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Java - For loop seems to execute past it's termination condition

I have debugged this code and it appears to run a for loop even though the termination condition is met.
This program takes two types of input:
Line 1 - How many data points there are following (N)
Lines 2 to N - The data points
The program should then print the smallest difference between all of the data points.
So, for instance, a sample input would be (on separate lines): 3 5 8 9
There are 3 data points (5 8 9), and the smallest difference is between 8 and 9, so the program should return 1.
I am trying to build the program in a way in which the data points are populated into an array at the same time as the comparisons are made. Obviously I could separate those concerns, but I am experimenting. Here it is:
package com.m3c.vks.test;
import java.util.*;
import java.io.*;
import java.math.*;
class Solution {
public static void main(String args[]) {
Scanner in = new Scanner(System.in);
int N = in.nextInt(); //How many data points there will be
int strengthArray[] = new int[N]; //Initialise the array to be of length = N, the first input line
int tempStrengthDifference=99999; //junk difference set arbitrarily high - bad practice I know
int lowestStrengthDifference=99999;
for (int i = 0; i < N; i++) //Go through all elements of array
{
strengthArray[i] = in.nextInt(); //read a value for the ith element
System.out.println("i: " + i); //test
if (i > 0) //do not execute the next for loop if i = 0 as cannot evaluate sA[-1]
{
for (int j = i - 1; j < 1; j--) // **this is line 20** from wherever the array has populated up to, work backwards to compare the numbers which have been fed in thus far
{
System.out.println("j: " + j); //test
tempStrengthDifference = Math.abs(strengthArray[i] - strengthArray[j]); //finding the difference between two values
if (tempStrengthDifference < lowestStrengthDifference) //store the lowest found thus far in lowestSD
{
lowestStrengthDifference = tempStrengthDifference;
}
}
}
}
System.out.println(lowestStrengthDifference);
}
}
Everything is fine up until when i = 1 on line 20. At this point, j is set to i - 1 = 0 and the difference is found. However, when the for loop comes back around again, the termination condition of j < 1 is not met, and instead the loop continues to set j = -1, at which point it throws an out of bounds error as it clearly cannot evaluate strengthArray[-1]
Any ideas? Thanks

Have a look at your loop: for (int j = i - 1; j < 1; j--)
You start with j = 0 when i == 1 and thus j < 1 is ok.
The next iteration has j = -1 (0-1) and hence you get the problem.
Do you mean to use j >= 0 as your loop condition instead? Note that the second parameter is not a termination condition but a continuation condition, i.e. as long as that condition is met the loop will execute.

The reason behind your failure is the inner loop variable change.
When i=1, j=0 and after executing the loop once, J is decremented, and thus j becomes - 1. The condition j<1 is satisfied since you have written j--, change it to j++ and you should be fine.

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.

Why does this loop display this output?

for (int i = 1; i < 5; i++) {
int j = 0;
while (j < i) {
System.out.println("i = " + i + "\tj = " + j);
j++;
}
}
The output is:
i = 1 j = 0
i = 2 j = 0
i = 2 j = 1
i = 3 j = 0
i = 3 j = 1
i = 3 j = 2
i = 4 j = 0
i = 4 j = 1
i = 4 j = 2
i = 4 j = 3
My question is, why is the value of j=0 in the second line, even though we increment it after the first line was displayed?
Also, why is the value of i=2 twice, and i=3 thrice?

you declared int j=0; within the first loop so whenever the outer loop runs value of j set to be 0.
And reason of repeated value of i is the inner loop.For every time the loop runs the value of i is same for it.

Because you said that "until j is < than i" you increment j but the increment stops when j is = i so you have that i repeats for each increment of j.
I'm a bit confused about what your program should do but if I understood you should resolve your code like this:
for (i = 0; i < 5; i++) {
int j;
for (j = 0; j < i; j++) {
System.out.println("i = "+i+"\tj = "+j);
}
}

Outer for loop re-initialzes j to 0 in second iteration of the outer loop as it does in every iteration of the outer loop. Hence second line shows j as 0.
Since in second and third iteration of outer for loop, inner loop gets executed twice and thrice respectively, but outer loop variable value remains the same during the iteration of the outer loop. Hence, i=2 and i=3 is shown twice and thrice.

