Except where otherwise noted, the contents of this document are Copyright 2012 Stuart Reges and Marty Stepp.
lab document created by Marty Stepp, Stuart Reges and Whitaker Brand
Goals for today:
for
loops for repeating
lines of code
Recall that Java has expressions to represent math and other computations. Expressions may use operators, which are evaluated according to rules of precedence. Every expression produces a value of a given type.
Type | Description | Example | Result |
---|---|---|---|
int |
integers (up to 231 - 1) | 3 + 4 * 5 |
23 |
double |
real numbers (up to 10308) | 3.0 / 2.0 + 4.1 |
5.6 |
String |
text characters | "hi" + (1 + 1) + "u" |
"hi2u" |
Write the results of each of the following expressions. If you're stuck, ask a TA or neighbor.
12 / 5 + 8 / 4
|
4 |
|
3 * 4 + 15 / 2
|
19 |
|
-(1 + 2 * 3 + (1 + 2) * 3)
|
-16 |
|
42 % 5 + 16 % 3
|
3 |
|
2.5 * 2 + 17 / 4
|
9.0 |
|
4.5 / 3 / 2 + 1
|
1.75 |
Write the results of each of the following expressions.
2 + 6 + "cse 142"
|
"8cse 142" |
|
"cse 142" + 2 + 6
|
"cse 14226" |
|
1 + 9 / 2 * 2.0
|
9.0 |
|
46 / 3 / 2.0 / 3 * 4/5
|
2.0 |
|
50 / 9 / 2.0 + 200 / 10 / (5.0 / 2)
|
10.5 |
continued on the next slide...
In this exercise, you'll use the Interactions Pane to quickly discover the result of some expressions that would be difficult to evaluate by hand. Copy/paste each expression below into the Interactions Pane to evaluate it, then input the answer into this slide.
123 * 456 - 789
|
55299 |
|
3.14 + 1.59 * 2.65
|
7.3535 |
|
2 * 2 * 2 * 2 * 2 * 2 * 2 * 2 * 2 * 2
|
1024 |
|
2 + 2 + "xyz" + 3 + 3
|
"4xyz33" |
(For the last expression, the Interactions Pane doesn't put ""
quotes around Strings when displaying results, so you must add those yourself if needed. For example, if the Interactions Pane gives you a result of abc123
, it should be written here as "abc123"
.)
Recall that you can use a variable to store the results of an expression in memory and use them later in the program.
type name; // declare name = value or expression; // assign a value ... type name = value or expression; // declare-and-initialize together
Examples:
double iphonePrice; iPhonePrice = 299.95; int siblings = 3; System.out.println("I have " + siblings + " brothers/sisters.");
Which of the following choices is the correct syntax for declaring a real
number variable named grade
and initializing its value
to 4.0
?
Suppose you have a variable named grade
, set
to 1.6
:
double grade = 1.6; // uh-oh
Suppose later in the program's code, we want to change the value
of grade
to 4.0
. Which is the correct syntax to
do this?
Suppose you have a variable named balance
, set
to 463.23
:
double balance = 463.23
Suppose later in the program's code, we want to add 5 to the account balance. Which is a correct statement to do this?
a
,
b
, and c
What are the values of a
, b
, and c
after the following statements? Write your answers in the boxes on the
right.
int a = 5; int b = 10; int c = b; a = a + 1; // a? 6 b = b - 1; // b? 9 c = c + a; // c? 16
for
loops
A for
loop repeats a group of statements a given number of times.
for (initialization; test; update) { statement(s) to repeat; }
Example:
for (int i = 1; i <= 10; i++) { System.out.println("We're number one!"); }
for
loop
public class Count2 { public static void main(String[] args) { for ( fill me in! ) { System.out.println( fill me in! ); } } }
2 times 1 = 2 2 times 2 = 4 2 times 3 = 6 2 times 4 = 8
Our Practice-It! system lets you solve Java problems online.
What output is produced by the following Java program? Write the output in the box on the right side.
public class OddStuff { public static void main(String[] args) { int number = 32; for (int count = 1; count <= number; count++) { System.out.println(number); number = number / 2; } } } |
Output: 32 16 8 4 |
public static void stars() { for (int i = 1; i <= 10; i++) { for (int j = 1; j <= i; j++) { System.out.print("*"); } for (int j = 1; j <= 20 - 2 * i; j++) { System.out.print(" "); } for (int j = 1; j <= i; j++) { System.out.print("*"); } System.out.println(); } }
* * ** ** *** *** **** **** ***** ***** ****** ****** ******* ******* ******** ******** ********* ********* ********************
ComputePay
The following program redundantly repeats the same expressions many times. Download it and open it in jGRASP, then modify the program to remove the redundancy using variables. Use an appropriate type for each variable.
The program's output should be the same after your modifications. No expression should be computed more than once in the code.
