The for loop statement Syntax Semantics Nested Loops continue, break, and exit Statements

1. CS 120
The for-loop and nested
loops
Jubail Industrial College
Computer Science & Engineering Department

2. 2
Outline
 The for loop statement
 Syntax
 Semantics
 Nested Loops
 continue, break, and exit Statements

3. 3
The for Statement
 for loops are used if the number of iterations are known.
 It begins with the keyword for, followed by three expressions in
parentheses that describe what to do with one or more
controlling variables
 It consists of 3 parts separated by a semicolon (;):
 Part 1: initialize control variable. Only executed once.
 This part tells how the controlling variable or variables are
initialized or declared and initialized before the first iteration
 Part 2: condition (boolean expression).
 This part determines when the loop should end, based on the
evaluation of a Boolean expression before each iteration
 Part 3: update of control variable. Executed at the end of each
iteration.
 This part tells how the control variable or variables are updated after
each iteration of the loop body

4. 4
Syntax of the for Statement
 General form:
for(initialize; condition; update)
statement
for(initialize; condition; update){
block_statement
}
OR

5. 5
Semantics of the for Statement
 How it works:
Initialize control
variable
boolean_exp
Execute
statements
Update control
variable
true
false

6. 6
for Statement Syntax and Alternate Semantics
 Syntax:
 Example:
 Example output:
for(initialize; Boolean_Expression; update)
Body
for(number = 100; number >= 0; number--)
System.out.println(number + " boxes.");
100 boxes.
99 boxes.
.
.
.
0 boxes.

7. 7
for Statement Syntax and Alternate Semantics
 Equivalent while Loop
Syntax :
 Equivalent example:
 Equivalent output: 100 boxes.
99 boxes.
.
.
.
0 boxes.
number = 100;
while(number >= 0)
{ System.out.println(number + “
boxes.");
number--; }
Initialization;
while ( Boolean_Expression)
{ Body ;
Update; }

8. 8
The for Statement
 The following loop calculates:

10
1
i
i
int sum = 0;
for(int i = 1; i <= 10; i++)
sum = sum + i;
System.out.println(“sum = “ + sum);
i = 1
i ≤ 10
sum = sum + i
i++
True
System.out.
println(“sum =
“ + sum);
False
sum
i
1 1
2 3
3 6
4 10
5 15
6 21
7 28
8 36
9 45
10 55
11
Loop
stops

9. 9
The for Statement
 To calculate: 1 + 2 + 3 + 4 + . . . + n
 To calculate: 1 × 2 × 3 × 4 × . . . × n
int sum = 0;
for(int i = 1; i <= n; i++)
sum = sum + i;
int product = 1;
for(int i = 1; i <= n; i++)
product = product * i;

10. 10
The for Statement
 The control variable of the for loop can only be used inside the
loop. If needed after the loop, then it must be declared before the
loop.
 count should be declared before loop if needed afterwards.
double sum = 0;
for(int count = 1; count <= 10; count++){
System.out.print(“Enter score “ + count);
sum = sum + kb.nextInt();
}
System.out.println(“average = “ + sum / (count – 1));
Error:
count is
undefined
double sum = 0;
int count;
for(count = 1; count <= 10; count++){
System.out.print(“Enter score “ + count);
sum = sum + kb.nextInt();
}
System.out.println(“average = “ + sum / (count – 1));

11. 11
The Comma in the for Statement
 A for loop can contain multiple initialization actions separated
with commas
 Caution must be used when combining a declaration with multiple
actions
 It is illegal to combine multiple type declarations with multiple
actions, for example
 To avoid possible problems, it is best to declare all variables outside
the for loop
 A for loop can contain multiple update actions, separated with
commas, also
 However, a for loop can contain only one Boolean expression to
test for ending the loop

12. 12
The Comma in the for Statement
 If another initialization or update is needed, they are separated
by a comma (,).
 The following code display odd and even numbers in 1 – 20.
 Initialization could also be done before the loop. This means part
1 can be empty:
int x, y;
for(x = 1, y = 2; x <= 20 && y <= 20 ; x+=2, y += 2)
System.out.println(x+ " " + y);
int x = 1, y = 2;
for( ; x <= 20 && y <= 20 ; x += 2, y += 2)
System.out.println(x+ " " + y);

13. 13
Extra Semicolon in the for Statement
 You normally do not place a semicolon after the closing
parenthesis at the beginning of a for loop.
 Well, if you place a semicolon by mistake, the compiler won’t
throw an error, but it will be considered as a full for statement
whose body is empty.
 Example:
 Output:
 This same problem can arise with a while loop. Remember, we
only place a semicolon after parenthesis that encloses the
Boolean_Expression in a do_while loop.
for ( int count = 1; count <= 10; count++);
System.out.println("Hello");
Hello

14. 14
Nested Loops
 A loop structure can be used in the body of another loop
structure.
 The inner loop will iterate completely for each single iteration of
the outer loop.
 It outputs:
int row, column;
for(row = 1; row <= 3; row++){
for(column = 1; column <= 3; column++)
System.out.print(row + " " + column + " ");
System.out.println();
}
1 2
1 1
2 1
1 3
2 2 2 3
3 1 3 2 3 3

15. 15
Nested Loops
 The inner loop can use the control variable of the outer loop.
for(int i = 1; i <= 4; i++){
for(int j = 1; j <= i; j++)
System.out.print(" * " );
System.out.println();
}
*
**
***
****

16. 16
continue & break
 continue statement ends the current iteration of the loop and
moves to update part.
 Its useful to skip an iteration.
 This code finds summation of positive numbers less than 12 but
not multiple of 3.
int sum = 0;
for(int i = 1; i <= 12; i++){
if(i % 3 == 0){
continue;
}
sum = sum + i;
}
sum
i
1 1
2 3
3
Skip
4
5
6
7
8
9
10
11
12
7
12
Skip
19
27
Skip
37
48
Skip

17. 17
continue & break
 break statement terminates the loop.
 Its useful to stop the loop completely if something happens.
 This code stops calculating factorial of a number entered by user
when it becomes more than 2147483647.
 If break or continue is in nested loops, it will apply to the
closest loop.
int f = 1;
System.out.print(“Enter a number to find its factorial:”);
int n = kb.nextInt();
for(int i = 1; i <= n; i++){
if(f >= 2147483647) {
break;
}
f = f * i;
}

18. 18
exit Statement
 exit statement in System class is used to end the program.
 Its useful to abort the program if something happens.
 This code calculates factorial of a number entered by user. It
aborts the program if it is negative.
int f = 1;
System.out.print(“Enter a number to find its factorial:”);
int n = kb.nextInt();
if(n < 0){
System.out.println(“factorial of negative is unknown”);
System.out.println(“Program aborting”);
System.exit(0);
}
else {
for(int i = 1; i <= n; i++){
if(f >= 2147483647) break;
f = f * i;
}
}

19. 19
The end
Important to do at home :
- read section 3.3