- Java
provides 3 types of
loop-statements:
- The while-statement
- The for-statement
- The do-while-statement
- We will now study the for-statement
- Algorithm to find
all divisors of
a number n:
for (x = 1, 2, 3, ..., n) do { if ( n is divisible by x ) { print x; // x is a divisor of n } }The variable x takes on the values 1, 2, 3, ..., n one at a time
For each value, we check whether n is divisible by x
If so, we print the value x (and obtain all divisors)
- The for-statement is
ideally suited for
making the following type of program:
- Let some
variable x take
on a series of value
one at a time
- For each value
taken on
by a variable x,
the body of the for-statement is
executed once.
Example:
/* --------------------------------------------------------- Execute a statement for each of the value 1, 2, ..., 10 --------------------------------------------------------- */ repeat ( for x = 1, 2, 3, .... 10 ) execute this statement;
- Let some
variable x take
on a series of value
one at a time
- The for-statement can best be explained with an example....
- The for-statement was
originally invented to
let some variable take
on a series of value
- The most common form
of the for-statement is
as follows:
for ( var = START_VALUE ; var <= STOP_VALUE ; var = var + INCR ) { /* for-body (statements) */ }Meaning:
- The variable var will
take on the following values:
- START_VALUE
- START_VALUE + INCR
- START_VALUE + 2×INCR
- ...
- Up to the largest value that is ≤ STOP_VALUE
- For each of the value,
the statements in the for-body
are executed
- In other words:
repeat ( for var = START_VALUE, START_VALUE+INCR, START_VALUE+2×INCR, ..., STOP_VALUE ) execute statements in the for-body (one time for each value in the list);
- The variable var will
take on the following values:
- Example 1:
public class For01 { public static void main(String[] args) { int a; // Example "for-statement" for ( a = 1 ; a <= 10 ; a = a + 1 ) { System.out.println(a); // Print a } System.out.println("Done"); } }Output: 1 2 (1+1×1) 3 (1+2×1) 4 (1+3×1) 5 (1+4×1) 6 (1+5×1) 7 (1+6×1) 8 (1+7×1) 9 (1+8×1) 10 (1+9×1) Done
Notice that in the for-statement:
- a = 1 specifies the starting value
- a <= 10 specifies when the for-loop will stop
- a = a + 1 specifies how the variable a will change each time through the loop
- Example Program:
(Experiment with it yourself outside class !)
- Prog file: click here
How to run the program:
- Right click on link and
save in a scratch directory
- To compile: javac For01.java
- To run: java For01
- Example 2: changing the
START_VALUE
public class For01 { public static void main(String[] args) { int a; // Changing the starting value: for ( a = 5; a <= 10; a = a + 1 ) { System.out.println(a); // Print a } System.out.println("Done"); } }Output: 5 6 (5+1×1) 7 (5+2×1) 8 (5+3×1) 9 (5+4×1) 10 (5+5×1) Done
Notice that in the for-statement:
- a = 5 specifies the starting value
- a <= 10 specifies when the for-loop will stop
- a = a + 1 specifies how the variable a will change each time through the loop
- Example 3: changing
the STOP_VALUE
public class For01 { public static void main(String[] args) { int a; // Changing the continuation condition: for ( a = 1; a <= 3; a = a + 1 ) { System.out.println(a); // Print a } System.out.println("Done"); } }Output: 1 2 (1+1×1) 3 (1+2×1) Done
Notice that in the for-statement:
- a = 1 specifies the starting value
- a <= 3 specifies when the for-loop will stop
- a = a + 1 specifies how the variable a will change each time through the loop
- Example 4:
changing the INCR (increment)
value
public class For01 { public static void main(String[] args) { int a; // Changing the increment statement: for ( a = 1; a <= 10; a = a + 2 ) { System.out.println(a); // Print a } System.out.println("Done"); } }Output: 1 3 (1+1×2) 5 (1+2×2) 7 (1+3×2) 9 (1+4×2) Done
Notice that in the for-statement:
- a = 1 specifies the starting value
- a <= 10 specifies when the for-loop will stop
- a = a + 2 specifies how the variable a will change each time through the loop: increment by 2!!
