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How would I add two int that are in the same array to each other and convert them into an int. In the Luhn Algorithm

I am trying to add two parts of an array together to go into an int value. I am using Luhn algorithm to figure out of a credit card is a valid credit card. We are only using 6 digit credit card's just to make sure no one enter's a real credit card number. The part I am confused on is when I go to split a number that is above 10 and add it together. Example if the algorithm was to give me 12 I would need to separate it into 1 and 2 and then add them together to equal 3. I believe I am splitting it currently in the code but when I go to add them together I get some number that makes no since. here is a section of the code with some notes about it.
I have printed out numbers in certain places to show myself what is going on in certain places. I have also added in some comments that say that either the number that is printed out is what is expected, and some comments for when there isn't something I expected
int[] cardNumber = new int[]{ 1,2,3,4,5,5};
int doubleVariablesum = 0;
int singleVariablesum = 0;
int totalSum = 0;
int cutOffVar = 0;
String temp2;
for (int i = cardNumber.length - 1; i >= 0;) {
int tempSum = 0;
int temp = cardNumber[i];
temp = temp * 2;
System.out.println("This is the temp at temp * 2: " + temp);
temp2 = Integer.toString(temp);
if (temp2.length() == 1) {
System.out.println("Temp2 char 0: "+ temp2.charAt(0));
// this prints out the correct number
// Example: if there number should be 4 it will print 4
tempSum = temp2.charAt(0);
System.out.println("This is tempSum == 1: " + tempSum);
// when this goes to add temp2.charAt(0) which should be 4 it prints out //something like 56
} else {
System.out.println("TEMP2 char 0 and char 1: " + temp2.charAt(0) + " " + temp2.charAt(1));
// this prints out the correct number successfully spited
tempSum = temp2.charAt(0) + temp2.charAt(1);
System.out.println("This is tempSum != 1: " + tempSum);
// but here it when I try to add them together it is giving me something
// like 97 which doesn't make since for the numbers I am giving it
}
doubleVariablesum = tempSum + doubleVariablesum;
System.out.println("This is the Double variable: " + doubleVariablesum);
System.out.println();
i = i - 2;
}

Since you are converting the number to a string to split the integer, and then trying to add them back together. You're essentially adding the two characters numerical values together which is giving you that odd number. You would need to convert it back to an integer, which you can do by using
Integer.parseInt(String.valueOf(temp2.charAt(0)))

When adding char symbols '0' and '1' their ASCII values are added - not numbers 0 and 1.
It is possible to use method Character::getNumericValue or just subtract '0' when converting digit symbol to int.
However, it is also possible to calculate sum of digits in a 2-digit number without any conversion to String and char manipulation like this:
int sum2digits = sum / 10 + sum % 10; // sum / 10 always returns 1 if sum is a total of 2 digits

Seems like charAt() type casts into integer value, but the ascii one. Hence for the characters '0' and '1', the numbers 48 and 49 are returned resulting in a sum of 97. To fix this, you could just assign temp2 to (temp / 10) + (temp % 10). Which actually splits a two digit integer and adds their sum.

You need to be aware of the following when dealing with char and String
Assigning the result of charAt(index) to an int will assign the ASCII value and not the actual integer value. To get the actual value you need to String.valueOf(temp2.charAt(0)).
The result of concatenating chars is the sum of the ASCII values.
eg if char c = '1'; System.out.println(c + c); will print "98" not "11".
However System.out.println("" + c + c); will print "11". Note the "" will force String concatenation.

