I'm trying to reverse an array but the transaction keeps getting reverted even though the logic seems right. Can someone help me out?

Question

Here is the code :

// SPDX-License-Identifier: MIT
pragma solidity >=0.5.0 <0.9.0;

contract Day1 {

function reverseArray(uint[] memory arr1, uint length) public pure returns(uint[] memory){

  uint[] memory arr2;
  uint i = length-1;
  uint j = 0;

  while(i>=0 && j<length){
    arr2[j] = arr1[i];    
    i--;
    j++;
  } 

  return arr2;

  }
}

The error:

Answer 1

So, two things:

1. you need to change how you declare your arr2
2. in your while loop, i gets to 0 and goes into the loop where you do i-- so you are doing '0 - 1' and you underflow, which reverts your transaction.

I got it to work by doing the last swap outside of the loop. I'm sure there is a better way but I don't have enough time to find it right now.

here is the code

pragma solidity >=0.5.0 <0.9.0;

contract Day1 {

  function reverseArray(uint[] memory arr1, uint length) public pure returns(uint[] memory){

    uint[] memory arr2 = new uint[](length);
    uint i = length-1;
    uint j = 0;

    while(i>0 && j<length){
      arr2[j] = arr1[i]; 
      i--;
      j++;
    }

    arr2[j] = arr1[i];

    return arr2;

    }
}

Hope this helps!

Answer 2

There are number of syntax issues in the code you posted. But this should help;

// SPDX-License-Identifier: MIT
pragma solidity >=0.5.0 <0.9.0;

contract RevertArray {

    function reverseArray(uint[] memory array) public pure returns(uint[] memory) {

    uint length = array.length;
    uint numberOfElements = length -1;
    uint[] memory reversed = new uint[](length);
    
    uint i = numberOfElements;
    uint j;

    while( j < numberOfElements) {
        reversed[j] = array[i];    
        i--;
        j++;
    } 

    reversed[j] = array[i];

    return reversed;

    }

}