Programming loop

Repetition Statements
• Repetition statements allow us to execute a cstatement
multiple times
• Often they are referred to as loops
• Like conditional statements, they are controlled by boolean
expressions
• Java has three kinds of repetition statements:
• the while loop
• the do loop
• the for loop


 A while statement has the following syntax:
while ( condition ){
statement;
}
• If the condition is true, the statement is executed
• Then the condition is evaluated again, and if it is still true, the
statement is executed again
• The statement is executed repeatedly until the condition becomes
false
The while Statement


Logic of a while Loop
condition
evaluated
statement
true false


 An example of a while statement:
int count = 1;
while (count <= 5){
System.out.println (count);
count++;
}
• If the condition of a while loop is false initially, the statement is never
executed
• Therefore, the body of a while loop will execute zero or more times
The while Statement


 Let's look at some examples of loop processing
 A loop can be used to maintain a running sum
 A sentinel value is a special input value that
represents the end of input
 A loop can also be used for input validation, making a
program more robust
The while Statement


 The body of a while loop eventually must make the
condition false
 If not, it is called an infinite loop, which will execute
until the user interrupts the program
 This is a common logical (semantic) error
 You should always double check the logic of a
program to ensure that your loops will terminate
normally
Infinite Loops


 An example of an infinite loop:
int count = 1;
while (count <= 25){
count = count - 1;
}
 This loop will continue executing until interrupted (Control-C)
or until an underflow error occurs
Infinite Loops


 Similar to nested if statements, loops can be nested
as well
 That is, the body of a loop can contain another loop
 For each iteration of the outer loop, the inner loop
iterates completely
 Your second course project involves a while loop
nested inside of a for loop
Nested Loops

 How many times will the string "Here" be printed?
 count1 = 1;
while (count1 <= 10){
count2 = 1;
while (count2 <= 20) {
System.out.println ("Here");
count2++;
}
count1++;
}
10 * 20 = 200
Nested Loops


 A do-while statement (also called a do loop) has the following
syntax:
do{
statement;
}while ( condition )
• The statement is executed once initially, and then the
condition is evaluated
• The statement is executed repeatedly until the condition
becomes false
The do-while Statement


Logic of a do-while
Loop
statement
condition
evaluated
true
false


 An example of a do loop:
 int count = 0;
do{
count++;
} while (count < 5);
 The body of a do loop executes at least once
The do Statement


Comparing while and
do
statement
true false
condition
evaluated
The while Loop
true
condition
evaluated
statement
false
The do Loop


 for ( initialization ; condition ;
increment ){
statement;
}
The for Statement
The statement is
executed until the
condition becomes false
The increment portion is
executed at the end of each
iteration
The initialization
is executed once
before the loop begins


Logic of a for loop
condition
evaluated
initialization
statement
true
increment
false


 A for loop is functionally equivalent to the following
while loop structure:
initialization;
while ( condition ){
statement;
increment;
}
The for Statement


 An example of a for loop
for (int count=1; count <= 5; count++){
}
• The initialization section can be used to declare a variable
• Like a while loop, the condition of a for loop is tested prior to
executing the loop body
• Therefore, the body of a for loop will execute zero or more times
The for Statement


 The increment section can perform any calculation
for (int num=100; num > 0; num -=
5){
System.out.println (num);
}
 A for loop is well suited for executing statements
a specific number of times that can be calculated
or determined in advance
The for Statement


 Each expression in the header of a for loop is optional
 If the initialization is left out, no initialization is performed
 If the condition is left out, it is always considered to be true,
and therefore creates an infinite loop
 We usually call this a “forever loop”
 If the increment is left out, no increment operation is
performed
The for Statement


 Remember the break keyword that we used to stop
a switch statement from executing more than one
statement?
 break can also be used to exit an infinite loop
 But it is almost always best to use a well-written
while loop
break revisited

Programming loop

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