Formatting with Array[i] Logic

I'm trying to build a program that prints 75 random caps and lowercase letters, 25 per line. I think I have all the logic worked out, but whenever I run it the formatting is all off and rather than printing 25 characters per line, it prints a random number. Here's my code so far:
char allLetters[] = new char[3700];
for(int i = 1; i <= 75; i++) { //Begin for loop
int max = 122;
int min = 65;
allLetters[i] = (char)(Math.random() * (max - min) + min);
if(i % 25 != 0){
if (allLetters[i] <= 90) {
System.out.printf("%s,",allLetters[i]);
}
if (allLetters[i] >= 97) {
System.out.printf("%s,",allLetters[i]);
}
} //Close if
else {
if (allLetters[i] <= 90) {
System.out.printf("%s\n",allLetters[i]);
}
if (allLetters[i] >= 97) {
System.out.printf("%s\n",allLetters[i]);
}
}
} //End for
Currently, the output is something like:
U,i,y,e,v,T,G,p,P,a,U,G,e,B,w,U,o,F,G,w,j,m,R
O,X,w,w,u,p,t,g,X,J,R,c,w,I,d,H,R,m,y,b,o
C,p,M,F,X,U,v,O,a,Y,F,E,x,s,x,k,C,b,D,R,r,H
I've tried using different variables besides i, playing around with numbers and such but I can't seem to find the exact flaw in the logic that throws the formatting off. Any help would be greatly appreciated!

The problem with your logic is that you are counting each chosen character even when you do not print it. The ASCII characters falling between 90 and 97, exclusive, are not characters, and you rightfully skip printing them. Yet the loop is still counting those iterations as if a valid letter has been printed. This is resulting in an incorrect count in the output.
The workaround used in the code snippet below is to keep picking characters in a loop until we actually get a lowercase or uppercase letter. Only then do we continue with your previous logic.
char allLetters[] = new char[3700];
int max = 122;
int min = 65;
for (int i = 1; i <= 75; i++) {
char next;
do {
next = (char)(Math.random() * (max - min) + min);
} while (next > 90 && next < 97);
allLetters[i] = next;
if (i % 25 != 0) {
System.out.printf("%s,", next);
}
else {
System.out.printf("%s\n", next);
}
}
Demo

Your two conditions that you put on your random characters, namely allLetters[i] <= 90 and allLetters[i] >= 97, do not cover the entire interval of possible characters, which in your program is 65 to 122, inclusive. When a character between 91 and 96 gets generated, your program does not print anything. The probability of getting one of these six random characters is roughly 10%, so you get 21..23 characters printed.
If you really want to skip these six characters, fix the problem by using a while loop instead of a for loop, and increment the counter of printed characters only when you print something:
int printed = 0;
while (printed != 75) {
char ch = (char)(Math.random() * (max - min) + min);
if (ch >= 91 && ch <= 96) continue;
printed++;
System.out.print(ch);
if (printed % 25 == 0 {
System.out.println();
} else {
System.out.print(',');
}
}

I was looking at your problem, and there's a "clever" solution using Java 8+ IntStream and lambdas. Generate 75 random int(s) between 0 and 26, map each value to a one-character String offset from either 'a' or 'A' by using nextBoolean() from Random. Collect that to a single seventy-five character String. Then print the three twenty-five character substring(s) we're interested in. Like,
Random rand = new Random();
String s = IntStream.generate(() -> rand.nextInt(26)).limit(75)
.mapToObj(i -> Character.toString(
rand.nextBoolean() ? (char) (i + 'a') : (char) (i + 'A')))
.collect(Collectors.joining());
System.out.println(s.substring(0, 25));
System.out.println(s.substring(25, 50));
System.out.println(s.substring(50));

