The for-statement

1. The for-statement

2. Previously discussed: different loop-statements in Java • Java provides 3 types of loop-statements: We will now study the for-statement 1. The while-statement 2. The for-statement 3. The do-while-statement

3. Previously discussed: find all divisor of a number n • Algorithm to find all divisors of a number 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) for (x = 1, 2, 3, ..., n) do { if ( n is divisible by x ) { print x; // x is a divisor of n } }

4. The for-statement • The for-statement is ideally suited for making the following type of program: • The for-statement can best be explained with an example.... • Let some variable x take on a series of valueone at a time • For each value taken on by a variable x, the body of the for-statement is executed once.

5. Example for-statement: the most common way to use a for-statement • The for-statement was originally invented to let some variable take on a series of value

6. Example for-statement: the most common way to use a for-statement (cont.) • The most common form of the for-statement is as follows: for ( var = START_VALUE ; var <= STOP_VALUE ; var = var + INCR ) { /* for-body (statements) */ }

7. Example for-statement: the most common way to use a for-statement (cont.) • Meaning: • The variable var will take on the following values: • For each of the value, the statements in the for-body are executed • START_VALUE • START_VALUE+ INCR • START_VALUE+ 2×INCR • ... • Up to the largest value that is ≤ STOP_VALUE

8. Example for-statement: the most common way to use a for-statement (cont.) • 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

9. Example for-statement: the most common way to use a for-statement (cont.) • 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

10. Example for-statement: the most common way to use a for-statement (cont.) • Example Program: (Experiment with it yourself outside class !)               • Prog file: http://mathcs.emory.edu/~cheung/Courses/170/Syllabus/07/Progs/For01.java • How to run the program:             • Right click on link and save in a scratch directory • To compile:   javac For01.java • To run:          java For01

11. Example for-statement: the most common way to use a for-statement (cont.) • 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

12. Example for-statement: the most common way to use a for-statement (cont.) • 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

13. Example for-statement: the most common way to use a for-statement (cont.) • 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

14. Example for-statement: the most common way to use a for-statement (cont.) • 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

15. Example for-statement: the most common way to use a for-statement (cont.) • 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

16. Example for-statement: the most common way to use a for-statement (cont.) • 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!!

17. Example for-statement: the most common way to use a for-statement (cont.) • 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"); } }

18. Example for-statement: the most common way to use a for-statement (cont.) • Answer: 4 7 (4 + 1×3) 10 (4 + 2×3) 13 (4 + 3×3) Done

19. Syntax and meaning of the for-statement • The general syntax of the for-statement is as follows:

20. Example for-statement: the most common way to use a for-statement (cont.) 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

21. Syntax and meaning of the for-statement (cont.) • Explanation: • The keyword for announces (to the Java compiler) that we started an for-statement • Following the keyword for, there are 3 clauses: 1. An init-statement: this is a statement that is executed once at the start of the for-statement

22. Syntax and meaning of the for-statement (cont.) 2. 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) 3. A INCR-STATEMENT: this is a statement that is executedat the end of the loop.

23. Syntax and meaning of the for-statement (cont.) The INCR-STATEMENT is executed: • After the for-loop body • Before the for-statement repeats to test the • LOOP-CONTINUATION-CONDITION

24. Syntax and meaning of the for-statement (cont.) • The for-statement is completed with one statement • This is the body of the for-statement • The body will be executedas long as the loop-continuation-condition is true !!!       • After the body has been executed, the INCR-STATEMENT is executed before repeating the check of the LOOP-CONTINUATION-CONDITION

25. Common practice in for-statements • Common practice in for-loops: • Use a block as body of for-statements

26. Common practice in for-statements (cont.) • A typical for-loop looks like this:

27. Flow chart of a for-statement • For-statement: (I have used colors to highlight the correspondence of the pieces in the Java language syntax and in the flow chart)

28. Flow chart of a for-statement (cont.) • Flow chart representing the for-statement:

29. Flow chart of a for-statement (cont.) • This is the program flow when LOOP-CONTINUATION-CONDITION is true:

30. Flow chart of a for-statement (cont.) • In this case, the for-statement will: • Execute the statements in the body • Execute the INCR-STATEMENET • Repeat the for-statement from the LOOP-CONTINUATION-CONDITION

31. Flow chart of a for-statement (cont.) • This is the program flow when LOOP-CONTINUATION-CONDITION is false:

32. Flow chart of a for-statement (cont.) • In this case, the for-statement will: • Terminate • The program will proceed with the statement that follows the for-statement

33. Flow chart of a for-statement (cont.) • Example: for ( a = 1 ; a <= 10 ; a++ ) { System.out.println(a); } System.out.println("Done");

34. Flow chart of a for-statement (cont.) • Flow chart of this program:

35. Flow chart of a for-statement (cont.) • 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");

36. Flow chart of a for-statement (cont.) • Execution: • Program "loops" as long as a ≤ 10 • Program exits loop when a > 10

37. Structure diagram of a for-statement • For-statement:

38. Structure diagram of a for-statement (cont.) • Structure diagram representing a while-statement:

39. Structure diagram of a for-statement (cont.) • How to "read" the diagram: • The outer rectangle containing the LOOP-CONTINUATION-CONDITION represents the for-statement:

40. Structure diagram of a for-statement (cont.) • The for-statement is one (single) statement) • The for-statement will only complete execution when the LOOP-CONTINUATION-CONDITION becomes false

41. Structure diagram of a for-statement (cont.) • The inner rectangle (shaded with a green color) is the while-loop body When the LOOP-CONTINUATION-CONDITION is true, the for-statement will execute: • The statements in the inner rectangle (the for-statement's body) • The INCR-statement