c - 如何输出随机生成的整数数组中第一个重复数字的索引 (C)

Question

我正在尝试用 C 语言编写一个程序，它将生成 0-99 之间的 50 个随机整数，然后打印出来或抛出到第一个重复数字出现的索引处的变量。我认为使用数组是解决这个问题的最佳方法。我似乎成功地生成了一个随机(足够随机)整数数组并将它们放入一个数组中。

但是，在比较数组的每个元素时，我的 for 循环遇到了问题。看来我最终将索引 1 与索引 1 进行了比较并获得了匹配。
这是我当前的代码供引用。任何帮助/建议将不胜感激 - 我是编程新手:)

#include <stdio.h>
#include <stdlib.h>

int main()
{
//define the test pool(N - people in the birthday problem)
//and possible unique attributes(M - birthdays in the birthday problem)
int N = 50;
int M = 100;
int i;
int x;
int y;
int arr[N];
int count = 1;

//use for loop to populate an array of length N with numbers ranging
//from 0 to 99
for(i = 0; i < N; i++)
        {
                arr[i] = rand() % (M-1);
                printf("%d\n", arr[i]);
        }

//just formatting the output a little
printf("\n\n\n");


//iterate through the array at each position, comparing it to all other
//positions in the array.  if array[x] does not equal array[y] increment
//the counter.  if it does match, print the counter to the screen.
for(x = 0; x < N; x++)
        {
                for(y = 0; y < N; y++)
                        {
                                if(arr[x] != arr[y])
                                        {
                                                count++;
                                        }
                                else
                                        {
                                                printf("the first repeat is  at:  ");
                                                printf("%d\n", count);
                                                break;
                                        }
                                break;
                        }
                break;
        }

return 0;

}

Answer 1

你必须改变

 for(y = 0; y < N; y++)

至

 for(y = x + 1; y < N; y++)

因为，当您选择一个元素x时，您必须在x的剩余数组前面中搜索其重复项，因此， y=x+1 到 N。如果您再次开始从 y=0 开始搜索重复项，您最终会在 x 或之前找到 x 的重复项。

更好的方法:

在您的实现中，您必须运行两个循环来检查重复项。更快的方法是记录 counter 数组中出现的每个数字的计数，并仅运行一个循环，检查 counter[ arr[i] ] > 1 ，这意味着元素 arr[i] 已至少重复一次。

代码:

#include <stdio.h>
#include <stdlib.h>

int main()
{
//define the test pool(N - people in the birthday problem)
//and possible unique attributes(M - birthdays in the birthday problem)
        int N = 50;
        int M = 100;
        int i;
        int x;
        int y;
        int arr[N];
        //Counter[M] will store the number of occurences
        //of M in the program;
        int counter[M];
        int count = 1;

        for(int i = 0;i < 100; ++i)
        {
                counter[i] = 0;
        }
//use for loop to populate an array of length N with numbers ranging
//from 0 to 99
        for(i = 0; i < N; i++)
        {
                arr[i] = rand() % (M-1);
                printf("%d\n", arr[i]);
                counter[arr[i]]++; 
                //^^^^^^^^^^^^^^^ Incrementing the count array
                //of the particular element arr[i].
        }

//just formatting the output a little
        printf("\n\n\n");


//iterate through the array at each position, comparing it to all other
//positions in the array.  if array[x] does not equal array[y] increment
//the counter.  if it does match, print the counter to the screen.
        for(x = 0; x < N; x++)
        {
                if (counter[arr[x]] > 1)
                {
                        printf("the first repeat is  at:  ");
                        if (youWantTheIndex)
                        {
                            printf("%d\n", x); //This prints the index
                        }
                        else if (youWantTheElement)
                        {
                        printf("%d\n", arr[x]); //This prints the element.
                        }
                        break;
                }
        }
return 0;

}