Develop Reference

Create a 2D array with first number being the number of rows

1.Create a square 2D array of a size that is decided by user input - the first number being the number of rows.
2.Allow the user to fill the array with integers of their choice.
3. Print to the screen the sum of the diagonals. For example see below
Example 2D Array:
| 1 2 3 4 |
| 5 4 6 9 |
| 11 13 16 6 |
| 2 4 18 20 |
Diagonal 1 = 1, 4, 16, 20. Diagonal 2 = 4, 6, 13, 2. The sum of Diagonal 1 is 41. (1 + 4 + 16 + 20) The sum of Diagonal 2 is 25. (4 + 6 + 13 + 2) The sum of both diagonals is 66. You would return 66, the sum of both diagonals only.
The code I have written is as follows but it is missing the following :
I know I need to fill the array somewhere - but not sure where ???
when I run the sum diagonals it doesn't let me fill array properly ??
import java.util.Scanner;
public class SumOfDiagonals{
public static void main(String [] args){
Scanner input = new Scanner (System.in);
int [][] matrix;
System.out.print ("Enter row number:");
int row = input.nextInt();
int col = row;
matrix = new int [row][col];
for (int i=0; i<matrix.length; i++){
for (int j = 0; i<matrix[0].length; j++){
System.out.print ("Enter a number for ( "+i+", "+j+")");
matrix[i][j]=input.nextInt();
}
}
int sum1=0;
int sum2=0;
for (int i=0;i<matrix.length;i++){
sum1 += matrix[i][i];
sum2 += matrix[i][matrix.length-1-i];
}
System.out.println ("Diagonal 1: "+sum1);
System.out.println ("Diagonal 2: "+sum2);
System.out.println ("Sum of two diagonals: " + (sum2+sum1));
}
}
My error when the test code is as follows:
Your program tested with:
5 5 4 3 2 1 1 2 3 7 5 1 1 1 1 1 11 12 13 14 15 15 14 13 12 11
Your answer:
Enter row number:Enter a number for ( 0, 0)
Enter a number for ( 0, 1)
Enter a number for ( 0, 2)
Enter a number for ( 0, 3)
Enter a number for ( 0, 4)
Enter a number for ( 0, 5)
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 5
at SumOfDiagonals.main(SumOfDiagonals.java:13)
Expected answer:
Sum of diagonals = 69
You got a grade of 52

You have a typo in your second for-loop.
for (int i = 0; i < matrix.length; i++) {
for (int j = 0; *HERE* i < matrix[0].length; j++) {
System.out.print("Enter a number for ( " + i + ", " + j + ")");
matrix[i][j] = input.nextInt();
}
}
should be
for (int i = 0; i < matrix.length; i++) {
for (int j = 0; j < matrix[0].length; j++) {
System.out.print("Enter a number for ( " + i + ", " + j + ")");
matrix[i][j] = input.nextInt();
}
}

Formatting This Magical Square

I am looking to format the output of this code so that the totals of the rows and columns show up next to/below said row/column. I am getting the numbers correct, but spaces and char counts seem to be too unpredictable to format it correctly for any size square. Here is the code i currently have.
public void printSquare()
{
for (int x = 0; x < square.length; x ++)
{
System.out.println();
for (int j = 0; j < square.length; j++)
{
System.out.print(" " + square[x][j]);
}
System.out.println(" | " + sumRow(x));
}
System.out.print(sumOtherDiag());
for (int x = 0; x < square.length; x ++)
{
System.out.print(" ^");
}
System.out.println(sumMainDiag());
for (int x = 0; x < square.length; x ++)
{
System.out.print(" " + sumCol(x));
}
}
And the output for this is..
8 1 6 | 15
3 5 7 | 15
4 9 2 | 15
15 ^ ^ ^15
15 15 15
Are there any strategies in order to more reliably format something like this?

Instead of
System.out.printf(" " + square[x][j]);
use
System.out.printf("%4d", square[x][j]); // 4 positions for right-aligned decimal
and so on for other similar statements.

Print triangle in java using single for loop

1
2 3
4 5 6
I have to print the triangle using single for loop.
I have tried using two for loops and i did it successfully but i have to solve it using single for loop.

