1. Loops A loop is: Java provides three forms for explicit loops:
a block of code that executes repeatedly while some condition holds true.
Java provides three forms for explicit loops:
while
for
do..while
The conditions in loops are written as in if statements
See examples on next slides
Fundamentals of Software Development 1

2. Loops while loop while (condition) { statement; … statement; }
for (start; condition; step) { statement; …
statement; }
for loop
do {
statement; …
statement;
} while (condition);
do-while loop
Fundamentals of Software Development 1

3. for-loop example
Problem: Display the square of all numbers from 1 to 99
Solution idea: Use a counting variable
What should the variable start at?
The loop should continue while …?
Each time through the loop, increment the variable by …?
for (int k = 1; k < 100; ++k) {
System.out.println(k*k); }
Alternative solution:
for (int k = 1; k <= 99; ++k) {
System.out.println(k*k); }
Both solutions are correct;
the first risks round-off error in its stopping condition
Fundamentals of Software Development 1

4. while-loop example
Problem: Input numbers from the console, displaying the square of each, stopping when the user inputs a negative number
Solution: First do an input,
then use a while loop that runs while inputs are nonnegative
Fundamentals of Software Development 1

5. Problem: Input numbers from the console, displaying the square of each, stopping when the user inputs a negative number
Scanner inputStream = new Scanner(System.in);
double input;
input = inputStream.nextDouble();
while (input >= 0) {
System.out.println(input*input); }
Fundamentals of Software Development 1

6. for loops versus while loops
for (int i = 0; i < 7; i = i + 1) { System.out.println (i + " " + i*i); }
int i = 0;
while (i < 7) { System.out.println (i + " " + i*i);
i = i + 1; }
The two boxes are equivalent.
Typically we use:
for when we know in advance how many times the loop will execute
while when something that happens in the loop determines when the loop exits
do..while when we want a while loop that always does at least one iteration
Fundamentals of Software Development 1