1 2 3 4 5 6 7 8 9 10 11 12 13 |
public class Oops { public static void main(String[] args) { int x; System.out.println("x is" x); int x = 15.2; // set x to 15.2 System.out.println("x is now + x"); int y; // set y to 1 more than x y = int x + 1; System.out.println("x and y are " + x + and + y); } } |
answer on next slide...
+
between "x is"
and x
x
before assigning it a
value
15.2
into a variable of
type int
"
mark should be between now
and +
int
here
y
should be same type as x
y
to be 1 more
than x
(should not write the word int
here)
and
should be in quotes with surrounding spaces
public class Oops { public static void main(String[] args) { double x = 0.0; System.out.println("x is" + x); x = 15.2; // set x to 15.2 System.out.println("x is now " + x); double y; // set y to 1 more than x y = x + 1; System.out.println("x and y are " + x + " and " + y); } }
i
,
j
, and k
What are the values of i
, j
, and k
after the following statements?
int i = 2; int j = 3; int k = 4; int x = i + j + k; i = x - i - j; // i? 4 j = x - j - k; // j? 2 k = x - i - k; // k? 1
Suppose you have a real number variable x
. Write a Java
expression that computes a variable named y
storing the
following value:
y = 12.3x4 - 9.1x3 + 19.3x2 - 4.6x + 34.2
(We haven't learned a way to do exponents yet, but you can simulate them using several multiplications.)
Use the example program on the next slide to test your code.
Copy/paste this program into jGRASP to test your solution.
// expected output: // y is 7043.7 public class EquationY { public static void main(String[] args) { double x = 5; double y = put your expression for y here ; System.out.println("y is " + y); } }
(answer on next slide)
double y = 12.3*x*x*x*x - 9.1*x*x*x + 19.3*x*x - 4.6*x + 34.2;
If you want an added challenge, try to come up with a way to compute the
above value while using the *
operator no more than 4 times.
(click Next → for answer)
double y = (((12.3 * x - 9.1) * x + 19.3) * x - 4.6) * x + 34.2;
Bday
that
declares four variables and assigns appropriate values to them.
My birthday is 9/19, and Suzy's is 6/14.
for
loop
practicepublic class Triangle { public static void main(String[] args) { for (int line = 1; line <= 4; line++) { for (int stars = 1; stars <= expression ; stars++) { System.out.print("*"); } System.out.println(); } } }
continued on the next slide...
We want to produce the following output:
******* ***** *** *
Fill in the table below indicating how many stars appear on each line of output.
Line | Stars |
---|---|
1
|
7 |
2
|
5 |
3
|
3 |
4
|
1 |
continued on the next slide...
We need an expression for the number of stars on each line of this form:
multiplier * line + constant
for (int stars = 1; stars <= expression ; stars++) {Your program should now produce the correct output. You can verify with the Output Comparison Tool.
for
loops to produce the
following output:
000111222333444555666777888999 000111222333444555666777888999 000111222333444555666777888999
99999888887777766666555554444433333222221111100000 99999888887777766666555554444433333222221111100000 99999888887777766666555554444433333222221111100000 99999888887777766666555554444433333222221111100000 99999888887777766666555554444433333222221111100000
999999999888888887777777666666555554444333221 999999999888888887777777666666555554444333221 999999999888888887777777666666555554444333221 999999999888888887777777666666555554444333221
Write a program to produce the following output using nested for
loops. Use a table to help you figure out the patterns of characters on each line.
-----1----- ----333---- ---55555--- --7777777-- -999999999- |
|
|
Test your loop expressions in Practice-It! using the checkmark icon above. Use your expressions in the loop tests of the inner loops of your code.
Write a Java program in a class named SlashFigure
to produce the following output with nested for
loops. Use a loop table if necessary to figure out the expressions.
!!!!!!!!!!!!!!!!!!!!!! \\!!!!!!!!!!!!!!!!!!// \\\\!!!!!!!!!!!!!!//// \\\\\\!!!!!!!!!!////// \\\\\\\\!!!!!!//////// \\\\\\\\\\!!////////// |
|
Test your code in Practice-It! or the Output Comparison Tool.
Add a class constant to your slash figure program so that it can be resized from its default of 6:
size 4 | size 7 |
---|---|
!!!!!!!!!!!!!! \\!!!!!!!!!!// \\\\!!!!!!//// \\\\\\!!////// |
!!!!!!!!!!!!!!!!!!!!!!!!!! \\!!!!!!!!!!!!!!!!!!!!!!// \\\\!!!!!!!!!!!!!!!!!!//// \\\\\\!!!!!!!!!!!!!!////// \\\\\\\\!!!!!!!!!!//////// \\\\\\\\\\!!!!!!////////// \\\\\\\\\\\\!!//////////// |
Test your code in the Output Comparison Tool.
If you finish all the exercises, try out our Practice-It web tool. It lets you solve Java problems from our Building Java Programs textbook.
You can view an exercise, type a solution, and submit it to see if you have solved it correctly.
Choose some problems from the book and try to solve them!