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Why does my loop increment 1 over the value needed?

This is my first CS class ever, and I'm trying to learn everything the best that can. In this loop I am trying to find valid mastercard numbers using Luhn's formula. The program works, but it's been bugging me that my final for loop outputs 1 number over the correct value, unless I subtract 1 from z after the loop finishes iterating. For example: if the sum = 56 , then z would = 5 , when the correct answer would be 4. How can I fix this going forward?
import java.util.Random;
import java.util.Scanner;
import java.util.concurrent.ThreadLocalRandom;
public class MCGenerator {
public static void main(String [] args) {
Scanner scnr = new Scanner(System.in);
Random rand = new Random();
System.out.print("How many Mastercard numbers would you like to generate? ");
int quantity = scnr.nextInt();
int x;
int use;
int range;
int appendI;
Long appendL;
String firstDigits;
String append;
String preliminary;
int y;
int c;
int sum;
int z;
int findDigit;
String lastNum;
String cardNumber;
System.out.println("\nHere you go, have fun: ");
for(x = 0; x < quantity; x++ ) {
use = rand.nextInt(2 - 1 + 1) +1;
if(use == 1) {
range = rand.nextInt(55 - 51 + 1) + 51;
}
else {
range = rand.nextInt(272099 - 222100 +1) + 222100;
}
if(range < 56) {
appendL = ThreadLocalRandom.current().nextLong(1000000000000L, 10000000000000L);
append = String.valueOf(appendL);
}
else {
appendI = rand.nextInt(999999999 - 100000000 + 1) + 100000000;
append = String.valueOf(appendI);
}
firstDigits = String.valueOf(range);
preliminary = firstDigits + append;
for(y = 0, sum = 0; y < 15; y++ ) {
c = preliminary.charAt(y) - '0';
if(y % 2 == 0){
c *= 2;
}
if(c > 9) {
c -= 9;
}
sum += c;
}
for(z = 0, findDigit = sum; findDigit > 0; z++){
findDigit = sum + z;
findDigit %= 10;
}
z -= 1;
lastNum = String.valueOf(z);
cardNumber = preliminary + lastNum;
System.out.println(cardNumber);
}
}
}

Java long division without / operator having problem with quotient, remainder

I wrote a division method that does long division without / operator but when I run a unit-test it gives me correct quotient but the wrong remainder.
This is my division method:
public MyBigInteger dividedBy(MyBigInteger divisor) throws Exception {
int x = 0;
int temp = 0;
int count = 0;
int y = 10;
int total = 0;
int total2 = 0;
reverse(coefficients);
reverse(divisor.coefficients);
int current = 1;
int quotient = 0;
for (Integer i : coefficients) {
total = 10 * total + i;
}
for (Integer j : divisor.coefficients) {
total2 = 10 * total2 + j;
}
if (total2 > total) {
throw new Exception("The answer is 0");
}
if (total2 == total) {
throw new Exception("The answer is 1");
}
while (total2 <= total) {
total2 <<= 1;
current <<= 1;
}
total2 >>= 1;
current >>= 1;
while (current != 0) {
if (total >= total2) {
total -= total2;
quotient |= current;
}
current >>= 1;
total2 >>= 1;
}
MyBigInteger answer = new MyBigInteger(quotient, this.base);
return answer;
}
And this is the test-code:
quo = big1.divide(big2).toString(base);
quo = "(" + quo + ")_" + base;
System.out.print("divide: big1/big2 = "); // BigInteger
System.out.println(quo);
long s_time = System.currentTimeMillis();
System.out.print("divide: n1/n2 = "); // MyBigInteger
try {
quo_mybig = n1.dividedBy(n2).toString();
System.out.println(quo_mybig);
System.out.println("Time Required (Divide): " + (System.currentTimeMillis() - s_time) + " ms");
System.out.println(remarks);
if (quo.contentEquals(quo_mybig))
System.out.println("Test passed.");
else
System.out.println("Test failed.");
}
And this is what I got
big1: (3956)_10 (BigInteger)
big2: (27)_10 (BigInteger)
n1: (3956)_10 (MyBigInteger)
n2: (27)_10 (MyBigInteger)
divide: big1/big2 = (146)_10
divide: n1/n2 = (146)_10
Time Required (Divide): 0 ms
Remarks: An efficent implementation finds a solution within 1 ms.
Test passed.
big1 mod big2 = (14)_10
n1 mod n2 = (1499)_10
Test failed.
When I put MyBigInteger answer = new MyBigInteger(146,this.base); instead of MyBigInteger answer = new MyBigInteger(quotient,this.base); I get the mod test passed even though the quotient is 146 (checked with system.out.println)
I have no idea what is wrong here. Please help...

