1. Department of Computer Science
COM S 207
For-Loop Statement
Instructor: Ying Cai
Department of Computer Science
Iowa State University

2. The for loop condition checked before each iteration
loop variable, initialized before the loop starts
update extended after each iteration
no semi-column here
for (int i=5; i<=10; i++) {
Statement1;
Statement2;
:::
} // end for i
body of loop
A for loop is a count-controlled loop whereas a while loop is an event-controlled loop

3. 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

4. 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;

5. 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

6. 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

7. 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

8. Write Loops that computes
the sum of all integers between -120 and 2312 (inclusive)
the sum of all EVEN integers between 78 and 3090 (exclusive)
the sum of all SQUARES of integers between -300 and (inclusive)
the average of all ODD integers between -123 and 321 (inclusive)

9. Ex. 1: Given a number, check if it is a prime number
Pseudocode
ask for number
test against denominator, starting from 2, 3, ..., number/2
if number % denominator = 0, then number is not a prime number
otherwise, number is a prime number

10. More examples Given a number, check if it is a prime number
Given a range, find out all prime numbers in between
Given a number, print its binary digits (e.g., 15 -> 1111, 20 ->11000)
Given a number, print its hexadecimal digits (e.g., 9->9, 10->A, 15 -> F, 20 -> 14)

11. Sentinel loop (sec. 4.4)
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
while (salary != 1) {
salary = in.nextDouble();
if (salary != -1)
sum = sum + salary;
count++;
} // end if
} // end for while
if (counter > 0)
double average = sum / count;
System.out.println(“Average salary is “ + average); }
else
System.out.println(“no data”);

12. Nested Loop A loop inside another loop X1 X2 X3 X4 1 1 1 1 2 4 8 16
: : : :