The trick is that the last number of the current row is the sum of the previous row last number and the number of the row
In other words: lastNum = prevLastNumber + rowNum
int row = 1;
int last = 0;
for (int i = 1; i < 37; i++) {
if (i < (row + last)) {
System.out.print(i + " ");
} else {
System.out.print(i + "\n");
row++;
last = i;
}
}
And the output is the following:
1
2 3
4 5 6
7 8 9 10
11 12 13 14 15
16 17 18 19 20 21
22 23 24 25 26 27 28
29 30 31 32 33 34 35 36

Try this.
public class Pyramid7Floyds {
public static void main(String[] args) {
int nextNumber = 1;
for (int i = 1; i <= 10; i++) {
for (int j = 1; j <= i; j++) {
System.out.print(nextNumber<10 ? (" " + nextNumber++) : (" " + nextNumber++) ); //2spaces in single digit & 1 space in double digit.
//System.out.format("%3d",nextNumber++ ); //You may use this line for formatting as a replacement of above line. (comment above line before using this)
}
System.out.println();
}
}
}

public class HelloWorld{
public static void main(String []args){
int j=1;
for(int i=1;i<=6;i++){
System.out.print(i+" ");
if (i==j){
System.out.print('\n');
j=2*j+1;
}
}
}
}

Printing odd numbers in odd sequences

There's this problem from my programming class that I can't get right... The output must be all odd numbers, in odd amounts per line, until the amount of numbers per line meets the odd number that was entered as the input. Example:
input: 5
correct output:
1
3 5 7
9 11 13 15 17
If the number entered is even or negative, then the user should enter a different number. This is what I have so far:
public static void firstNum() {
Scanner kb = new Scanner(System.in);
int num = kb.nextInt();
if (num % 2 == 0 || num < 0)
firstNum();
if (num % 2 == 1)
for (int i = 0; i < num; i++) {
int odd = 1;
String a = "";
for (int j = 1; j <= num; j++) {
a = odd + " ";
odd += 2;
System.out.print(a);
}
System.out.println();
}
}
public static void main(String[] args) {
Scanner kb = new Scanner(System.in);
firstNum();
}
The output I'm getting for input(3) is
1 3 5
1 3 5
1 3 5
When it really should be
1
3 5 7
Can anyone help me?

Try this:
public static void firstNum() {
Scanner kb = new Scanner(System.in);
int num = kb.nextInt();
while (num % 2 == 0 || num < 0) {
num = kb.nextInt();
}
int odd = 1;
for (int i = 1; i <= num; i += 2) {
String a = "";
for (int j = 1; j <= i; j++) {
a = odd + " ";
odd += 2;
System.out.print(a);
}
System.out.println();
}
}

if (num % 2 == 1) {
int odd = 1;
}
for (int i = 0; i < num; i++) {
String a = "";
for (int j = 1; j <= odd; j++) {
a = odd + " ";
odd += 2;
System.out.print(a);
}
System.out.println();
}
You should assign odd before for loop.
In inner for loop compare j and odd together.

For questions like this, usually there is no need to use and conditional statements. Your school probably do not want you to use String as well. You can control everything within a pair of loops.
This is my solution:
int size = 7; // size is taken from user's input
int val = 1;
int row = (size / 2) + 1;
for (int x = 0; x <= row; x++) {
for (int y = 0; y < (x * 2) + 1; y++) {
System.out.print(val + " ");
val += 2;
}
System.out.println("");
}
I left out the part where you need to check whether input is odd.
How I derive my codes:
Observe a pattern in the desired output. It consists of rows and columns. You can easily form the printout by just using 2 loops.
Use the outer loop to control the number of rows. Inner loop to control number of columns to be printed in each row.
The input number is actually the size of the base of your triangle. We can use that to get number of rows.
That gives us: int row = (size/2)+1;
The tricky part is the number of columns to be printed per row.
1st row -> print 1 column
2nd row -> print 3 columns
3rd row -> print 5 columns
4th row -> print 7 columns and so on
We observe that the relation between row and column is actually:
column = (row * 2) + 1
Hence, we have: y<(x*2)+1 as a control for the inner loop.
Only odd number is to be printed, so we start at val 1 and increase val be 2 each time to ensure only odd numbers are printed.
(val += 2;)
Test Run:
1
3 5 7
9 11 13 15 17
19 21 23 25 27 29 31
33 35 37 39 41 43 45 47 49

