Recognizing Patterns
• Counting: continually updating a value by a
fixed amount
• Counting raindrops
int dropCount = 0; //Raindrop counter
while (dropCount < MAX) {
new Raindrop(…);
dropCount++;
}
Counting Bricks
while ( count < TOTAL ) {
new Brick(…);
count++;
}
The Counting while Loop
int i = initialValue;
while (i < stopVal) {
...
i++;
}
// initialize
// test
// do stuff
// increment
The for loop
• Especially useful for counting
• Ex:
for ( int i=initialVal;
i<stopVal;
i++;) {
…
}
//initialize
//test
//increment
//do stuff
Counting Raindrops with for Loop
for (int dropCount = 0;
dropCount <MAX;
dropCount++) {
new Raindrop (…);
}
More General Start and End Points
• Loops can take whatever starting point, end
point, and increment
Ex:
for (int i=23; i <= 1728; i=i+591;){
//do stuff
}
• But one should avoid using a double for any
of the three values
General Syntax of for Loop
• for (initialization; condition; update) {
//Do something
}
Initialization: gen’ly creates a counting variable
Condition: a boolean expression to stop the loop
Updating: updates the variable created
Counting Backwards with for Loop
Ex: Printing a countdown
for (int count = 10; count >= 1; count--) {
System.out.println(count);
}
Update Values
• Can increment loop index by any value
• Ex: Drawing grass blades
for (int pos = 0;
pos < WIDTH;
pos = pos + GAP) {
new Line (pos, TOP, pos, GROUND, canvas);
}
Debugging Loops
• You may find the following useful to fix
your loops when they are not working right:
– Memory table (trace)
– Inserting System.out.println( ) statements
– Using the BlueJ debugger
Debugging using the
Memory Table (Trace) method
• The purpose of this exercise is to help
reinforce how loops actually work
• The computer follows a specific set of rules
involving loops. Until you get an intuition
for it, you may have to periodically trace a
loop to understand how it works (or does
not work)
• The code examples are arbitrary but reflect
what comes up often in loops
Trace the following statements
sum
m
k
k< 5?
Trace the following statements
Solution:
sum
0
10
11
12
13
14
m
10
12
14
16
18
20
k
0
1
2
3
4
5
k< 5?
T
T
T
T
T
F
done
Another trace example
j
j<=18?
Console
Another trace example--SOLN
j
10
13
16
19
j<=18?
T
T
T
F
Console
10
10 13
10 13 16
10 13 16
and
another
i
i<4 p
p%2 == 0 Console
and
another
SOLN
i
i<4 p
p%2 == 0 Console
10
1
T
17
F
17
2
T
24
T
17 24 even
3
T
31
F
17 24 even 31
4
F
(done )
Lab 13 Problem 8
•
•
•
•
Draw the row of windows first
The size of the windows is 30 (or so)
Separated by 15 ... gives 450 total + edges
Then draw the outline. Try to center row of
windows in buiilding
Nested Loops
• Any loop body can contain another loop
Ex: for ( … ) {
while (…) {
while (…) {
for(…) {
}
}
}
}
Tracing a nested loop
i
i<4 k
k>2 Console
Tracing a nested loop -- SOLN
i
0
i<4 k
T 5
4
3
2
1
T 5
4
etc. ..... ....
k>2
T
T
T
F
T
T
....
Console
5
4
3
6
5
....
Lab 13 Problem 14
sinitialize xpos, ypos
sset up outer loop to go 0 to 19 {
s set up inner loop 0 to 3 {
• draw a window
• change xpos
• }
• change ypos
• reset xpos
}
The do while Loop
• Syntax:
do {
<code to repeat>
} while (<condition>)
Craps Example
do while Loop vs while Loop
• do while
– Condition checked at the end
– Loop body executed at least once
• while
– Condition checked at the beginning
– Loop body may never execute
Avoiding Loop Errors
• Easier to find errors if you know where to
look
• Common loop errors include:
• Off by 1 in counting loops
• Infinite loops
Off by one errors
Suppose we want to run a for loop 5 times:
for(int i=0;i<=5; i++){ for(int i=0;i<5;i++) {
}
}
The left hand version will run it 6 times, not 5.
Infinite Loops
Ex:
while ( count< TOTAL ) {
new Brick (…);
}
Since value of count is not updated, the
condition in while will stay true forever.