为什么这些代码给我一个sigsegv错误？

Question

Why is these code giving me a RUNTIME SIGSEGV ERROR. I have tried running the code and works perfectly in codeblocks but some IDE is giving me these error.

It takes a Fibonacci series then modulus each number in the series and only takes up the numbers at eve places till a single number is obtained.

for example:input 1 9 {0 1 1 2 3 5 8 13 21}->{0 1 1 2 3 5 8 3 1}->{1 2 5 3}->{2 3}->3

#include <stdio.h>

int main(void) {
    // your code goes here
    int n,j,k,r,o;
    o=0;


//  printf("enter the number of test cases: ");
    scanf("%d",&n);
    int s[n];
    int a;
    a=n;

    while(n!=0)
    {


         r=k;
         int e[k/2];
         int m;
         scanf("%d",&k);//enter say 9
      if(k!=1)
            {
              int i[k];
              i[0]=0;
              i[1]=1;

              for(j=0;j<k;j++)
                  {
                     if(j>1)
                       {
                         i[j]=(i[j-2]+i[j-1])%10;
                       }


                  }


              while(r!=1)
                {
                 m=0;

                 for(j=0;j<r;j++)
                    {

                      if(j!=0)
                       {

                        if(j%2!=0)
                         {
                           e[m]=i[j];
                           m++;
                         }
                       }
                    }

                 for(j=0;j<k/2;j++)
                    {
                      i[j]=e[j];
                    }
                 r=r/2;
                }
             s[o]=e[0];
             o++;
             n--;
           }
        else
           {
             return 0;
           }


    }
    if(k!=1)
      {
        for(j=0;j<a;j++)
         {
             printf("%d\n",s[j]);
         }
      }


    return 0;
}

I want to know which point in the code is trigerring the error i know little about these error(like acessing array beyond bonds) can you explain me that?

Answer 1

Your k variable is uninitialized when you first access it at

r=k;
int e[k/2];

This means that it is equal to whatever happened to be stored at that memory location before. This could be any random number and is very bad.

On the next line you declare an array e[k/2] with size k/2 , but since k was never initialized, this could be any size. If k happens to be negative, I get a segmentation fault at that line.

To fix this issue, you need to initialize all your variables before using them.

A segmentation fault (SIGSEGV) is what occurs if you try to access memory that isn't allocated to your program by the operating system.

To help with debugging these errors, run your code in the debugger. This can then take you directly to the line where the segmentation fault occured.

Answer 2

just put the scanf("%d",&k); above r=k;

A SIGSEGV is an error(signal) caused by an invalid memory reference or a segmentation fault. You are probably trying to access an array element out of bounds or trying to use too much memory.

also. i am not sure about the return 0; in the else statement. i haven't run the code so i may be mistaken but it should be return 1; instead of return 0;

if(j!=0)
{
      if(j%2!=0)
      {
            e[m]=i[j];
            m++;
      }
}

you can replace it with if(j != 0 && j%2 != 0)

Answer 3

我知道您指的是这些问题...这不是您应该解决的方式...您必须找到O（1）时间中剩下的数字...由于去除了奇数位，我们只需要担心偶数位置...现在，那些偶数位置被重复除以2可能会变得很奇怪...因此，有固定数量的步骤可以将整个数组转换为单个元素...在此的每一点上一步偶数放置数字除以2，因此在这些步骤之间或之后只有一个偶数放置的数字不会变得奇怪...您必须通过看N才能找到该数字，至少我做了一样...这只是一个提示，如果不清楚，我可以在评论中解释更多的逻辑...Upvote😝