-
Class exercise:
what is the output of this
for-loop
public class For01 { public static void main(String[] args) { int a; // Changing the increment statement: for ( a = 4; a <= 14; a = a + 3 ) { System.out.println(a); // Print a } System.out.println("Done"); } }Answer:
4 7 (4 + 1×3) 10 (4 + 2×3) 13 (4 + 3×3) Done
Because:
- We start with the value 4
- We increase the value with 3 each time
- Hence: 4, 7, 10, 13
- The next value in the series is 16, but 16 is not ≤ 14... So we stop
- The general syntax of
the for-statement is as follows:
Explanation:
- The keyword for
announces (to the Java compiler) that
we started an for-statement
- Following the keyword for,
there are 3 clauses:
- An init-expression:
this is an expression that is
evaluated once
at the start of
the for-statement
- Recall that the = symbol is
an operator that returns a value
We saw assignment expressions in this webpage: click here
- The init-expression is
usually an
assignment expression
of the following form (but it can be very complicated):
i = 0 i = 1 i = a + 1 and so on....
- Recall that the = symbol is
an operator that returns a value
- A LOOP-CONTINUATION-CONDITION:
this is
a Boolean expression
that determines whether the
for-statement will
be terminated
(This clause has the same function as in the LOOP-CONTINUATION-CONDITION of an while-statement)
- The incr-expression:
this is an expression that
is evaluated at the end
of the loop.
When is the incr-expression is evaluated:
- After the statements in the for-loop body
- Before the for-statement repeats to test the LOOP-CONTINUATION-CONDITION
The incr-expression is used to make the for-loop advance to the next (possible solution) case
To make the advancement, the incr-expression is usually an assignment expression of the following form (but it can be very complicated):
i = i + 1 (move up by 1) i = i - 1 (move down by 1) i = i + 2 (move up by 2) i = i + a (move up by a) and so on....
- An init-expression:
this is an expression that is
evaluated once
at the start of
the for-statement
- The for-statement is
completed with
one statement
- This is the body of the
for-statement
- The body will
be
executed
as long as the
loop-continuation-condition is true
!!!
After the body has been executed, the incr-expression is evaluated before repeating the check of the LOOP-CONTINUATION-CONDITION
- This is the body of the
for-statement
- The keyword for
announces (to the Java compiler) that
we started an for-statement
- Common practice in
for-loops:
- Use a block as body of for-statements
- A typical for-loop
looks like this:
- For-statement:
(I have used colors to highlight the correspondence of the pieces in the Java language syntax and in the flow chart)
- Flow chart representing the
for-statement:
- This is the program flow when
LOOP-CONTINUATION-CONDITION
is true:
In this case, the for-statement will:
- Execute the statements in the
body
- Evaluate the incr-expression
- Repeat the for-statement from the LOOP-CONTINUATION-CONDITION
- Execute the statements in the
body
- This is the program flow when
LOOP-CONTINUATION-CONDITION
is false:
In this case, the for-statement will:
- Terminate
- The program will proceed with the statement that follows the for-statement
- Terminate
- Example:
for ( a = 1 ; a <= 10 ; a++ ) { System.out.println(a); } System.out.println("Done");Flow chart of this program:
Note: this is the same flow chart we saw before belonging to this program fragment:
a = 1; while ( a <= 10 ) { System.out.println(a); a++; } System.out.println("Done");
- Execution:
- Program "loops" as long as a ≤ 10
- Program exits loop when a > 10
- For-statement:
- Structure diagram representing a
while-statement:
How to "read" the diagram:
- The outer rectangle
containing the
LOOP-CONTINUATION-CONDITION
represents the
for-statement:
The for-statement is one (single) statement)
The for-statement will only complete execution when the LOOP-CONTINUATION-CONDITION becomes false
- The inner rectangle (shaded with a green color) is the while-loop body
When the LOOP-CONTINUATION-CONDITION is true, the for-statement will:
- First execute the statements in the
inner rectangle
(the for-statement's body)
- Then (always followed by) evaluate the incr-expression
- The inner rectangle (shaded with a green color) is the while-loop body
- The outer rectangle
containing the
LOOP-CONTINUATION-CONDITION
represents the
for-statement: