition while (cond)
Statement incr
?
for (init cond incr) Statement
Mostly for convenience and readability
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Do-while loops
Statement while (cond) Statement
do Statement while (cond)
?
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While loops

• Syntax while (cond) statement
• What it means
• Test cond.If false, then while loop is
  finished go on to next statementIf true,
  execute statement, then go back and start
  again
• Continue like this until cond becomes false or
  a break is executed.

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More while loop examples
x 5 while (xgt0) echo x x x-1
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More while loop examples
x 5 s 0 while (x gt 0) s s x
x x-1
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More while loop examples
s 0 x 1 while (x lt 10) s s
x x x 1
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While loop examples
while (true) echo "I will do the reading
assignments"
Output?
while (false) echo "I will start my HW early"
Output?
Do lab exercise 1
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The exit Function

• If you have a situation where it is pointless to
  continue execution you can terminate the program
  with the exit() or die() functions
• A string is often used as an argument to identify
  if the program ended normally or abnormally

Do lab exercise 2
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for loops

• Convenient syntax for loops with loop variables
  that are incremented or decremented each time
  through the loop (as in previous examples).
• Recall

init while (cond) statement incr
?
for (init cond incr) statement
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for loop examples

• Previous examples

s 0 x 5 while (x gt 0) s s
x x x-1
s 0 for (x 5 x gt 0 x x-1)
s s x
?
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for loop examples
s 0 x 1 while (x lt 10) s s
x x x 1
s 0 for (x 1 x lt 10 x
x1) s s x
?
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for loop usage
One use of for loops is simply to execute a
statement, or sequence of statements, a fixed
number of times
for (i 1 i lt n i) statement
executes statement exactly n times.
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for loop usage (cont.)
Example print I will not turn HW in late a
hundred times.
for (i 1 i lt 100 i) echo "I will not
turn HW in late")
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for loop usage (cont.)
Keep in mind that loop bodies can be as
complicated as you like. This loop prints My
robot loves me and My robot loves me not on
alternate lines, ten times each
for (i 0 i lt 20 i) if (i2
1) echo "My robot loves me\n" else
echo "My robot loves me not\n"
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for loop usage (cont.)
An alternate approach. Which is more elegant?
for (i 1 i lt 20 i) echo "\nMy
robot loves me" if (i2 0) echo "
not"
Do lab exercise 3
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Practical ConsiderationsWhen Using Loops

• The most common loop errors are unintended
  infinite loops and off-by-one errors in counting
  loops
• An off-by-one error is an error that occurs when
  a loop is executed one too many or one too few
  times.
• Sooner or later everyone writes an unintentional
  infinite loop
• To get out of an unintended infinite loop enter
  C (control-C) or stop button on browser
• Loops should tested thoroughly, especially at the
  boundaries of the loop test, to check for
  off-by-one and other possible errors

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Tracing a Variable in a Loop

• Tracing a variable print out the variable each
  time through the loop
• A common technique to test loop counters and
  troubleshoot off-by-one and other loop errors
• Some systems provide a built-in tracing system
  that allows you to trace a variable without
  having to change your program.
• If no built-in utility is available, insert
  temporary output statements to print values.