Develop Reference

Counting Sort issue

public static void countingSort(Integer[] a, int n) {
//TODO
//COMPLETE THIS METHOD
int[] counter = new int[n+1];
int[] sorted = new int[a.length];
Arrays.fill(counter,0);
// fills counter array with each number count
for ( int i = 0 ; i < a.length; i++){
counter[a[i]] += 1;
} // adds n-1 index + n index
for (int i = 1; i < counter.length; i++) {
counter[i] = (counter[i] + counter[i-1]);
} // shifts array to the right
for (int i = counter.length-1; i > 0; i--) {
counter[i] = counter[i-1];
} // fills sorted array with the sorted out counts
for (int i = 0; i < a.length; i++ ){
sorted[counter[a[i]]] = a[i];
counter[a[i]]++;
}
}
When ran it throws an array out of bound exception in the body of the first for loop. Im having trouble seeing it, if anyone could help guide me, it'd be much appreciated.

When ran it throws an array out of bound exception in the body of the
first for loop.
That would be this:
for ( int i = 0 ; i < a.length; i++){
counter[a[i]] += 1;
}
Im having trouble seeing it, if anyone could help
guide me, it'd be much appreciated.
An ArrayIndexOutOfBoundsException in that loop can only mean that you are exceeding the bounds of a or those of counter. It is clear from inspection that you are not exceeding the bounds of a (though you could both eliminate that possibility and make the code a bit cleaner by switching to an enhanced for loop). The only remaining possibility is that you are exceeding the bounds of counter.
The approach you've implemented supports only non-negative integers in the input array, and requires the method invoker to pass an upper bound on the element values as the second parameter. If those constraints are not satisfied then an exception such as you observe will be thrown.
That is, if the exception is thrown where you say it is, then the problem is with the input.

Iterating through an array of arrays in java [duplicate]

This question already has answers here:
How can I avoid ArrayIndexOutOfBoundsException or IndexOutOfBoundsException? [duplicate]
(2 answers)
Closed 5 years ago.
I am simply trying to iterate through every variable in an array of arrays:
Here's the working code:
int[][] mArray;
mArray = new int[2][2];
mArray[0][0] = 1;
mArray[0][1] = 2;
mArray[1][0] = 3;
mArray[1][1] = 4;
for (int i = 0; i < mArray.length; i++)
{
for (int x = 0; x < mArray[i].length; x++)
{
System.out.println(mArray[i][x]);
}
}
This prints out:
1
2
3
4
So Everything's fine.
However if I replace
"for (int x = 0; x < mArray[i].length; x++)"
with
"for (int x = 0; x < mArray[x].length; x++)"
It get the following error: "Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 2".
Can anyone explain why this error happens to occur? Both mArray[i].length and mArray[x].length result in a value of "2", so why does the second option not work?
Thanks in advance :)

Explanation is simple.
for (int x = 0; x < mArray[x].length; x++) {
// body
}
Before iterations: x=0
Statements execution order:
- 1st iteration: 0 < mArray[0].length == true; execute body; increment x
(x==1)
- 2nd iteration: 1 < mArray[1].length == true; execute body;
increment x (x==2)
- 3rd iteration: 2 < mArray[2].length cause
ArrayIndexOutOfBoundsException, because x==2 and length of mArray is 2
UPD: totally, it's wrong logic to replace i with x in inner cycle, because i - it's row counter, and x - counter for columns.

Understanding the for loop in Java

I'm new to Java.
I can't seem to understand why these two codes produce different outputs.
Please explain this to me.
What is the difference of y<=x; and y<=5;. As you can see the x is 5 too, I don't understand why I get different outputs.
for (int x = 0; x < 5; x++) {
for (int y = 1; y <=x ; y++) {
System.out.print("x");
}
for (int g = 4; g >= x; g--) {
System.out.print("*");
}
System.out.println();
}
Output:
*****
x****
xx***
xxx**
xxxx*
Code:
for (int x = 0; x < 5; x++) {
for (int y = 1; y <= 5; y++) {
System.out.print("x");
}
for (int g = 4; g >= x; g--) {
System.out.print("*");
}
System.out.println();
}
Output:
xxxxx*****
xxxxx****
xxxxx***
xxxxx**
xxxxx*

Basically the main difference is this line:
for(int y=1; y<=x; y++)
resp.
for(int y=1; y<=5; y++)
The number of times the loop is executed is different. Namely in the first case it is variable (so the number of 'x' increases), in the second case it is fixed (5 'x' printed each time).
(edit: typo)

xstarts at 0 so the first iteration has the condition y<=0, the second will have y<=1 and so on .. till y<=5
While the second one will have y<=5in every iteration, thats why you get xxxxx in every line.

In the first code you print x times the "x" String in each row.
for(int y=1; y<=x; y++) {
System.out.print("x");
}
BTW, it prints the following (which is different than what you claim in the question):
*****
x****
xx***
xxx**
xxxx*
In the second code you print 5 times the "x" String in each row.
for(int y=1; y<=5; y++) {
System.out.print("x");
}
As you can see the x is = 5 too
No, x iterates from 0 to 4, so in each iteration of the outer for loop, it has a different value.