Output of converting string into number does not come as expected

Given a string as input, convert it into the number it represents. You can assume that the string consists of only numeric digits. It will not consist of negative numbers. Do not use Integer.parseInt to solve this problem.
MyApproach
I converted string to char array and stored the original number but I am unable to convert it into a single number
I tried converting individual elements but the digits can be of any length.So,It was difficult to follow that approach.
Hint:I have a hint that the numbers can be added using place values
For e.g if the number is 2300.I stored each number in the form of arrays.Then it should be 2*1000+3*100+0*10+0=2300
But I am unable to convert it into code.
Can anyone guide me how to do that?
Note I cannot use any inbuilt functions.
public int toNumber(String str)
{
char ch1[]=str.toCharArray();
int c[]=new int[ch1.length];
int k=0;
for(int i=0;i<c.length;i++)
{
if(ch1[i]==48)
{
c[k++]=0;
}
else if(ch1[i]==49)
{
c[k++]=1;
}
else if(ch1[i]==50)
{
c[k++]=2;
}
else if(ch1[i]==51)
{
c[k++]=3;
}
else if(ch1[i]==52)
{
c[k++]=4;
}
else if(ch1[i]==53)
{
c[k++]=5;
}
else if(ch1[i]==54)
{
c[k++]=6;
}
else if(ch1[i]==55)
{
c[k++]=7;
}
else if(ch1[i]==56)
{
c[k++]=8;
}
else if(ch1[i]==57)
{
c[k++]=9;
}
}
}

You don't need to do powers or keep track of your multiplier. Just multiply your running total by 10 each time you add in a new digit. And use c - '0' to turn a character into a number:
int n = 0;
for (int i = 0; i < str.length(); i++) {
n = n * 10 + str.charAt(i) - '0';
}
So for example for 1234 it goes
0 * 10 + 1 = 1
1 * 10 + 2 = 12
12 * 10 + 3 = 123
123 * 10 + 4 = 1234

A digit character ('0'-'9') can be converted into an integer value (0-9) using:
ch - '0'
This is because the digit characters are all consecutive in ASCII/Unicode.
As for calculating the full number, for the input 2300, you don't want:
2 * 1000 + 3 * 100 + 0 * 10 + 0
Instead, you'll want a more incremental approach using a loop:
r = 0
r = r * 10 + 2 (2)
r = r * 10 + 3 (23)
r = r * 10 + 0 (230)
r = r * 10 + 0 (2300)
This is much better than trying to calculate 1000 (Math.pow(10,3)), which your formula would require.
This should be enough information for you to code it. If not, create a new question.

If you loop through the char array you have and take the last value, put it through an if statement and add to an to number integer whatever that number is (use 10 if statements). Next go to the second to last value, and do the same thing only this time multiply the resulting numbers by 10 before adding it to the total number. Repeat this using 1 * 10^(value away from end) being multiplied to the number gotten from the if statements.

Well what comes to my mind when seeing this problem is to multiply the numbers you are getting with your current code with the place they have in the charArray:
int desiredNumber = 0;
for(int k=1; k<=c.length; k++) {
desiredNumber += c[k] * (Math.pow(10, c.length - k));
}
If you are not allowed to use the Math.pow() function then simply write one yourself with aid of a loop.
Greetings Raven

You can do
int no = 0;
for(int i = 0; i < c.length; i++){
no += c[i] * Math.pow(10, c.length - 1 - i);
}

How this bitshift to build the number works?

I saw this method which is used for faster reading of positive value of Long.
public static long readLong(InputStream in) throws IOException {
long n = 0;
int c = in.read();
while (c < 48 || c > 57) {
c = in.read();
}
while (c >= 48 && c <= 57) {
n = (n<<3) + (n<<1) + (c-'0');
c = in.read();
}
return n;
}
While I understand all of the part, what I'm not able to get is this:
bit shifting by odd number to
build the number n = (n<<3) + (n<<1) + (c-'0');
Why ignore the 3rd bit and how it's able to build it?
If anyone of you could explain me in simple way, it would be very much helpful.
Thanks!

n << i is n * 2^i. So,
(n<<3) + (n<<1) = (n * 2^3) + (n * 2^1) = n * (2^3 + 2^1) = n * 10
Basically, it means to shift the value of n one digit to the left.
Adding it with c + '0' means adding the last digit with the integer value of c.

