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how do you find the sum of a sequence in Java

I really don't understand how to find the sum of a sequence in Java. For instance, the program will ask for the input of the first and last number of the sequence and add the sum of the sequence(3 + 4 + 5 = 12). My System.out.println() isn't working as well. Why is this?
import java.util.Scanner;
public class SumOfASequenceTheSequel {
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
System.out.println("First number?");
int first = Integer.valueOf(scanner.nextLine());
System.out.println("Second number?");
int second = Integer.valueOf(scanner.nextLine());
int sum = 0;
int i = first;
while (i <= second) {
sum = sum + i;
i = i++;
}
System.out.println("The sum is " + sum);
}
}

i = i++; This statement is incorrect. Because you used post incremental. In this statement first assign i's value to i after increment value. but i's value not changes. Then this while loop become infinity loop. So you need remove assignment part and only need i++;. Also you can use pre incremental (++i;). If you use pre incremental, then i = ++i; statement fine. But value assignment is not need.
If you can replace while loop using for loop, then you don't meet this problem. For loop solution below. Below Code sum += i; equals to sum = sum + i
for (int i = first; i <= second; i++) {
sum += i;
}

i++; is called Post Increment equal to i = i + 1; So you just need i++;, not i = i++;.

As others have pointed out, because you are doing post-increment and assignment in the same statement, (i = i++;), your program is running in an infinite loop.
You might want to change it to either a pre-increment assignment (i = ++i;) or even better a plain pre or post increment (i++;).
Look at this link to understand the difference between pre and post increment operators.
I will recommend a for loop instead of while loop for better readability. See the following snippet:
for(int i = first; i <= second; i++) {
sum = sum + i;
}
If you like Stream API instead of traditional loops, you can also print the sum the following way:
System.out.println("The sum is " + IntStream.rangeClosed(first, second).sum());
On a side note, you might want to close the Scanner resource after you are done using it to avoid resource leak, like so: scanner.close();

array index not incrementing (need to do casting in square brackets)

I have some doubts as to why the value of index is not incrementing here.
The reason why I have declared my array like that is because I need to store n natural numbers where (1 ≤ n ≤ 1012), so numbers are large which is why I have taken an array of type long, but then I get an error that I cannot put any long value in the group, which is why I cast it to int. Is there any way to declare an array for this type of condition, as I want a large number of indexes, and I can not put a long number in the [ ].
hope you guys understand my problem
CODE:
import java.util.Scanner;
class Error{
public static void main(String[] args) {
Scanner in = new Scanner(System.in);
long n = in.nextLong();
long array[] = new long[(int) n];
long index = 0;
for (int j = 1; j <= n; j++) {
if (j % 2 != 0) {//odd
array[(int) index++] = j;
System.out.print(" " + array[(int) --index]);
System.out.print(index);// index value -> always 0 why??
System.out.print(j);
}
}
System.out.println();
}
}
OUTPUT:
Unix-Box ~/Desktop$ javac Error.java
Unix-Box ~/Desktop$ java Error
10
101 303 505 707 909
Unix-Box ~/Desktop$
the middle value is of index and it is always 0
what i shout it to be like
10
101 313 525 737 949
Unix-Box ~/Desktop$

According to
https://www.quora.com/What-is-the-maximum-size-of-the-array-in-Java,
the max size of an array is 2147483647 theoretically, but in practice we would want to use 2147483600 to be safe. Declaring the array as type long will mean that long values can be stored inside. Maybe you can use a two dimensional array to store a long n amount of values. Something like--
public static void main(String[] args)
{
System.out.println("enter the size of the array:");
Scanner in = new Scanner(System.in);
long n = Long.parseLong(in.nextLine());
int secondIndex = 2147483600;
int firstIndex = ((int)(n/secondIndex))+1;
if(secondIndex > n)
{secondIndex = (int)n;
}
else{int leftover = (int)(n%secondIndex);
secondIndex = secondIndex - leftover;}
long[][] array = new long[firstIndex][secondIndex];
//loop through array
outerloop:
for(int i =0;i <firstIndex; i++)
{
for(int z = 0; z<secondIndex; z++)
{
System.out.println("do work with number here: " + array[i][z]);
if(z==(secondIndex-1))
{
z=0;
continue outerloop;
}
}
}
}
You might get a java.lang.OutOfMemoryError:, which can be resolved by reading this article https://plumbr.eu/outofmemoryerror/java-heap-space.

