c - 为什么 < 比 != 快得多？

Question

Problem : Consider the following algorithm to generate a sequence of numbers. Start with an integer n. If n is even, divide by 2. If n is odd, multiply by 3 and add 1. Repeat this process with the new value of n, terminating when n = 1. The input will consist of a series of pairs of integers i and j, one pair of integers perline. All integers will be less than 1,000,000 and greater than 0. For each pair of input integers i and j, output i, j in the same order in which they appeared in the input and then the maximum cycle length for integers between and including i and j. These three numbers should be separated by one space, with all three numbers on one line and with one line of output for each line of input.

示例输入:

1 10

示例输出:

1 10 20

所以我写了这个:

#include <stdio.h>
#include <string.h>

struct line{int in1;int in2;int result;};

int cycle(int in);

int main(int argc, char *argv[]) {
    int cycle(int in);
    char c;
    int firstIn=0;
    struct line l[500] ;
    int pointer=0;

    while(2<3){
        l[pointer].in1=0;
        l[pointer].in2=0;
        scanf("%u %u",&l[pointer].in1,&l[pointer].in2);
        if(l[pointer].in1<1||l[pointer].in2<1){
            break;
        }

        int maxCyc=0;
        int j,m;
        int min,max;
        if(l[pointer].in1>l[pointer].in2){
            max=l[pointer].in1;
            min=l[pointer].in2;
        }
        else{
            max=l[pointer].in2;
            min=l[pointer].in1;
        }
        for(j=min;j<=max;j++){

            m = cycle(j);
            if(m>maxCyc)
                maxCyc=m;
        }
        l[pointer].result=maxCyc;
        printf("%d %d %d\n",l[pointer].in1,l[pointer].in2,l[pointer].result);
        pointer++;
    }
}

int cycle(int in){
    int cyc = 1;
    while(in>1){
        if(in%2==0){
            cyc++;
            in=in/2;
        }
        else{
            cyc++;
            in=in*3+1;
        }
    }
    return cyc;
}

完全没问题，但是当您将循环方法中的 while(in>1) 更改为 while(in!=1) 时，它会变得更慢。我的问题是为什么？!

Time when its while(in>1) : 0.683 sec

and when its while(in!=1) : I waited more than 5 min nothing happened yet :)

for input : 1 1000000

不存在无限循环或其他东西，因为 in 根本无法低于 1(因为它必须已经是 1)。

致以诚挚的问候

Answer 1

当您使用输入值113383调用cycle时，该过程最终将n设置为 827370449，而3*827370449+1为2482111348，大于最大signed int，解释为-1812855948。所以这是你的第一个负数，而不应该有负数。

如果此过程最终将 n 设置为 -2，则此后它将在 -2 和 -1 之间无限循环。可能还有其他我没有考虑到的循环。

如果您要使用无符号整数，则有可能(我没有检查)最终也会溢出，这不会导致负值，但会导致不正确 值，使您的结果无效。

无论您使用哪种整数表示形式，最好在每个循环的顶部将 n 与 (maximum-1)/3 进行比较，其中 maximum 是整数类型的最大可能正值，只是为了确保不会溢出。