1. Lecture 4: Program Control Flow
Dr. Kyung Eun Park
Summer 2017

2. Default Control Flow: Top to Bottom
Flow Chart
start
statement 1
statement 2
statement 3
end

3. Method Invocation
From main(), call method1(), return from the method and continue …
start
main() {
int num;
num = 10; …
method1();
System.out }
method1 { … }
method2 { … } …
end

4. Conditional Execution : if Statement
if Conditional Statement
start
start
main() {
int num; …
if (num>50) { }
statements
test
test
Yes
redrawn
num>50
Yes_statements
Yes_statements …
statements
end
end

5. The if Statement Executes a block of statements only if a test is true
if (test) {
statement;
... }
Example:
double gpa = console.nextDouble();
if (gpa >= 2.0) {
System.out.println("Application accepted.");

6. The if-else Statement
Executes one block if a test is true, another if false
if (test) {
statement(s);
} else { }
Example:
double gpa = console.nextDouble();
if (gpa >= 2.0) {
System.out.println("Welcome to Mars University!");
System.out.println("Application denied.");

7. Conditional Execution: if-else Statement
start
if-else Conditional Statement
start
statements
test
main() {
int num; …
if (num>50) {
} else { } No
Yes
num>50
test
Yes_statements …
redrawn
No_statements
Yes_statements
No_statements …
statements
end
end

8. Nested if-else Statement
Chooses between outcomes using many tests
if (test1) {
statement1;
} else if (test2) {
statement2;
} else {
statement3; }
Example:
if (x > 0) {
System.out.println("Positive");
} else if (x < 0) {
System.out.println("Negative");
System.out.println("Zero");

9. Nested if-else-if Statement
If it ends with else, exactly one path must be taken.
If it ends with if, the code might not execute any path.
if (test1) {
statement1;
} else if (test2) {
statement2;
} else if (test3) {
statement3; }
Example:
if (place == 1) {
System.out.println("Gold medal!");
} else if (place == 2) {
System.out.println("Silver medal!");
} else if (place == 3) {
System.out.println("Bronze medal.");

10. Program I – if-else, return statement
Write a method quadrant that accepts a pair of real numbers x and y and returns the quadrant for that point:
Example: quadrant(-4.2, 17.3) returns 2
If the point falls directly on either axis, return 0 y+
quadrant 2
quadrant 1 x- x+
quadrant 3
quadrant 4 y-

11. Program I – if-else, return statement
quadrant method
public static int quadrant(double x, double y) {
if (x > 0 && y > 0) {
return 1;
} else if (x < 0 && y > 0) {
return 2;
} else if (x < 0 && y < 0) {
return 3;
} else if (x > 0 && y < 0) {
return 4;
} else { // at least one coordinate equals 0
return 0; }

12. Boolean Expressions Tests use relational operators:
if statements and for loops both use logical tests.
if (i <= 10) { ...
for (int i = 1; i <= 10; i++) { ...
These are boolean expressions
Tests use relational operators:
Operator
Meaning
Example
Value ==
equals
1 + 1 == 2
true !=
does not equal
3.2 != 2.5
<
less than
10 < 5
false
>
greater than
10 > 5
<=
less than or equal to
126 <= 100
>=
greater than or equal to
5.0 >= 5.0
Note that == tests equality, not = . The = is used for the assignment operator!

13. Logical Operators Tests can be combined using logical operators:
"Truth tables" for each, used with logical values p and q:
Operator
Description
Example
Result
&&
and
(2 == 3) && (-1 < 5)
false || or
(2 == 3) || (-1 < 5)
true !
not
!(2 == 3) p q
p && q
p || q
true
false p !p
true
false

14. Evaluating Logic (Boolean) Expressions
Relational operators have lower precedence than math.
5 * 7 >= * (7 - 1)
5 * 7 >= * 6
35 >=
35 >= 33
true
Relational operators cannot be "chained" as in algebra.
2 <= x <= 10
true <= (assume that x is 15)
error!
Instead, combine multiple tests with && or ||
2 <= x && x <= 10
true && false
false

15. Logical Questions
What is the result of each of the following expressions?
int x = 42;
int y = 17;
int z = 25;
y < x && y <= z
x % 2 == y % 2 || x % 2 == z % 2
x <= y + z && x >= y + z
!(x < y && x < z)
(x + y) % 2 == 0 || !((z - y) % 2 == 0)
Answers: true, false, true, true, false

