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COM S 207
For-Loop Statement
Instructor: Ying Cai
Department of Computer Science
Iowa State University
[email protected]
The for loop
loop variable,
initialized before
the loop starts
condition checked
before each iteration
for (int i=5; i<=10; i++)
{
Statement1;
Statement2;
body of loop
:::
} // end for i
update extended
after each iteration
no semi-column
here
A for loop is a count-controlled loop whereas a
while loop is an event-controlled loop
for loop vs. while loop
A for loop is a count-controlled loop whereas a while loop is
an event-controlled loop
for (int i=5; i<=10; i++)
{
Statement1;
Statement2;
:::
} // end for i
while (condition)
{
Statement1;
Statement2;
:::
} // end while
A count-controller loop is executed a definite number of times.
In an event-controlled loop, we do not know how many
iterations of the loop body may be executed – it is also called an
infinite loop
A for loop can be countdown instead of up
for (int counter = 10; counter >= 0; counter--)
{
System.out.println(counter);
} // end for counter
body executed 11 times
counter = 10;
counter = 9;
::
counter = 0;
The increment or decrement to the loop variable does not
have to be 1 -- it can be any other number, but the same
value is used for each iteration
for (int cntr = 10; cntr >= 0; cntr = cntr - 2)
{
System.out.println(counter);
} // end for cntr
Comprehension Checks
for (i = 0; i<=5; i++)
{
System.out.println(i);
} // end for i
for (i=5; i<=0; i--)
{
System.out.println(i);
} // end for i
for (i = 0; i<=9; i = i + 2)
{
System.out.println(i);
} // end for i
for (i = 0; i!=9; i = i + 2)
{
System.out.println(i);
} // end for i
Comprehension Checks
for (i = 1; i <= 20; i = i * 2)
{
System.out.println(i);
} // end for i
for (i = 1; i <= str.length(); i++)
{
System.out.println(i);
} // end for i
for (int n = 10; n >= 0; n--)
{
System.out.print(n + “ “);
} // end for n
Write Loops that computes
1) the sum of all integers between -120 and 2312
(inclusive)
2) the sum of all EVEN integers between 78 and 3090
(exclusive)
3) the sum of all SQUARES of integers between -300 and
+3300 (inclusive)
4) the average of all ODD integers between -123 and
321 (inclusive)
Ex. 1: Given a number, check if it is
a prime number
Pseudocode
1. ask for number
2. test against denominator, starting from 2, 3, ...,
number/2
3. if number % denominator = 0, then number is not a
prime number
4. otherwise, number is a prime number
More examples
1) Given a number, check if it is a prime number
2) Given a range, find out all prime numbers in between
3) Given a number, print its binary digits (e.g., 15 ->
1111, 20 ->11000)
4) Given a number, print its hexadecimal digits (e.g., 9>9, 10->A, 15 -> F, 20 -> 14)
Sentinel loop (sec. 4.4)
while (salary != 1)
{
salary = in.nextDouble();
if (salary != -1)
{
sum = sum + salary;
count++;
} // end if
} // end for while
loops that have a pre-determined
sentinel value for termination.
Example: compute the average of
a set of salary values and use -1
as a sentinel
if (counter > 0)
{
double average = sum / count;
System.out.println(“Average salary is “ + average);
}
else
{
System.out.println(“no data”);
}
Nested Loop
A loop inside another loop
X1
X2
X3
X4
1
2
1
4
1
8
1
16
27
81
3
9
: : : :
10
100
1000 1000