C＃-使用Int vs Long的相同计算速度变慢？

Question

I've run into something really strange while working my way through some practice problems using dotnetfiddle . I have a program that applies a mathematical sequence (different calculations each step depending on whether the current step is even or odd):

using System;

public class Program
{

    public static void Main()
    {
        int ceiling = 1000000;

        int maxMoves = 0;
        int maxStart = 0;
        int testNumber;

        for(int i = 1; i <= ceiling; i++){
            testNumber = i;
            int moves = 1;
            while(testNumber != 1){
                if(testNumber % 2 == 0){
                    testNumber = testNumber / 2;
                    moves++;
                } else {
                    testNumber = (3 * testNumber) + 1;
                    moves++;
                }
            }
            if(moves > maxMoves){
                maxMoves = moves;
                maxStart = i;
            }
        }

        Console.WriteLine(maxStart);
        Console.WriteLine(maxMoves);
    }

}

As written, the execution time limit gets exceeded. However, if I change the declaration of test number to a long instead of an int, the program runs:

int maxMoves = 0;
int maxStart = 0;
**long** testNumber;

Why would making this change, which requires recasting i from an int to a long each increment of the for loop (at testNumber = i ), be faster than leaving this as an int ? Is performing the mathematical operations faster on a long value?

Answer 1

The reason seems to be an overflow . If you run that code enclosed in a

checked
{
    // your code
}

you get an OverflowException when running with testNumber as int .

The reason is that eventually 3*testNumber+1 exceeds the boundary of an int . In an unchecked context this does not throw an exception, but leads to negative values for testNumber .

At this point your sequence (I think it's Collatz , right?) does not work anymore and the calculation takes (probably infinitly) longer, because you never reach 1 (or at least it takes you a whole lot more iterations to reach 1 ).