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Why is the for loop running even though the condition is false

I have been trying to write a code trace for this program. after many attempts on paper I thought I understood it but now I have stumbled again.
In the for loop i=counter ,because i=0 and the counter=0 so since it has to be less than counter how is this code even running?
I have included my incomplete code trace below.
The aim of the program is to print 50 unique random numbers between 1 and 999, in 10 rows of 5. The program works perfectly.I just want to figure out how its working with the help of a code trace. I know this might be a simple problem to you, but I am struggling with it. Could you please help me, thank you.
import java.util.Random;
public class Random50 {
public static void main(String[] args) {
int numbersNeeded = 50;
int[] randomNumbers = new int[numbersNeeded];
int counter = 0;
Random randomGenerator = new Random();
int max = 999;
int min = 1;
while (counter < numbersNeeded) {
int generated = min + randomGenerator.nextInt(max);
boolean found = false;
for (int i = 0; i < counter && !found; i++) {
if (randomNumbers[i] == generated) {
found = true;
}
}
if (!found) {
randomNumbers[counter++] = generated;
}
}
for (int i = 0; i < counter; i++) {
System.out.printf("%03d ",randomNumbers[i] );
if (i > 0 && (i+1) % 5 == 0) {
System.out.println("");
}
}
}
}

This line is the reason why:
randomNumbers[counter++] = generated;
By using counter++, you're using the ++ postfix increment operator, which evaluates to the value of counter, but then increments it by 1 after the evaluation. The equivalent code without use of this operator would look something like this:
randomNumbers[counter] = generated;
counter = counter + 1;
So on the first iteration where this line is executed, randomNumbers[0] is updated, but then counter gets set to 1 (because 0 + 1 = 1). On the second iteration, randomNumbers[1] is updated, then counter gets updated to 2 (because 1 + 1 = 2)...and so forth.
You can find more information about arithmetic operators here: https://docs.oracle.com/javase/tutorial/java/nutsandbolts/op1.html

Array is printing more numbers than intended?

I'm supposed to create and initialize a 100-element array, then make the 7th element the number "7", and finally print the array, starting a new line every 20 elements. I've been trying to figure this out for a long time and I can't.
My code right now is:
public class Array {
public static void main(String args[]) {
int [] array = new int[100];
for (int a = 0; a < array.length; a++) {
if (array[a] == 6) {
array[a]=7;
array[a] = a + 1;
}
printArray(array);
}
}
public static void printArray(int[] array){
for (int a=0; a < array.length; a++) {
System.out.print(" " + array[a]);
if ((a - 1) % 20 == 0) {
System.out.println("");
}
}
}
}
When I run this my output is a lot of zeros, far more than 100. They are separated every 20 characters as intended, but the seventh element is not 7. I think it has to do with the association between int "a" and my array, but I can't figure it out. I know the solution must be simple but I just cannot see it. Thank you all!

Proper indentation of your code, in particular the main method, reveals what is going on. You are calling printArray from within the for loop, so you are printing the array contents 100 times.
for (int a = 0; a < array.length; a++) {
if (array[a] == 6) {
array[a]=7;
array[a] = a + 1;
}
printArray(array);
}
Move the call to printArray after the } ending brace for the for loop.
Now you'll get 100 0s.
Also, I think you meant to have array[a] = a + 1; executed if the index was not 6, e.g.
if (array[a] == 6) {
array[a] = 7;
} else {
array[a] = a + 1;
}
Additionally, you will want to print a newline after 20 numbers, e.g. after indexes 19, 39, etc., so add 1 to a before calculating the remainder, instead of subtracting 1, so that 19 + 1 = 20, whose remainder is 0.

There are many things wrong. However, to answer your question, you are printing the array 100 times since printArray is inside your first loop.

You misplaced an end parenthesis in your main method. The properly formatted method looks like this:
public static void main(String args[]) {
int [] array = new int[100];
for (int a = 0; a < array.length; a++) {
if (array[a] == 6) {
array[a]=7;
}
array[a] = a + 1;
}
printArray(array);
}

First of all your code is organized very badly so it's very easy for u to miss what went where. You have 2 major mistakes, first of all you called printArray()
Inside your for loop and therefore printed it 100 times.
Second, you kept checking if the value inside the array in index a is 6.
You need to check if a is 6 since it is your index like this:
if(a == 6)
array[a] = 7;

