LOOPING
What are the 3 structures of writing code?
sequential
decision
repetition
Def. Looping is the repetition of statements
in a program.
There various types of loops:
controlled by a flag, count controlled, state controlled
Loop controlled by a boolean flag* and a pretest
The flow of logic of a loop is shown in a flowchart:
start
flag = false
end
false
!flag == ?
true
body of loop
update flag
*sometimes called a sentinel
boolean done = false;
while ( ! done )
{ statements
...
//initialization step
//loop condition
//body of loop
If (something)
//update flag based
done = true;
//on some decision
}
//sometimes from user
NOTE: this is a pretest loop
See ex.
p.241
the body is executed only when the
loop condition is TRUE.
Loop controlled by a boolean flag and a posttest
//no initialization necessary
do
{statements;
...
update flag;
}
while ( flag ); Note: that means flag is true
Note: the test is not made until the body has
executed once.
Example of a do - while loop :
char ch;
boolean ok;
do
{
ch = getChar(“Enter a digit”);
if (ch < ‘0’ || ch > ‘9’)
ok = false;
}
while (! ok );
Count controlled loops
int lcv = 0;
while ( lcv < 10 )
// initialization
// final value in condition
{println ( “ HI! ” ); // body of loop
lcv++;
// or lcv = lcv + 1 (update)
}
Note: each execution of the body of the loop is
called one iteration.
pretest
Is this a pre or posttest loop?
lcv = 10
What change would make it zero iterations?
Write a loop that will output the integers 1
to 10
Write a loop that will output the integers
10 to 1.
Summation is a common use of looping.
Let’s write a loop to find the sum of the integer 1 to 10.
int sum = 0;
// called the accumulator
int count = 0;
// initialization of
loop control variable
while ( count < 10 ) // loop condition
{sum = sum + count; // or sum += count
count ++;
} //end while
// or count = count + 1
State controlled loops (similar to flag-controlled)
The loop is ended when a state is reached that
causes the loop-repetition condition to become
false.
See ex bottom p 249
See ex bottom p 261
Question: What is the difference between a
sentinel-controlled loop and a state-controlled loop?
A sentinel is a “dummy data value” an end marker.
A state is reached that causes the boolean condition
to become false.
for loops
Note: this is an option that most languages provide to condense code
but is not necessary.
for ( lcv = initial value; lcv < final value; lcv ++ )
Notice: how 3 of the 4 parts of a loop are in this line.
Rewrite the while “count to 10” loop using for.
Question: can every while loop be written as a for loop?
No. Only when the number of repetitions is known ahead of time.
Therefore, not a flag controlled and only some state controlled loops.
Question: does the initial value have to be 1?
no
How many iterations of this loop? For (ct = 10; ct <= 20; ct++) { };
What does this loop do?
for (int x = 0; x < 32,768; x++);
nothing but count to 32,768 - this provides a pause.
(It is also a common error: do not put a semicolon after the parentheses unless you want a
loop that does nothing.)
What does this do?
For (ct = 10; ct != 20; ct +=3 )
{ println ( “Hi!”); }
Trace:
Caution:
infinite
loop
ct
output
10
Hi
13
Hi
16
Hi
19
Hi
22
Hi
Is a for loop a pretest loop, posttest loop or
neither?
Pretest.
Write a loop whose body will never be executed.
Can the lcv be any type besides int?
yes. Any ordinal type: any primitve data type
whose elements can be listed in sequential order.
(short, int, long, char)
What does this loop do:
for ( char ch = ‘A’; ch <= ‘Z’; ch++)
displayResult(ch + “ “ + (int) ch);
A 65
B 66
…
Z 90