As others have indicated,
array[(int) index++] = j; // You use index and then increment it
System.out.print(" " + array[(int) --index]); // You decrement the index here
That's why index will always be 0 when you print it.
Personally, I don't like mixing brackets with increment operators for the precise reason you're seeing here - it tends to be confusing and it lends itself to subtle (and not-so-subtle) bugs and off-by-one errors. In fact, I really don't like mixing them with any other syntax (with the exception of for loops) as it can quickly become very unclear. For example, if you had done something like
index++;
array[(int)index] = j;
index--;
System.out.print(" " + array[(int)index]);
the problem would've been obvious immediately.
In general, it's a bad idea to sacrifice clarity for brevity.
Also, just to review how the operators in question are working:
index++ - use the value and then increment it
index-- - use the value and then decrement it
++index - increment the value and then use it
--index - decrement the value and then use it.
Here's a C# code sample I put together (Java's behavior will be identical) to illustrate this:
int i = 0;
Trace.TraceInformation((i++).ToString()); // Prints 0
Trace.TraceInformation(i.ToString()); // Prints 1
Trace.TraceInformation((--i).ToString()); // Prints 0
Trace.TraceInformation((i--).ToString()); // Prints 0
Trace.TraceInformation(i.ToString()); // Prints -1
I'd encourage you to trace/step through this to convince yourself that that's the case and to understand exactly why the value is what it is at every point.
Either way, this syntax can be very confusing if overused.

How to exclude -1 sentinel value when finding minimum element in an array, java

I'm trying to find the minimum element in an array of size 25 elements are read by user and should stop when the user enters -1
import java.util.*;
public class LabSheet4{
public static void main (String[] args){
Scanner read = new Scanner (System.in);
double scores[] = new double[25];
int minIndex=0, i =0, sentinel=0;
do{
scores[i] = read.nextDouble();
if(scores[i]==-1)
sentinel=-1;
i++;}while(sentinel!=-1);
for(int k=0;k<scores.length;k++)
if(scores[minIndex] > scores[k] && scores[minIndex]>0)
minIndex = k;
lowestScore = scores[minIndex];}}
How do I exclude -1?

One way would be to not put the value in the list until after you know it isn't the sentinel.
Another would be to not increment the counter when you find you have added the sentinel. Note that this means you need a slightly bigger array.

One way is not to store it in the first place but another fix would be to ignore it when running on the array like this.
for(int k=0;k<scores.length;k++)
if(scores[minIndex] > scores[k] && scores[minIndex]>0 && scores[k] !=1)

One common idiom for this class of problems, is to use a while loop with an assignment and test. Something like,
double scores[] = new double[25];
int i = 0;
double score;
while ((score = read.nextDouble()) != -1) {
scores[i] = score;
i++;
}
Then you could start at 0 and iterate from 1 to i (because 0 is your initial value and i is the count of elements). Like,
int minIndex = 0;
for (int k = 1; k < i; k++) {
if (scores[minIndex] > scores[k]) {
minIndex = k;
}
}
double lowestScore = scores[minIndex];

Instead of
if(scores[i]==-1)
sentinel=-1;
use
if (scores[i]==-1 && i > 0) {
sentinel=-1;
scores[i] = scores[minIndex];
}
so you replace -1 with (so far) the lowest value - which is OK, isn't it?.

Do not store -1 in scores at all.
That way you won't have to exclude it later.
Most importantly,
it's not a functional value like the other values you store,
but a symbol with a special technical meaning in the way you process input.
As such, it doesn't belong in the scores array.
Change your loop that reads the user input,
so as to not store -1.
Check the input before you store it,
and break out of the loop if it is -1:
while (true) {
double input = read.nextDouble();
if (input == -1) {
break;
}
scores[i++] = input;
}
Btw, what will happen if the user enters 26 values that are not -1?
The program will crash, because scores can only store 25 values.
So you need further some improvements to prevent that from happening.
For example you could use a for loop instead:
for (int i = 0; i < scores.length; i++) {
double input = read.nextDouble();
if (input == -1) {
break;
}
scores[i] = input;
}

Print sum of unique values of an integer array in Java

I am yet again stuck at the answer. This program prints the unique values but I am unable to get the sum of those unique values right. Any help is appreciated
public static void main(String args[]){
int sum = 0;
Integer[] numbers = {1,2,23,43,23,56,7,9,11,12,12,67,54,23,56,54,43,2,1,19};
Set<Integer> setUniqueNumbers = new LinkedHashSet<Integer>();
for (int x : numbers) {
setUniqueNumbers.add(x);
}
for (Integer x : setUniqueNumbers) {
System.out.println(x);
for (int i=0; i<=x; i++){
sum += i;
}
}
System.out.println(sum);
}

This is a great example for making use of the Java 8 language additions:
int sum = Arrays.stream(numbers).distinct().collect(Collectors.summingInt(Integer::intValue));
This line would replace everything in your code starting at the Set declaration until the last line before the System.out.println.

There's no need for this loop
for (int i=0; i<=x; i++){
sum += i;
}
Because you're adding i rather than the actual integers in the set. What's happening here is that you're adding all the numbers from 0 to x to sum. So for 23, you're not increasing sum by 23, instead, you're adding 1+2+3+4+5+....+23 to sum. All you need to do is add x, so the above loop can be omitted and replaced with a simple line of adding x to sum,
sum += x;

This kind of error always occures if one pokes around in low level loops etc.
Best is, to get rid of low level code and use Java 8 APIs:
Integer[] numbers = {1,2,23,43,23,56,7,9,11,12,12,67,54,23,56,54,43,2,1,19};
int sum = Arrays.stream(numbers)
.distinct()
.mapToInt(Integer::intValue)
.sum();
In this way there is barely any space for mistakes.
If you have an int array, the code is even shorter:
int[] intnumbers = {1,2,23,43,23,56,7,9,11,12,12,67,54,23,56,54,43,2,1,19};
int sumofints = Arrays.stream(intnumbers)
.distinct()
.sum();