Well, I ran your code, and there are a few places that can be corrected.
As for your problem of the many things being printed, that's because you've placed your printarray() inside the for loop, so it's printing the array 100 times.
As for printing it out, i find this code to be more concise:
public static void printArray(int[] array){
int counter = 0;
for(int i = 0; i < array.length; i++){
System.out.print(array[i] + " ");
counter++;
if(counter == 20){
counter = 0;
System.out.print("\n");
}
}
}
Also, I'm not really sure why you're using a for loop to just change the 7th element. You could use this:
array[6] = 7;
I'm not really sure what you're doing in the for loop.
I hope this helped! Good luck!

triangle numbers in java

I am new to Java and now I want to learn better for loop. I made some examples , but I don't know how to do a triangle that look like this:
for n=6:
111111
22222
3333
444
55
6
My code until now:
class Pyramid
{
public static void main (String[] args)
{
int i,n=9,j;
for(i=1;i<=n;i++)
{
for(j=1;j<=i;j++) {
System.out.print(i); }
System.out.print("\n");
}}}
But what I managed to do it looks like this:
1
22
333
4444
55555
666666
How to make it in reverse order ?

We can use is a function int numberForRow(int row) that will perform a suitable transformation. Then the function can be used like r = numberForRow(i); print(r). It needs to map this:
row (i) -> display number (r)
6 1
5 2
4 3
3 4
2 5
1 6
I think you can write it :)
Look at the relationship between the input (i) and output (r) - it might be useful to note that they always add up to the same value so a little bit of math should do the trick.
(While a function isn't strictly required I find that such functions can help break down a problem and, especially in this case, illustrate a transformation well - it also works in case of a "more advanced" transformation, such as was in the original question ;-)

Your issue is that your outer for loop was going from 6 to 1, so you need to reverse that.
Change
for(i=n;i>=1;i--) {
To
for(i = 1; i <= n; i++) {
Further explanation, so you understand what is happening inside a for loop:
A for loop operates on three clauses: where you start, the condition that the loop runs, and what to do after it runs.
------v
for(i = 1; i <= n; i++) {
This is the assignment. You set a variable to a number, which is where the loop starts. In this case, we start with i = 1, since we want to print only one 1 on the first line. In the third clause, we will increment it (read: add one to it), and run the loop again.
--------------v
for(i = 1; i <= n; i++) {
This is the condition. The loop will run whenever this condition evaluates to true. In other words, if n = 6, this loop will run when i <= 6.
--------------------v
for(i = 1; i <= n; i++) {
This is what will happen each time the loop is executed. After it runs through once when i = 1, we will increment i, so now i = 2. This will happen until the condition (i <= n) evaluates to false, i.e. when i = 7. If the condition is false, the loop will terminate.

public class Pyramid {
public static void main (String[] args)
{
int i,n=9,j;
for(i=1;i<=n;i++)
{
//for(j=1;j<=i;j++) {
for(j=n;j>=i;j--) {
System.out.print(i);
}
System.out.print("\n");
}
}
}
This should help.

Can be done using below method:
public class Main {
public static void main(String[] args) {
int n = 6;
int m =n;
for (int i = 1; i <= n; i++,m--) {
for (int j = 1; j <= m; j++) {
System.out.print(i);
}
System.out.println();
}
}
}

If you want to print the triangular numbers then use the following code
`public class Triangular{
public static void main(String[] args) {
int i = 0;
int j =0;
int count =0;
for (i=1;i<=10;i++){
count = 0; // This is a program to print triangular numbers in JAVA
for(j=1;j<=i;j++){
count = count + j;
}
System.out.println(count);
}
}
}`

Efficient way of generating all combinations of 12 numbers that add to 100 in Java [duplicate]