You can use two nested loops (or streams) as follows: an outer loop through rows with an odd number of elements and an inner loop through the elements of these rows. The internal action is to sequentially print and increase one value.
a loop in a loop
int n = 9;
int val = 1;
// iterate over the rows with an odd
// number of elements: 1, 3, 5...
for (int i = 1; i <= n; i += 2) {
// iterate over the elements of the row
for (int j = 0; j < i; j++) {
// print the current value
System.out.print(val + " ");
// and increase it
val += 2;
}
// new line
System.out.println();
}
a stream in a stream
int n = 9;
AtomicInteger val = new AtomicInteger(1);
// iterate over the rows with an odd
// number of elements: 1, 3, 5...
IntStream.iterate(1, i -> i <= n, i -> i + 2)
// iterate over the elements of the row
.peek(i -> IntStream.range(0, i)
// print the current value and increase it
.forEach(j -> System.out.print(val.getAndAdd(2) + " ")))
// new line
.forEach(i -> System.out.println());
Output:
1
3 5 7
9 11 13 15 17
19 21 23 25 27 29 31
33 35 37 39 41 43 45 47 49
See also: How do I create a matrix with user-defined dimensions and populate it with increasing values?

Seems I am bit late to post, here is my solution:
public static void firstNum() {
Scanner scanner = new Scanner(System.in);
System.out.println("Enter the odd number: ");
int num = scanner.nextInt();
if (num % 2 == 0 || num < 0) {
firstNum();
}
if (num % 2 == 1) {
int disNum = 1;
for (int i = 1; i <= num; i += 2) {
for (int k = 1; k <= i; k++, disNum += 2) {
System.out.print(disNum + " ");
}
System.out.println();
}
}
}

Trouble with a java histogram issue

This is the question:
Design and implement an application that creates a histogram that allows you to
visually inspect the frequency distribution of a set of values. The program should read in
an arbitrary number of integers that are in the range 1 to 100 inclusive; then produce a
chart similar to the one below that indicates how many input values fell in the range 1
to 10, 11 to 20, and so on. Print one asterisk for each value entered.
1 - 10 | *****
11 - 20 | **
21 - 30 | *******************
31 - 40 |
41 - 50 | ***
51 - 60 | ********
61 - 70 | **
71 - 80 | *****
81 - 90 | *******
91 - 100 | *********
Source Code:
import java.io.*;
import java.util.Scanner;
public class histogram
{
public static void main(java.lang.String[] args)
throws IOException
{
Scanner scan = new Scanner (System.in);
final int min = 1;
final int max = 10;
final int limit = 10;
int[] a = new int[max];
for (int b = 0; b < a.length; b++)
{
a[b] = 0;
}
System.out.println("Enter Number between 1 and 100: \n (Or press 0 to stop)");
int number = scan.nextInt();
while (number >= min && number <= (limit*max) && number != 0)
{
a[(number-1)/limit] = a[(number - 1 ) / limit] + 1;
System.out.print("Please enter a value:");
number = scan.nextInt();
}
System.out.println("\n__Histogram__");
for (int y = 0; y < a.length; y++)
{
System.out.print(" " + (y * limit + 1) + " - " + (y + 1) * limit + "\t");
return;
}
for (int z = 0; z < a[b]; z++)
{
System.out.print("*");
}
System.out.println(0);
}
}
In the last for statement, it says that b cannot be resolved to a variable. When I use java for help it sets b to 0 and the program doesn't run correctly. Any ideas?

Since b was an array index in the earlier loop, I think you will need to put the last loop inside the loop to increment through the array indexes as you did in the beginning.
Changed b to y and moved z for loop inside y for loop.
for (int y = 0; y < a.length; y++)
{
System.out.print(" " + (y * limit + 1) + " - " + (y + 1) * limit + "\t");
for (int z = 0; z < a[y]; z++)
{
System.out.print("*");
}
System.out.println();
}