Tracing

Tracing loops and arrays

Passing arrays as parameters

Objects in java are passed by reference. Primitive data types are passed by value. This means that when you pass an Object as a method parameter, you are not giving the method the Object itself, but a reference to it.

String [] theirActualGrades= {"F", "F", "F", "F", "F", "A", "B", "C", "F", "D"}; myInstance.defaultFillGrade(theirActualGrades);

public void defaultFillGrade(String [] grades){ for (int student = 0; student < grades.length; student++) grades[student] = "A"; // note, pass by reference ... what does that mean? // objects in Java are always passed by reference // primitive data types are passed by value }

If I write another line after the call to the method:

myInstance.defaultFillGrade(theirActualGrades); for (int student = 0; student < theirActualGrades.length; student++) System.out.println(theirActualGrades[student]);

I get that all grades in the array theirActualGrades are A's

Another small method which accepts an array as a parameter:

int myArray [] = {1,2,3,4,5,6,7,8,9,10}; // ... somewhere in some method int myInstance.myArrayTotal = sum(myArray); // method definition public int sum( int [] array) { int total = 0; for (int s=0; s < array.length; s++) // when the index s equals the length, loop will stop total = total + array[s]; // find the sum of all of the elements in the array // total here is called an accumulator - it accumulates the sum return total; }

Again, somewhere an array was instatiated:
int[] table = new int[24];

And a sample invocation of the method looks like:

total = sum(table);

Some more array traces. How many lines of output would be in the following?:

for (int I =0; I < 4; I++) for (int K = 0 ; K < 5; K++) System.out.println("here I am again");

output
and

for (int I =0; I < 6; I++) for (int K = 0 ; K <1; K++) System.out.println("here I am again");

output
and

for (int I =0; I < 5; I++) for (int K = I ; K < 5-I; K++) System.out.println("here I am again");

output
and

for (int I =5; I < 0; I--) for (int K = 0 ; K < 1; K++) System.out.println("here I am again");

has no output

More Loop Examples:

int x = 1; do { System.out.println("Looping, round " + x); x++; } while (x <= 4);

Yields:
Looping, round 1
Looping, round 2
Looping, round 3
Looping, round 4

int n = 6; int counter = 0; do { counter = counter + 2; } while (counter <= n);

Yields: ?

What do these do?

y *= 2; x += 1; x++; ++x;

Write a for loop that generates the following output
(many middle terms are intentionally left out):

-1000
-996
-992
-988
//... output deleted
988
992
996
1000

Answer

an if question:
For what exact range of values of the variable x does the code below print the letter C? these

if (x <= 200 ) if (x < 100) if (x <= 0) System.out.println("A"); else System.out.println("B"); else System.out.println("C"); else System.out.println("D");

another loop (with a switch) to trace

int count; for (count = 1; count <= 12; count++) { switch (count % 3) { // mod 3 case 0: g.setColor(Color.red); break; case 1: g.setColor(Color.blue); break; case 2: g.setColor(Color.green); break; } g.drawLine(10, count * 10 , 80, count * 10); }

code and output

Specific to lab 7:

loops and sequencing (the order of sequencing) are very very very very very very very important to programming

Using the code I provided with editing so will compile:

public void setNewGame(){ int chosenColors[] = {0,0,0,0,0,0,0,0}; // explain why have it ... and why here // Box boxes[][] = new Box[4][4]; // what is problem with declaring this here? for (int i = 0; i <= 3; i++) { // do calculations to set/change x variables System.out.println("my col value is: " + i); for (int j = 0; j <= 3; j++){ System.out.println("my row value is: " + j); // do calculations to set/change y variables // generate a random color (actually a random // int) which then accesses a color // in your array colorArray[] (see below) AND int rint = (int)(Math.random() * 8); // this **could** generate more than 2 of the // same color - so a check is needed ... somewhere // you have to keep track of what colors have already // been chosen and not allow more than twice // another array of ints is a good idea boxes[i][j] = new Box(x, y, height, width, colorArray[rint]); // do you have what you think you have? System.out.println("coordinates: row " + j + " column " + i); } } repaint(); }

looks like. Why?

Set x and y coordinate in outer loop:

for(col = 0; col <=3; col++){ localX = initialBoxX + col*width; // what and why System.out.println("my col value is: " + col); localY = initialBoxY + row*height; // what and why for(row = 0; row<4; row++){ System.out.println("my row value is: " + row); // missing color stuff boxes[row][col] = new Box(localX, localY, height, width, colorChoices[rint]); System.out.println("coordinates: row " + row + " column " + col); } }

how this would increment and its applet
and

initialize chosenColor array

for(col = 0; col <=3; col++){ int chosenColors[] = {0,0,0,0,0,0,0,0}; // what and why localX = initialBoxX + col*width; System.out.println("my col value is: " + col); for(row = 0; row<4; row++){ System.out.println("my row value is: " + row); localY = initialBoxY + row*height; rint = (int)(Math.random() * 8); chosenColors[rint] = chosenColors[rint] +1; // some color stuff missing about choosing two colors // even if put here would result in the applet below boxes[row][col] = new Box(localX, localY, height, width, colorChoices[rint]); System.out.println("coordinates: row " + row + " column " + col); } }

Discussion on how this would look and its applet
and

where to change color

int chosenColors[] = {0,0,0,0,0,0,0,0}; for(col = 0; col <=3; col++){ localX = initialBoxX + col*width; rint = (int)(Math.random() * 8); // and the missing color lines again... System.out.println("my col value is: " + col); for(row = 0; row<4; row++){ System.out.println("my row value is: " + row); localY = initialBoxY + row*height; boxes[row][col] = new Box(localX, localY, height, width, colorChoices[rint]); System.out.println("coordinates: row " + row + " column " + col); } }

Discussion on how this would look and its applet

and

Simple placement of a line, in general, makes a difference

int chosenColors[] = {0,0,0,0,0,0,0,0}; for(col = 0; col <=3; col++){ localX = initialBoxX + col*width; System.out.println("my col value is: " + col); for(row = 0; row<4; row++){ System.out.println("my row value is: " + row); localY = initialBoxY + row*height; rint = (int)(Math.random() * 8); while (chosenColors[rint] >= 2) {rint = (int)(Math.random() * 8);} chosenColors[rint] = chosenColors[rint] +1; boxes[row][col] = new Box(localX, localY, height, width, colorChoices[rint]); System.out.println("coordinates: row " + row + " column " + col); } }

Think about how this above code differs from the following:

int chosenColors[] = {0,0,0,0,0,0,0,0}; for(col = 0; col <=3; col++){ System.out.println("my col value is: " + col); for(row = 0; row<4; row++){ System.out.println("my row value is: " + row); localY = initialBoxY + row*height; rint = (int)(Math.random() * 8); while (chosenColors[rint] >= 2) {rint = (int)(Math.random() * 8);} chosenColors[rint] = chosenColors[rint] +1; boxes[row][col] = new Box(localX, localY, height, width, colorChoices[rint]); System.out.println("coordinates: row " + row + " column " + col); } localX = initialBoxX + col*width; }

and its applet

It is a very subtle difference in the placement of the line
localX = initialBoxX + col*width;

Consider what it does to the output...