In your first code your for(int y=1; y<=x; y++) for iterations of outer for loop is -
for(int y=1;y<=0;++y) (for first iteration of outer loop)
for(int y=1;y<=1;++y) (for second iteration of outer loop)
for(int y=1;y<=2;++y) (for third iteration of outer loop)
for(int y=1;y<=3;++y) (for fourth iteration of outer loop)
for(int y=1;y<=4;++y) (for fifth iteration of outer loop)
But in your second code its always -
for(int y=1; y<=5; ++y)
for all iterations of outer for loop.

It is very simple the will run for 5 time times and every itreation its value will be increamented by 1 i.e. from 0 to 4.
So in first loop inner loop will have the condition like this:
for (int y = 1; y <= x; y++) {
System.out.print("x");
}
But since in first loop the value of x is 0 hence it literally means:
for (int y = 1; y <= 0; y++) {
System.out.print("x");
}
But in the last iteartion of outer loop the value of x is 4 hence this is equivalent to:
for (int y = 1; y <= 4; y++) {
System.out.print("x");
}
So it iterates 4 times.

In your first example y is first less than x = 1 and during the next iteration it will be less x= 2 ... Because x values changes with your first for loop.
For the second example however you state that y have to be less than 5 which doesn't change at all.

They are different because in the first case your x varies from 0 to 4 based on :
for(int x=0; x<5; x++)
In the case second case x is fixed at 5.

I have replaced your two inner for loops with a new stringRepeat function. It might be easier to understand this way.
public class ForLoopStarsAndCrosses {
public static void main(String[] args) {
/// the total length ot the x's and stars
final int length = 5;
// start with 1 x until one less than the length (in this case 4)
for(int x = 1; x < length; x++) {
// the number of stars equals the total length minus the number of x's
final int nbStars = length - x;
// add the x's and the stars
final String output = stringRepeat("x", x) + stringRepeat("*", nbStars);
System.out.println(output);
}
System.out.println();
}
/** Repeat the string s n times */
private static String stringRepeat(String s, int n) {
String result = "";
for (int i = 0; i < n; i += 1) {
result += s;
}
return result;
}
}

Firstly, you should understand the difference between value and variable. Value is a constant as you write 5 but variable can be changeable.
For your question, first code and first round:
x = 1, y<= 1 and output: x
for(int y=1; y<=x; y++){
System.out.print("x");
}
but for the second code and first round:
y<=5 so output is: xxxxx
for(int y=1; y<=5; y++){
System.out.print("x");
}
it is very simple.

The purpose of loops (can be for, while, do-while) is that, how many number of times the same set of statements to be executed under a specific condition. "for" is a definite loop where there will be an index starting with an integer, keep increment it until the condition is achieved. "while" is a indefinite loop where there is no index and it will be executed until the condition is achieved. "do-while" is a loop similar to "while", which will be executed atleast once and then validates the condition for the next iteration.
Based on the above details,
for(int x=0; x<5; x++){
for(int y=1; y<=x; y++){
for(int x=0; x<5; x++){
for(int y=1; y<=5; y++){
The diff between these two conditions is that,
First condition:
In the first iteration, value of x is 0 and for the second loop y is started with 1.
Here when it compares the value with x which is 0 and 1<=0 which is false, this condition is failed and the statements under Y will not be executed and the control will go back control of x.
Second condition:
In the first iteration, value of x is 0 and for the second loop y is started with 1.
Here when it compares the value with 5, 1<=5 which is true, this condition is valid and the statements under Y will be executed and the control will go back control of x until y becomes 6 and condition checked is 6<=5 which is false, this condition is failed and the statements under Y will not be executed and the control will go back control of x
Note: Due to low reputation, I can't add comments and had to contribute this as an answer. Don't downvote this, instead suggest corrections by comment on the same.

Questions about array

Here is my code and it works perfectly fine.
import java.util.Random;
class apples
{
public static void main(String args[])
{
Random rand = new Random();
int frequency[] = new int[7];
for(int roll = 1;roll < 1000;roll++){
++frequency[1+rand.nextInt(6)];
}
System.out.println("Face\tFrequency");
for(int face = 1;face < frequency.length;face++){
System.out.println(face + "\t" + frequency[face]);
}
}
}
I do not understand this line of code
++frequency[1+rand.nextInt(6)];
When I removed the "++" operator, it couldn't be compiled. I know that it will add 1 to the randon numbers generated from 0 to 5 but why there is a "++" in front of frequency ? Why is it neccessary to put the "++" operator there ?

