Simple array question.

Programming Questions

brokenbeep

in Programming Questions • 2 years ago

Hi everyone, found this nice example for storing mouse movements into an array. There are however a couple of lines of code I don't get. Lines 10 - 12 move the values to the right but I just don't understand how. I just can't seem to put my finger on it, could anyone break the logic down a little more for me? Thanks in advance!

int[] y; // Declare array
void setup() {
size(200, 100);
y = new int[100]; // Create array
}
void draw() {
background(204);
// Shift the values to the right
for (int i = y.length-1; i > 0; i--) {
y[i] = y[i-1];
}
// Add new values to the beginning
y[0] = constrain(mouseY, 0, height-1);
// Display each pair of values as a line
for (int i = 1; i < y.length; i++) {
line(i, y[i], i-1, y[i-1]);
}
}

Replies(4)

v.k.

Re: Simple array question.

2 years ago

Once i read in a book (i think is called how to think like a computer scientist) something like this "to read a code you must do it as the processor will do". This code, at the beginning, was weird for me also.

Basically you are getting the last number in your array and throwing away, also at the same time every number shifts towards the end of the array, geting closer to be discarded.

Thinking with small numbers:

Get an array of ten ints. Fill it with 0,1,2,3...9.

then read your code...

for (int i = y.length-1; i > 0; i--) {
y[i] = y[i-1];
}

for int i = 9 // <<(array.length - 1 as the first index is 0 . Note that this time we will transverse the array from the end to // the beginning. Usually we do it the other way, writing - for int i=0(zero).

while 9 is greater than zero // << again Unusually ;i>0 to stop at the beginning.

do this:

start with i=9:

get number at index 8 // y[9-1]

//at index 8 we have value 8

save it at index 9 // y[9] = y[9-1]

//now at index 8 and 9 we have value 8 (index 9 just trashed it's previous value. i.e there is no more the value 9 in the array.)

DEcrement i // i - -

//i now is 8

start over...

get number at index 7 // y[8-1]

//at index 7 we have value 7

save it at index 8 // y[8] = y[8-1]

//now at index 8 and 7 we have value 7 and at index 9 we have value 8

decrement i...

when reaching i = 1 you wil have all values shifted right except made dy index 0 wich will be holding 0 as will be index 1.

but, then next line you put there (index 0) a brand new value:

y[0] = constrain(mouseY, 0, height-1);

And now you have your shifted array.

Hope that helps.

brokenbeep

Re: Simple array question.

2 years ago

Thank you for taking the time to reply to this question. I'm still a little confused, here's how I'm reading the logic, trying my best to "read a code... as the processor will do"!

for (int i = y.length-1; i > 0; i--) {
y[i] = y[i-1];
}

so we start with:

for (int i = 9; // (array.length - 1) as the first index is 0 - this I get just fine

i > 0; i-- // 9 is greater than 0 so we subtract 1 from it making i =8, onto the next line

y[8] = y[7]; //so i = 8 now, it looks like we are going to copy the contents of array 8 into array 7. Now this is what I just don't get, aren't we copying the data to the left as opposed to the right? Shouldn't this line of code be y[i] = y[i+1] or y[8] = y[9] so that we could copy the array content into the space above it? Logically this would eventually end with y[0] being copied to y[1] so leaving y[0] free to be written to, no?

I know that something pretty simple is just not clicking, I hope I've illustrated why this is confusing for me! Thanks again for trying to talk me through this! :)

brokenbeep

Re: Simple array question.

2 years ago

Now I've written it out like that I can see where I'm going wrong! Thanks again for your help! :)

v.k.

Re: Simple array question.

2 years ago

Yeah, it seems backwards. But it is not.

y[8] = y[7]; //so i = 8 now, it looks like we are going to copy the contents of array 8 into array 7. No,no...

number at 8 will be equal to number at 7, or copy seven to 8 not the inverse :-)

i > 0; i-- // 9 is greater than 0 so we subtract 1 from it making i =8, onto the next line

Keep in mind that at first we start with 9, after code inside for is executed the var "i" is decremented... and so on. The code are executed once for each value of "i", starting with the number stated at for( int i = XX;;). The increment/decrement starts after first run not before.

In this code:

http://en.wikipedia.org/wiki/Fisher%E2%80%93Yates_shuffle

there is an other confusing (and cool) use of index of arrays. I had some work to decipher this...

void shuffle(int[] array) {
// i is the number of items remaining to be shuffled.
for (int i = array.length; i > 1; i--) {
// Pick a random element to swap with the i-th element.
int j = rng.nextInt(i); // 0 <= j <= i-1 (0-based array)
// Swap array elements.
int tmp = array[j];
array[j] = array[i-1];
array[i-1] = tmp;
}
}

Below the full code

that's a implementation i've tried of this Fischer Yates shuffle in case u like to see it runing.

I'm not sure if did not mess up, but seems ok to me. It's a teste i did to see if i could spot a "bias" in graphic resulting from this shuffle. It goes over and over and over...

// with code from WikiPedia; Fisher–Yates shuffle
//@ http://en.wikipedia.org/wiki/Fisher–Yates_shuffle
import java.util.Random;
Random rng = new Random();
int[] numbers = new int[240];
int[] index = new int[2];
int[] indexReto = new int[2];
color c1 = color(69,31,90);
PFont f;
float leng=0;
int xisp=-40;
void setup() {
size(1500,480);
for (int i = 0; i < numbers.length; i++) {
numbers[i]=i+1;
}
for (int i = 0; i < index.length; i++) {
index[i]=i;
indexReto[i]=i;
}
background(c1);
f = loadFont("DialogInput-30.vlw");
}
void draw() {
//fill(c1);
//rect(0,420,width/2,height);
stroke(0);
strokeWeight(2);
line(0,height/2,width,height/2);
shuffle(numbers);
shuffle(index);
// noStroke();
//fill(c1,20);
// rect(xisp,0,20,height);
for (int i = 0; i < index.length; i++) {
fill(115,139,11);
noStroke();
ellipse(xisp+i,height/2-numbers[index[i]],1,1);
stroke(0);
//line(xisp+i,height/2,xisp+i,height/2-numbers[index[i]]);
fill(120,142,18);
noStroke();
int ind=int(random(0,60));
ellipse(xisp+i,height-numbers[ind],1,1);
stroke(0);
//line(xisp+i,height,xisp+i,height-numbers[ind]);
//textFont(f,30);
///text(numbers[index[i]],70+i*60,height);
//println(numbers[index[i]]+" reto>> "+numbers[indexReto[i]]);
}
if( xisp<=width){
xisp=(xisp+1);}else{xisp=-40;}
}
void shuffle(int[] array) {
// i is the number of items remaining to be shuffled.
for (int i = array.length; i > 1; i--) {
// Pick a random element to swap with the i-th element.
int j = rng.nextInt(i); // 0 <= j <= i-1 (0-based array)
// Swap array elements.
int tmp = array[j];
array[j] = array[i-1];
array[i-1] = tmp;
}
}