Java array rotation

Question

I need help rotating an array by a distance according to keyIndex.

Here is my code:

char[] arr = new char[] {'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z'};
int keyIndex = 2;

char[] newArr = new char[26];

int last = arr[arr.length -1];
for (int i = 0; i < arr.length; i++) {
    arr[i] = arr[i + keyIndex];
}

but I get an error and the last two characters are not being saved in elements 0, 1.

My teacher said this can be solved somehow by using modulus division in one line.

What part of your code is even trying to handle this case? To even detect it? — Scott Hunter
– Scott Hunter, Commented Jan 27, 2022 at 0:27
What is last used for? (it's nothing at the moment, so you should delete it from your code) — Bohemian
– Bohemian ♦, Commented Jan 27, 2022 at 0:34
okay. Do you have a solution for this? I am new to java and stack overflow and I don't know what I'm doing. — lightning345
– lightning345, Commented Jan 27, 2022 at 0:38
@ScaryWombat I just removed it, I was trying to create a new array but decided to rotate the original one. — lightning345
– lightning345, Commented Jan 27, 2022 at 0:42

Anmol · Accepted Answer · 2022-01-27 00:59:48Z

2

Replace your code's assignment line with this:

newArr[i] = arr[(i + keyIndex)%26];

The logic behind this is: % (i.e. modulus) gives us the remainder on the division by a given number.

let i = 25 and keyIndex=2, so arr[(25 + 2) % 26] => arr[27 % 26] => arr[1]

so we can say modulus26 bounds a number in the range of 0 to 26, which is the need for this rotation operation

Your Corrected Code:

char[] arr = new char[] {'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z'};
int keyIndex = 2;
char[] newArr = new char[arr.length];
for (int i = 0; i < arr.length; i++) {
    newArr[i] = arr[(i + keyIndex )%arr.length];
} 
//Print Array

edited Jan 27, 2022 at 0:59

answered Jan 27, 2022 at 0:29

Anmol

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15 Comments

Bohemian Over a year ago

or even % newArr.length for a more general solution. And how about some explanation?

Bohemian Over a year ago

So why should OP use modulus here? Please explain why this is necessary.

Scary Wombat Over a year ago

should not the assignment be to newArr ?

Bohemian Over a year ago

@passer-by if OP writes to newArr (which seems the original intention), instead of doing an in-place rotation, this problem is not an issue.

Bohemian Over a year ago

@Sharar294 this might be a good time to use a debugger. All Java IDEs support debugging. Halt execution and step through the code to see what happens.

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