This question already has an answer here:
How to iterate through array combinations with constant sum efficiently?
(1 answer)
Closed 9 years ago.
I have 12 products at a blend plant (call them a - l) and need to generate varying percentages of them, the total obviously adding up to 100%.
Something simple such as the code below will work, however it is highly inefficient. Is there a more efficient algorithm?
*Edit: As mentioned below there are just too many possibilities compute, efficiently or not. I will change this to only having a maximum of 5 or the 12 products in a blend and then running it against the number of ways that 5 products can be chosen from the 12 products.
There is Python code that some of you have pointed to that seems to work out the possibilities from the combinations. However my Python is minimal (ie 0%), would one of you be able to explain this in Java terms? I can get the combinations in Java (http://www.cs.colostate.edu/~cs161/Fall12/lecture-codes/Subsets.java)
public class Main {
public static void main(String[] args) throws FileNotFoundException, UnsupportedEncodingException {
for(int a=0;a<=100;a++){
for(int b=0;b<=100;b++){
for(int c=0;c<=100;c++){
for(int d=0;d<=100;d++){
for(int e=0;e<=100;e++){
for(int f=0;f<=100;f++){
for(int g=0;g<=100;g++){
for(int h=0;h<=100;h++){
for(int i=0;i<=100;i++){
for(int j=0;j<=100;j++){
for(int k=0;k<=100;k++){
for(int l=0;l<=100;l++){
if(a+b+c+d+e+f+g+h+i+j+k+l==100)
{
System.out.println(a+" "+b+" "+c+" "+d+" "+e+" "+f+" "+g+" "+h+" "+i+" "+j+" "+k+" "+l);
}}}}}}}}}}}}}
}
}

Why make it so difficult. Think simple way.
To explain the scenario simpler, consider 5 numbers to be generated randomly. Pseudo-code should be something like below.
Generate 5 random number, R1, R2 ... R5
total = sum of those 5 random number.
For all item to produce
produce1 = R1/total; // produce[i] = R[i]/total;

Please, don't use nested for loops that deep! Use recursion instead:
public static void main(String[] args) {
int N = 12;
int goal = 100;
generate(N, 0, goal, new int[N]);
}
public static void generate(int i, int sum, int goal, int[] result) {
if (i == 1) {
// one number to go, so make it fit
result[0] = goal - sum;
System.out.println(Arrays.toString(result));
} else {
// try all possible values for this step
for (int j = 0; j < goal - sum; j++) {
// set next number of the result
result[i-1] = j;
// go to next step
generate(i-1, sum + j , goal, result);
}
}
}
Note that I only tested this for N = 3 and goal = 5. It absolutely makes no sense to try generating all these possibilities (and would take forever to compute).

Let's take your comment that you can only have 5 elements in a combination, and the other 7 are 0%. Try this:
for (i = 0; i < (1<<12); ++i) {
if (count_number_of_1s(i) != 5) { continue; }
for (j = 0; j < 100000000; ++j) {
int [] perc = new int[12];
int val = j;
int sum = 0;
int cnt = 0;
for (k = 0; k < 12; ++k) {
if (i & (1 << k)) {
cnt++;
if (cnt == 5) {
perc[k] = 100 - sum;
}
else {
perc[k] = val % 100;
val /= 100;
}
sum += perc[k];
if (sum > 100) { break; }
}
else { perc[k] = 0; }
}
if (sum == 100) {
System.out.println(perc[0] + ...);
}
}
}
The outer loop iterates over all possible combinations of using 12 items. You can do this by looping over all numbers from 1:2^12, and the 1s in the binary representation of that number are the elements you're using. The count_number_of_1s is a function that loops over all the bits in the parameter and returns the number of 1s. If this is not 5, then just skip this iteration because you said you only want at most 5 mixed. (There are 792 such cases).
The j loop is looping over all the combinations of 4 (not 5) items from 0:100. There are 100^4 such cases.
The inner loop is looping over all 12 variables, and for those that have a 1 in their bit-position in i, then it means you're using that one. You compute the percentage by taking the next two decimal digits from j. For the 5th item (cnt==5), you don't take digits, you compute it by subtracting from 100.
This will take a LONG time (minutes), but it won't be nearly as bad as 12 nested loops.

for(int a=0;a<=100;a++){
for(int b=0;b<=50;b++){
for(int c=0;c<=34;c++){
for(int d=0;d<=25;d++){
for(int e=0;e<=20;e++){
for(int f=0;f<=17;f++){
for(int g=0;g<=15;g++){
for(int h=0;h<=13;h++){
for(int i=0;i<=12;i++){
for(int j=0;j<=10;j++){
for(int k=0;k<=10;k++){
for(int l=0;l<=9;l++){
if(a+b+c+d+e+f+g+h+i+j+k+l==100)
{
// run 12 for loops for arranging the
// 12 obtained numbers at all 12 places
}}}}}}}}}}}}}
In Original approach(permutation based), the iterations were 102^12 = 1.268e24. Even though the 102th iteration was false, it did check the loop terminating condition for 102th time.
So you had 102^12 condition checks in "for" loops, in addition to "if" condition checks 101^12 times, so in total, 2.4e24 condition checks.
In my solution(combination based),No of for loop checks reduces to 6.243e15 for outer 12 loops, &
if condition checks = 6.243e15.
Now, the no of for loops(ie inner 12 for loops) for every true "if" condition, is 12^12 = 8.9e12.
Let there be x number of true if conditions. so total condition checks
=no of inner for loops*x
= 8.9e12 * x + 6.243e15
I'm not able to find the value of x. however, I believe it wouldnt be large enough to make total conditon checks greater than 2.4e24