The ++ operator is incrementing the frequency at the specified index. In this case, it's the same as saying:
for(int roll = 1;roll < 1000;roll++){
int index = 1+rand.nextInt(6);
frequency[index] = frequency[index] + 1;
}
Removing the ++ operator, you're left with:
for(int roll = 1;roll < 1000;roll++){
frequency[1+rand.nextInt(6)];
}
The line frequency[1+rand.nextInt(6)] makes no sense; it is not an operation, it does not do anything.
EDIT:
Perhaps a better illustration: let x be the value looked up in the frequency array. Then the original is equivalent to:
for(int roll = 1;roll < 1000;roll++){
int x = frequency[1+rand.nextInt(6)];
++x; // Equivalent to "x = x + 1"
}
Whereas if you remove the increment operator, your resulting loop would be:
for(int roll = 1;roll < 1000;roll++){
int x = frequency[1+rand.nextInt(6)];
x; // ...what?
}

The statement without the ++ looks at the contents of the array, but does not do anything with the value. Adding the ++ operator tells it to increment the value found there.
You probably are thinking that frequency[1+rand.nextInt(6)] is adding a value to the array. It is not. It is looking up a location in memory from a random location.

++ is a shorthand for
int i = 1+rand.nextInt(6);
frequency[i] = frequency[i] + 1;
Removing it causes no increment expression is executed.

that line is equivalent to this line of code
int index = 1+rand.nextInt(6);
frequency[index] = frequency[index]+1 ;
so if you removed '++' you make the statement incomplete like the following:
int x= 5;
x;
so it gives you error

Beginner: Trying to run an sum/average class getting an error

When I run this class I am not getting a the sum/average. I am getting an error listed below. I am not sure why I am getting it. I am a student very new at this.
public class DebugEight2
{
public static void main(String[] args)
{
int[] someNums = {4,17,22,8,35};
int tot;
int x;
int sum = 0;
for(x = 0; x <= someNums.length; ++x)
tot = someNums[x];
sum = sum + someNums[x];
System.out.println("Sum is " + sum);
System.out.println("Average is " + sum * 1.0 / someNums.length);
}
}
Error:
"Exception in thread "main" java.lang.ArrayIndexOutofBoundsException: 4 at DebugEight1.main (DebugEight1.java:17)"

Array index starts from 0 and goes till it's length -1 See that <= in your for loop. So this for(x = 0; x <= someNums.length; ++x) should be for(x = 0; x < someNums.length; ++x)

Problem is your loop condition, remove the equal sign like this
for(x = 0; x < someNums.length; ++x)
You should also wrapp the code which is supposed to be inside the loop into brackets (without them, only the next line after the for loop condition will be executed)
for(x = 0; x < someNums.length; ++x) {
tot = someNums[x];
sum = sum + someNums[x];
}

first you should remove = from <= someNums.length; i will look like this < someNums.length; and then change ++x to x++ if you put ++x when loop starts it will check condition < someNums.length; and then pluse one +1 to x and so on, currently your array length is 5 when loop goes on last increment its size is 6 and there is no item on 6 location thats why it is throwing exception so you have you change it ++x to x++
public class DebugEight2
{
public static void main(String[] args)
{
int[] someNums = {4,17,22,8,35};
int tot;
int x;
int sum = 0;
for(x = 0; x <someNums.length; x++) /// just change here
tot = someNums[x];
sum = sum + someNums[x];
System.out.println("Sum is " + sum);
System.out.println("Average is " + sum * 1.0 / someNums.length);
}
}

Arrays are zero based and their indices go from 0 to someNums.length -1
You not only exceeding the bounds of the array but the variable sum is being assigned outside of scope of the loop. Adjust the upper bounds of the array and add braces:
for (x = 0; x < someNums.length; x++) {
tot = someNums[x];
sum = sum + someNums[x];
}
Hope this helps

You want to iterate while x is lesser than someNums.length; since max index in array is arraysLength - 1, so change x <= someNums.length; to x < someNums.length; in for loop
There is no need for tot (I assume it should store total numbers) variable because you already have someNums.length which represents how many numbers you summed. BTW you already are calculating avg using sum and someNums.length. So you can safely remove
tot = someNums[x];
from your code.

What you did is you looped beyond the scope of the array. Try doing someNums.length, you will get the number 5.
Going to your for loop we now know that it starts at 0 and increments until it gets to 5. And the for loop is traversing over the array and is pointing at the positions in your array.
So let's traverse through your loop.
At x = 0, someNums[x] = 4.
At x = 1, someNums[x] = 17,
At x = 2, someNums[x] = 22,
At x = 3, someNums[x] = 8,
At x = 4, someNums[x] = 35,
As you can see, your array has now ended, you no longer have any more elements to traverse over. But we have previously stated that your for loop increments until it gets to 5.
So at x = 5, someNums[x] will have to give you an error since you are pointing at a position and trying to retrieve information from that position that essentially does not exist.
There are two ways to solve your problem:
You change the loop to for(int x = 0; x < someNums.length; x++)
You change the loop to for(int x = 0; x <= someNums.length - 1; x++)
These will solve your problem because they will stop your loop at 4 and not at 5, therefore you will no longer receive an error.