calculating the sum between two integers (java)

What did I do wrong? I'm really not sure what else to try or where my mistake is. Thanks for any help. It's supposed to calculate the sum of integers between two numbers, e.g. between 3 and 6 it would be 3 + 4 + 5 + 6
import java.util.Scanner;
public class TheSumBetweenTwoNumbers {
public static void main(String[] args) {
Scanner reader = new Scanner(System.in);
System.out.println("First:");
int n = Integer.parseInt(reader.nextLine());
System.out.println("Second:");
int max = Integer.parseInt(reader.nextLine());
int sum = 0;
int i = 0;
int difference = max - n;
while (i < difference) {
sum = n + (n + 1);
n++;
i++;
}
System.out.println("Sum is " + sum);
}
}

Why all this you need just a piece of code like this :
public static void main(String args[]) {
int min = 3, max = 6, sum = 0;
for (int i = min; i <= max; i++) {
sum += i;
}
System.out.println(sum);
}
With while loop it should be :
...
int i = min;
while (i <= max) {
sum += i;
i++;
}
...

You don't need to find a difference and loop over it, just running a loop from n to max will do. Also, you need to add the value to sum (+=) instead of assigning a value to it (=, which overwrites previous value)
Try this:
int i = n;
while (i <= max) {
sum += i;
i++;
}

You're overwriting the previous sum value with the most recent n + (n + 1), instead of accumulating the previous sum. Also, your loop is one iteration short. Try this:
int sum = 0;
for (int i = n; i <= max; i++) {
sum += i;
}
System.out.println("Sum is " + sum);

Change this snippet
int sum = 0;
int i = 0;
int difference = max - n;
while (i < difference) {
sum = n + (n + 1);
n++;
i++;
}
In
int sum = 0;
int i = n;
while (i <= max) {
sum = sum + i;
i++;
}

You made it a little bit overcomplicated. All you really need is a for loop that runs from n to max that adds up the incrementing variables:
int sum = 0;
for(int i = n; i <= max; i++){
sum += i;
}

trying to find the sum of even fibonacci numbers to 4 million

I am trying to find the sum of the even Fibonacci numbers up untill 4 million.
I found the numbers but i can't get them add up... in the if(n % 2 ==0) loop
8
34
144
610
2584
10946
46368
196418
832040
3524578
public static void number2()
{
int number = 40;
int a, b, c;
int numLim = 0;
a = 1;
b = 2;
while(numLim < 4000000)
{
c = a + b;
a = b;
b = c;
numLim = b;
if(numLim > 4000000)
{
break;
}
int sum = 0;
if(numLim % 2 == 0)
{
System.out.println(numLim);
sum = sum + numLim;
System.out.println("sum :" +sum);
}
}
}

You must define sum outside the while loop, or it will become 0 each iteration.
int sum = 0;
...
while ...
Remember not to set sum to 0 each iteration.

public class Euler2 {
public static void main(String[] args) {
int fibonacci;
int num = 0;
int num2 = 1;
int loop;
int sum = 0;
System.out.println(num2);
for (loop = 0; loop <= 32; loop++) {
fibonacci = num + num2;
num = num2;
num2 = fibonacci;
System.out.println("Fibonacci number : " + fibonacci);
sum += fibonacci;
System.out.println("This is the sum " +sum);
}
}
}
So I solved it like this, it's a little more efficient and the math works but Euler hates me, hope this helps.

