How do I replace a digit in an Int32 without turning it into a string?-CodePudding

var digitToSubstitute = 5;

var targetNumber   = 999999999;
var expectedOutput = 995999999;

How do I replace the n-th digit in an int32 without converting it to/from a string?

While this is a small sample, in real-life application I use this in a tight loop (to create all possible number combinations and check against existing ones), so I'd like to avoid creating strings.

CodePudding user response：

You could do something like this. Not sure how this performs compared to string manipulation.

int ReplaceNthDigit(int value, int digitPosition, int digitToReplace, bool fromLeft = false)
{
    if (fromLeft) {
        digitPosition = 1   (int)Math.Ceiling(Math.Log10(value)) - digitPosition;
    }

    int divisor = (int)Math.Pow(10, digitPosition - 1);
    int quotient = value / divisor;
    int remainder = value % divisor;
    int digitAtPosition = quotient % 10;
    return (quotient - digitAtPosition   digitToReplace) * divisor   remainder;
}

Console.WriteLine(ReplaceNthDigit(999999999, 7, 5, fromLeft: true)); // 999999599
Console.WriteLine(ReplaceNthDigit(999999999, 7, 5, fromLeft: false)); // 995999999

Note: won't work for negative numbers.