Consider this code,
while (c >= 48 && c <= 57) {
n = (n<<3) + (n<<1) + (c-'0');
System.out.println(n);
c = System.in.read();
}
If I enter 123456, it prints
1
12 // 1 * 10 + 2
123 // 12 * 10 + 3
1234 // 123 * 10 + 4
12345 // 1234 * 10 + 5
123456 // 12345 * 10 + 6
So what it basically does is making space at the units place for the next number by just multiplying it by 10 and adding the int value of the character.

This is what happens:
(n<<3) means (n*8)
(n<<1) means (n*2)
(c-'0') gives you the character as int value
Together this means
n = n*10 + theNewDigit

Why is myArrayList.size(); producing incorrect/gigantic numbers?

Basically, I'm trying to write a program that converts a number from base 2 to base 10. What I tried doing was translating the process listed on this website under the "Doubling method" into a for loop, but for some reason the numbers I'm getting are way to big.
The basic formula is (2 * previousTotal) + (currentDigit of the ArrayList that holds the user's inputted binary number) = previousTotal.
So for 1011001 in binary, the math would be:
(0 x 2) + 1 = 1
(1 x 2) + 0 = 2
(2 x 2) + 1 = 5
(5 x 2) + 1 = 11
(11x 2) + 0 = 22
(22 x 2) + 0 = 44
(44 x 2) + 1 = 89
The console however, prints out 6185 as the result. I'm thinking it might have something to do with me using an ArrayList of characters, but the charWhole.size() returns 7, which is how many digits are in the user's binary number. As soon as I do charsWhole.get(w); however, I start getting big numbers such as 49. I'd really appreciate some help!
I wrote out this loop, and according to some print statements that I placed throughout the code and my variable addThis seems to be where the problem is. The console prints out a final total of 6185, when 1011001 in base 10 is actually 89.
public static void backto2(){
System.out.println("What base are you coming from?");
Scanner backToB10 = new Scanner(System.in);
int bringMeBack = backToB10.nextInt();
//whole
System.out.println("Please enter the whole number part of your number.");
Scanner eachDigit = new Scanner(System.in);
String theirNumber = eachDigit.nextLine();
String str = theirNumber;
ArrayList<Character> charsWhole = new ArrayList<Character>();
for (char testt : str.toCharArray()) {
charsWhole.add(testt);
}
System.out.println(theirNumber); // User's number
System.out.println(charsWhole); // User's number separated into elements of an ArrayList
System.out.println(charsWhole.size()); // Gets size of arrayList, comes out as 7 which seems fine.
int previousTotal = 0, addThis = 0, q =0;
for( int w = 0; w < charsWhole.size(); w ++) {
addThis = charsWhole.get(w); //current digit of arraylist PROBLEM
q = previousTotal *2;
previousTotal = q + addThis; // previous total gets updated
System.out.println(q);
System.out.println(addThis);
System.out.println(q + " and " + addThis + "equals " + previousTotal);
}
System.out.println(previousTotal);

You are attempting to add a character to an integer. The implicit conversion uses the ASCII value of the character, so that '1' gets converted to 49, not 1, because 49 is the code for the character '1'. Subtract '0' to get the actual integer value.
addThis = charsWhole.get(w) - '0';
This works because the digits 0-9 are represented in ASCII as the codes 48-57, so in effect you will, for '1', subtract 49 - 48 to get 1.
You'll still have to handle cases when the character is outside the range of allowable characters.
EDIT
Java uses Unicode, but for the purposes of the codes for the digits 0-9, the codes are the same (48 thru 57, or 0x30 thru 0x39) in both ASCII and Unicode.

The problem is that you're using the chars rather than the number value they represent. In the line
addThis = charsWhole.get(w);
the value of addThis is the ascii value of the character. For '0', this is 48. Use this instead:
addThis = Integer.parseInt(charsWhole.get(w));

Another suggestion to solve the same problem:
addThis = charsWhole.getNumericValue(w);
See here for more information.