The j-variable and the i-variable in your code are going like this:
i=2:j=0->1: prints 2 times
i=3:j=0->1->2 prints 3 times
...

This concept is called nested loops. The for-loop is the outer loop, which has value of i from 1 to 4. In the inner while-loop, the value of j is initialized to 0 and it goes till j = i-1 (since the condition mentioned in while-loop is j < i).
You enter the first iteration of for-loop (i = 1). Now you enter the first iteration of while-loop (j = 0). Condition is true (0 < 1). i = 1 j = 0 is printed. In next iteration of while loop, j = 1, condition is false (1 not < 1). Hence while-loop is now exited.
Now in the second iteration of for-loop, i value is incremented (i = 2). Now you enter the first iteration of while-loop (j = 0). Condition is true (0 < 2). i = 2 j = 0 is printed. In next iteration of while loop, j = 1, condition is true (1 < 2). i = 2 j = 1 is printed. In next iteration of while loop, j = 2 , condition is false (2 not< 2). Hence while loop is exited.
As you can see, in the first iteration of for-loop print is executed only once. And in the second iteration of for-loop print is executed twice. This is because execution of print is dependent on the value of j being lesser than i (i increases by 1 every time). This pattern will repeat and for i = 3 print statement will be executed thrice and so on.
Hope you understood. If not, feel free to clear your doubts in the comments.

What is this Nested For Loop doing? (Asterisk Right Triangle)

I just started learning about "nested for loops". But my question what is the nested for loop exactly doing every time it loops? I'm still confused as to what is variable "i" is doing and what is variable "j" is doing. If you can explain what is happening after every loop, it would make nested for loops much clearer.
import java.util.Scanner;
public class LoopStatement {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
int numStar;
System.out.print("Number of stars: ");
numStar = input.nextInt();
while (numStar < 1 && numStar > 20) {
System.out.println("Enter a valid number ");
numStar = input.nextInt();
}
for (int i = 1; i <= numStar; i++) {
for (int j = 1; j <= i; j++) {
System.out.print("*");
}
System.out.println();
}
}
}
So this is the output I get if I input 4 for numStar
*
**
***
****

There is actually nothing special about nested loops, they behave exactly the same like regular loop.
Specifically here the outer loop loops from i being 1 to numStar and therefore prints numStar rows.
The nested loop loops from j being 1 to i where i is the number of current row being printed. In each iteration it prints one star and therefore altogether it prints i stars.
Therefore here it prints 1 star on row number 1, 2 stars on row number 2, etc.

Try writing down in a paper like this :
i=1 j=1 => *
i=1 j=2 => (j<=i) is false
.
.
i=2 j=1 => *
*
i=2 j=2 => *
**
.
.
You'll get the output if you try to write it in a paper.

In order to understand the nested loop you must understand the for structure. It sounds like you did not get it.
from the basics:
for (int i = 1; i <= numStar; i++) {
System.out.println(i);
}
the code above iterate exactly numStart following the steps:
a-Starts with i=1: int i = 1
b-Test condition: i <= numStar
c-Print the value of i in the screen: System.out.println(i)
d-increments i value: i++
It execute while the conditions is satisfied: i <= numStar. The steps c and d are executed only the condition is satisfied.
the tested loop follows the same logic:
for (int i = 1; i <= numStar; i++) {
for (int j = 1; j <= i; j++) {
System.out.print("*");
}
System.out.println();
}
when i=1 -> j iterates from 1 to 1. So it prints "*"
when i=2 -> j iterates from 1 to 2. So it prints "**"
when i=3 -> j iterates from 1 to 3. So it prints "***"
and so on...
After the nested iteration finish there is line break: System.out.println();

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.