java.lang.ArrayIndexOutOfBoundsException: 2 >= 2

I'm brand new to Java and my first assignment was to implement a "for" loop. I wrote this program in C++ and it compiles in Java, but I got an error at runtime. Can anyone tell me what's wrong?
import java.util.Scanner;
import java.util.Vector;
public class GlobalMembersMain
{
public static Vector<Integer> get_prime_factors(int number)
{
Vector<Integer> primefactors = new Vector<Integer>();
for (int j = 2; j <= number; j++)
{
if (number % j == 0)
{
primefactors.add(j);
number = number / j;
j = 1;
}
}
return primefactors;
}
public static void main(String[] args)
{
int number;
int count = 1;
System.out.print("Enter integer to analyse:");
System.out.print("\n");
Scanner scan = new Scanner(System.in);
number = scan.nextInt();
Vector<Integer> primefactors = new Vector<Integer>();
primefactors = get_prime_factors(number);
System.out.print("Prime factors are ");
for (int a = 0; a < primefactors.size() + 1; a++)
{
if (primefactors.elementAt(a) == primefactors.elementAt(a+1))
{
count++;
}
else
{
System.out.print(primefactors.elementAt(a));
System.out.print(" (");
System.out.print(count);
System.out.print(") ");
count = 1;
}
}
System.out.print("\n");
}
}
The output:
Enter integer to analyse:
10
Prime factors are 2 (1) Exception in thread "main" java.lang.ArrayIndexOutOfBoun
dsException: 2 >= 2
at java.util.Vector.elementAt(Unknown Source)
at GlobalMembersMain.main(GlobalMembersMain.java:36)

for (int a = 0; a < primefactors.size() + 1; a++)
{
if (primefactors.elementAt(a) == primefactors.elementAt(a+1))
{
count++;
}
Is exceeding the size of the primefactors collection. By 2, in fact.
Change to primefactors.size() - 1 to avoid this error.

Arrays are zero based, which I imagine you are aware of. What you may not be aware of is that in Java a List is backed by an array as well. When you invoke primefactors.size() +1 you are getting one more than you would possibly want. For instance is pf is of size 1 your loop will do the following:
pf.get(0); //returns the only value in the list
pf.get(1); // element doesn't exist
Now the other thing is you do not want to use Vector, generally speaking in Java. It is a synchronized collection. What you want is List/ArrayList.
OTHER CODE ISSUES
public static Vector<Integer> get_prime_factors(int number)
this does not need to be static. Also naming convention is camel case in Java so your function name should be getPrimeFactors(int number)
GlobalMembersMain
Should most likely be named GlobalMember as classes are to be singular in nature and I believe you added Main to indicate it was the class that holds the main function.
In your main function you would do this:
GlobalMember member = new GlobalMember();
member.getPrimeFactors(number);

This is where the problem is:
for (int a = 0; a < primefactors.size() + 1; a++)
{
if (primefactors.elementAt(a) == primefactors.elementAt(a+1))
{
count++;
}
//...
primefactors.elementAt(a+1) for the last element in your collection will throw the exception (AIOB).

Remember that arrays, lists and vectors in Java are zero-based. In your case, the primefactors vector will consist of two elements, available at index 0 and 1 respectively.
The problem you are facing is that you try to access the element primefactors.elementAt(2) which does not exist.
One problem is the break condition in the loop:
for (int a = 0; a < primefactors.size() + 1; a++) {
// ...
}
The first time, a will be 0, the second time 1 which are both fine. However, the loop will not break the third time, because a will equal 2, which is less than primefactors.size() + 1. Consequently, there will be a call to primefactors.elementAt(2) which does not exist and the program will blow up.
There is also a second problem inside the loop since you increment the loop variable by one during the comparison:
if (primefactors.elementAt(a) == primefactors.elementAt(a+1)) {
// ...
}
Yet again, your program will fail if you pass 2 as an argument to primefactors.elementAt(...)