So this is my first time commenting anywhere and I just really wanted to share my way of printing out only the unique values in an array without the need of any utilities.
//The following program seeks to process an array to remove all duplicate integers.
//The method prints the array before and after removing any duplicates
public class NoDups
{
//we use a void static void method as I wanted to print out the array without any duplicates. Doing it like this negates the need for any additional code after calling the method
static void printNoDups(int array[])
{ //Below prints out the array before any processing takes place
System.out.println("The array before any processing took place is: ");
System.out.print("{");
for (int i = 0; i < array.length; i++)
{
System.out.print(array[i]);
if (i != array.length - 1)
System.out.print(", ");
}
System.out.print("}");
System.out.println("");
//the if and if else statements below checks if the array contains more than 1 value as there can be no duplicates if this is the case
if (array.length==0)
System.out.println("That array has a length of 0.");
else if (array.length==1)
System.out.println("That array only has one value: " + array[0]);
else //This is where the fun begins
{
System.out.println("Processed Array is: ");
System.out.print( "{" + array[0]);//we print out the first value as it will always be printed (no duplicates has occured before it)
for (int i = 1; i < array.length; i++) //This parent for loop increments once the all the checks below are run
{
int check = 0;//this variable tracks the amount of times an value has appeared
for(int h = 0; h < i; h++) //This loop checks the current value for array[i] against all values before it
{
if (array[i] == array[h])
{
++check; //if any values match during this loop, the check value increments
}
}
if (check != 1) //only duplicates can result in a check value other than 1
{
System.out.print(", " + array[i]);
}
}
}
System.out.print("}"); //formatting
System.out.println("");
}
public static void main(String[] args)
{ //I really wanted to be able to request an input from the user but so that they could just copy and paste the whole array in as an input.
//I'm sure this can be done by splitting the input on "," or " " and then using a for loop to add them to the array but I dont want to spend too much time on this as there are still many tasks to get through!
//Will come back and revisit to add this if I remember.
int inpArray[] = {20,100,10,80,70,1,0,-1,2,10,15,300,7,6,2,18,19,21,9,0}; //This is just a test array
printNoDups(inpArray);
}
}

the bug is on the line
sum += i;
it should be
sum += x;

not a statement error, illegal start of type

import java.util.*;
public class ulang {
public static void main(final String[] args) {
int a;
int b;
int sum;
Scanner scan = new Scanner(System.in);
System.out.println("Enter num 1: ");
a = in.nextLine();
System.out.println("Enter num 2: ");
b = in.nextLine();
{
sum = a + b;
}
for (i = 0; i < 5; i++) {
(sum >= 10)
System.out.println("Congratulations");
else
System.out.println("Sum of the number is Less than 10");
}
}
}
I'm weak on looping especially in Java. So I need some corrections on my coding, but I have no idea how to fix it.
The coding should run like this: User need to insert 2 numbers and the program will calculate the sum of both number. After that, the program will determine if the total of sum is >=10 or <10. If the sum >=10, "Congratulations" will appear but if it is <10, then "The sum of number less than 10" will appear. How to fix it?

This is the immediate problem:
(sum>=10)
I believe you meant that to be an if statement:
if (sum>=10)
Additionally:
You're trying to use an in variable, but the Scanner variable is called scan
Scanner.nextLine() returns a String - I suspect you wanted Scanner.nextInt()
Your for loop uses a variable that hasn't been declared. You probably meant:
for (int i = 0; i < 5; i++)
A few other suggestions though:
The sum isn't going to change between the loop iterations... why are you looping at all?
You've got a new block in which you're calculating the sum, but for no obvious reason. Why?
It's generally a good idea to declare variables at the point of initialization, e.g.
Scanner scan = new Scanner(System.in);
System.out.println("Enter num 1: ");
int a = scan.nextInt();
System.out.println("Enter num 2: ");
int b = scan.nextInt();
int sum = a + b;
Given that you want to take the same basic action (writing a message to the screen) whether or not the user was successful, you might consider using the conditional operator like this:
String message = sum >= 10 ? "Congratulations"
: "Sum of the number is Less than 10";
System.out.println(message);
That would then allow you to refactor the loop to only evaluate the condition once:
String message = sum >= 10 ? "Congratulations"
: "Sum of the number is Less than 10";
for (int i = 0; i < 5; i++)
{
System.out.println(message);
}

(sum>=10)
This line needs an if at the beginning, or it won't be read as a branch.
if (sum >= 10)
You also should name your main-class Ulang, because java class identifiers should start with an upper case letter, for readability.
The loop should look like the following:
for (int i = 0; i < 5; i++) {
The first part defines the counter and assigns zero to it. The second is your condition and the last counts for you.

for (int i = 0; i < 5; i++) {
if (sum >= 10)
System.out.println("Congratulations");
else
System.out.println("Sum of the number is Less than 10");
}