ITM 352 - © Port, Kazman1 ITM 352 Flow-Control: Loops

ITM 352 - © Port, Kazman 1

ITM 352

Flow-Control: Loops


Iteration

• Summary of loops• while-do• for• do-while avoid using this!

• while loop examples• for loop examples


Iteration• Traditional method of repeating statements

(not using recursion)

loop control { --------------- ---------------}

repeat these statementszero or more times,controlled by loop control


Iteration (cont.)

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


While loops

while (condition) statement

contains variablesthat are changed

in loop

repeat untilcondition

becomes false

Keep in mind that statement can be a compound or block statement.


For loops

for (init; cond; incr) Statement

init; //for conditionwhile (cond) { Statement incr;}

Mostly for convenience and readability


Do-while loops

do Statementwhile (cond)

Statement;while (cond) Statement


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.


More While loop Examples

$x = 5;$s = 0;while ($x > 0) { $s = $s + $x; $x = $x-1;}


More While loop Examples

$s = 0; $x = 1;while ($x <= 10) { $s = $s + $x; $x = $x + 1;}


Infinite and Empty Loopswhile (true); is an infinite loop and you will have to waita long time! while (false){//stuff} and while ($n++ < 10);

{//stuff} are empty loops and will never execute anything

You should take care to ensure your loop is good by (1) Checking that the initial condition is true(2) Checking that the loop condition will eventually change to

false(3) Do NOT EVER use ; after while()(4) Make sure you initialize loop variables BEFORE the loopGenerally PHP has a maximum execution time to help avoid crashing the system (default 60 secs). You can set this to a lower value such as 5 sec by using set_time_limit (5);


While loop examples

while (true)echo "I will do the reading assignments";

while (false) echo "I will start my HW early";

Output?

Output?

Do Lab Exercise 1


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


exit,die example

$x = 5;while ($x++>0) {// terminate program at 3 if($x == 3) die(“done!”); echo “$x<br>”;}echo “I will not execute ”;


break and continue • If you have a situation where need to exit

the loop but not terminate the program use break

• If want to stop executing the statements in the loop for a given cycle and immediately jump to the top of the loop to start the next cycle use continue


break example

$x = 5;while ($x++>0) {// stop early at 3 if($x == 3) break; echo “$x<br>”;}echo “I will always execute”;


continue example

$x = 5;while ($x++>0) {// skip 3 if($x == 3) continue; echo “$x<br>”;}echo “I will always execute”;

Do Lab Exercise 2


for loops• Convenient syntax for loops with loop variables that

are incremented or decremented each time through the loop (as in previous examples).

• Recall:

for (init; cond; incr) statement

init;while (cond) { statement incr;}


for loop examples

Previous examples:

$s = 0; $x = 5;while ($x > 0) { $s = $s + $x; $x = $x-1;}

$s = 0; for ($x = 5; $x > 0; $x = $x-1) $s = $s + $x;


for loop examples

$s = 0; for ($x = 1; $x <= 10; $x = $x+1) $s = $s + $x;

$s = 0; $x = 1;while ($x <= 10) { $s = $s + $x; $x = $x + 1;}


for loop usage

for ($i = 1; $i <= n; $i++) statement

One use of for loops is simply to execute a statement, or sequence of statements, a fixed number of times:

executes statement exactly n times.


for loop usage (cont.)

for ($i = 1; $i <= 100; $i++) echo "I will not turn HW in late");

Example: print "I will not turn HW in late" a hundred times.



for ($i = 0; $i < 20; $i++) {if ($i%2 == 1)

echo "My robot loves me\n"; else echo "My robot loves me not\n";}

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 = 1; $i <= 20; $i++) { echo "\nMy robot loves me"; if ($i%2 == 0) echo " not"; }

An alternate approach. Which is more "elegant"?

Do Lab Exercise #3


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


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.

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ITM 352 - © Port, Kazman1 ITM 352 Flow-Control: Loops