public class Euler2 {
public static void main(String[] args) {
int fibonacci;
int num = 0;
int num2 = 1;
int loop;
int sum = 0;
System.out.println(num2);
for (loop = 0; loop <= 31; loop++) {
fibonacci = num + num2;
num = num2;
num2 = fibonacci;
System.out.println("Fibonacci number : " + fibonacci);
if (fibonacci%2 == 0) {
sum += fibonacci;
System.out.println(sum);
}
}
}
Sorry, this code works.

Tried doing the above in Java and here is my solution that works
public static void main(String[] args) {
int first = 1;
int second = 2;
int sum = 0;
int sumOfEvenValuedTerms = second;
for (int i = 0; i < 30; i++) {
sum = first + second;
if (sum <= 4000000) {
if (sum % 2 == 0) {
sumOfEvenValuedTerms += sum;
}
first = second;
second = sum;
}
}
System.out.println(sumOfEvenValuedTerms);
}
Output is 4613732

public static int getSumOfEvenNumbers(int n) {
int prev = 0;
int i =1;
int sum = 0;
while (i<n){
int nextNumber = i + prev;
if(nextNumber %2 ==0) {
System.out.println(nextNumber);
sum +=nextNumber;
}
prev = i;
i = nextNumber;
}
return sum;
}

public class evenFib {
public static void main(String[] args) {
double a = 1, b = 2, c = 0, sum = 0;
for (double i = 0; i <= 1000; i++) {
c = a + b;
a = b;
b = c;
if (c % 2 == 0 && sum < 4000000) {
sum = sum + c;
}
}
System.out.println(sum + 2);
}
}

nested for() loops && using the "i" count from the first loop in the second so i loops just once using the value?

i am trying to not import the math class to use but i am still trying to estimate the constant "e". it is said e= 1+(1/1!)+(1/2!)+(1/3!)+(1/4!)+(1/5!)+.....
these are what i have int at the top
String userInput;
int uIp; // this converts the string into int type
double e = 2;
then i ask some questions then i check to see not zero to exit and non negative to continue
While(uIp >0){
final int endTheLoop = 15;
int factorialNumber = 1;
double e2TheUserInput=0;
for(int i = 2; i < endTheLoop; i++){
for(int j = 1; j < i; j++){
factorialNumber = ((i - 1) * factorialNumber);
}
e = (1/factorialNumber) + e;
e2TheUserInput = Math.pow(e,uIp);
}
}

You are doing integer division(but e is a double right?):
e = (1/factorialNumber) + e;
Correct that to:
e = (1.0/(double)factorialNumber) + e;
It was counting all the loops, but changes are zero according to the integer division. :)
e= 2+(0)+(0)+(0)+(0)+.....

I am not sure what your code is trying to do but if you want to compute exp(x) this is how I would do it.
public static void main(String... args) {
for (int i = -4; i <= 4; i++)
System.out.println(i + ": " + exp(i) + " cf " + Math.exp(i));
}
private static double exp(double d) {
if (d < 0)
return 1 / exp(-d);
double e = 1, term = 1;
for (int i = 1; i < 20 || term > e * 1e-16; i++) {
term *= d / i;
e += term;
}
return e;
}
For large exponents, it more efficient to evaluate the integral powers without using a taylor series.
public static final double E = 2.7182818284590452354;
private static double exp(double d) {
if (d < 0)
return 1 / exp(-d);
long num = (long) d;
double numE = 1;
double mult = E;
while (num > 0) {
if ((num & 1) != 0)
numE *= mult;
num >>>= 1;
mult *= mult;
}
double fract = d - (long) d;
double fractE = 1, term = 1;
for (int i = 1; i < 20; i++) {
term *= fract / i;
fractE += term;
}
return numE * fractE;
}