16. Repetition with for Loops
We don’t like repetition, but computer does well.
System.out.println("Homer says:");
System.out.println("I am so smart");
System.out.println("S-M-R-T... I mean S-M-A-R-T");
Keeping repeating statements is not efficient!!!  Loop Structure
for (int i = 1; i <= 4; i++) { // repeat 4 times }

17. for loop syntax for (initialization; test; update) { statement; ... }
Perform initialization once.
Repeat the following:
Check if the test is true. If not, stop.
Execute the statements.
Perform the update
loop body
loop header

18. Initialization Tells Java what variable to use in the loop
for (int i = 1; i <= 6; i++) {
System.out.println("I am so smart"); }
Tells Java what variable to use in the loop
Performed once as the loop begins
The variable, i is called a loop counter
can use any name, not just i
can start at any value, not just 1
visible only within the for loop : Variable scope

19. Test Tests the loop counter variable against a limit
for (int i = 1; i <= 6; i++) {
System.out.println("I am so smart"); }
Tests the loop counter variable against a limit
Uses comparison operators:
< less than
<= less than or equal to
> greater than
>= greater than or equal to

20. Update: Increment and Decrement
shortcuts to increase or decrease a variable's value by 1
Shorthand Equivalent longer version
variable++; variable = variable + 1;
variable--; variable = variable - 1;
int x = 2;
x++; // x = x + 1;
// x now stores 3
double gpa = 2.5;
gpa--; // gpa = gpa - 1;
// gpa now stores 1.5

21. Modify-and-Assign shortcuts to modify a variable's value
Shorthand Equivalent longer version
variable += value; variable = variable + value;
variable -= value; variable = variable - value;
variable *= value; variable = variable * value;
variable /= value; variable = variable / value;
variable %= value; variable = variable % value;
x += 3; // x = x + 3;
gpa -= 0.5; // gpa = gpa - 0.5;
number *= 2; // number = number * 2;

22. Repetition over a Range
System.out.println("1 squared = " + 1 * 1);
System.out.println("2 squared = " + 2 * 2);
System.out.println("3 squared = " + 3 * 3);
System.out.println("4 squared = " + 4 * 4);
System.out.println("5 squared = " + 5 * 5);
System.out.println("6 squared = " + 6 * 6);
Intuition: "I want to print a line for each number from 1 to 6“
 The for loop does exactly that!
 "For each integer i from 1 through 6, print ...“
for (int i = 1; i <= 6; i++) {
System.out.println(i + " squared = " + (i * i)); }

23. Loop Walkthrough Output: for (int i = 1; i <= 4; i++) {2 4
for (int i = 1; i <= 4; i++) {
System.out.println(i + " squared = " + (i * i)); }
System.out.println("Whoo!");
Output:
1 squared = 1
2 squared = 4
3 squared = 9
4 squared = 16
Whoo! 3 5 1 2 3 4 5

24. Multi-line Loop Body Output: System.out.println("+----+");
for (int i = 1; i <= 3; i++) {
System.out.println("\\ /");
System.out.println("/ \\"); }
Output:
+----+
\ /
/ \

25. Degree Conversion: Celsius to Fahrenheit
int highTemp = 5;
for (int i = -3; i <= highTemp / 2; i++) {
System.out.println(i * ); }
Output:

26. Cumulative Algorithms
Find the sum of all integers from :
// This may require a lot of typing
int sum = ;
System.out.println("The sum is " + sum);
What if we want the sum from 1 - 1,000,000?
Or the sum up to any maximum?
 How can we generalize the above code?

27. Cumulative Sum Loop
cumulative sum: A variable that keeps a sum in progress and is updated repeatedly until summing is finished.
Cumulative sum variable, sum must be declared outside the loop that updates it, so that it will still exist after the loop.
int sum = 0;
for (int i = 1; i <= 1000; i++) {
sum = sum + i; }
System.out.println("The sum is " + sum);

28. Homework I – Cumulative Product
Write a method cumProduct that accepts a positive number and returns the cumulative product up to the number starting from 1.
Example: cumProduct(10) returns
If the number in negative, please make it positive and continue…

29. Homework II – File and Cumulative Sum
Write a program which generates the grade statistics including the average grade, the highest, and the lowest grade with the names of student.
Given data (student.txt) 10
Kim
Eric 86
Stef 79
Joe 50
Lauren
John
Emily 89
Grace
Tim
Amy 87
Output (grade.txt)
/*Statistics from student data */
Avg. Grade = 83.9
Max. Grade = 98 (Lauren)
Min. Grade = 50 (Joe)