Holidays make people become pigs. In fact, there have been several impulses to write blogs in more than a month, but each time they write half, they don't stick to it, and school starts in a twinkling of an eye.

During the holiday, I saw two interesting topics: the problem of counters and the problem of prime arithmetic sequence.

1. Counter problem

Inadvertently, I saw such a problem and suddenly became interested.

#include<iostream> using namespace std; int main(){ int count; count=0; int x; x=0; while(x<=8){ int ra=rand()%2; if(ra=0){ x++; } else{ x--; } count++; } cout<<count<<endl; return 0; }

The output result is negative, indicating that the number of operations is large, and a problem is exposed at the same time. It is a huge amount of calculation to calculate the average number of events.

Replacing int with double can finally give me a number:

#include<iostream> using namespace std; int main(){ double count; count=0; int x; x=0; while(x<=8){ int ra=rand()%2; if(ra=0){ x++; } else{ x--; } count++; } cout<<count<<endl; return 0; }

But the unsolved problem is, how to get an average value? This may have to be solved in the way of probability theory. At the same time, a very simple mathematical problem also reveals an imperfection in c + + that I think is the rand function.

At the end of last month, the card game "undetermined event book" was born. I scratched my hand and wrote a simulator to simulate card drawing, but I didn't expect to hit a wall here in random number generation. The simulator written by my friend vb became, so I gave up to overcome this problem.

It's sad that a quick forming applet was abandoned. I tried to use seeds and srand (unsigned) time (null), but if the button clicks fast, I still can't generate the desired random number.

2. Prime arithmetic sequence problem

This is a question I saw when watching the real question of the Blue Bridge Cup. At first glance, I may feel that it is not difficult, but if this question is regarded as an equation problem, there is actually an unknown number, but I want to write it in a main function and give up the idea of solving it violently in the final triple cycle.

This is the failure code I wrote. I'm going to find the problem and then go back to find my own problem:

#include <iostream> using namespace std; int p1[100010]; int main() { for(int m=0;m<20000;m++){ p1[m]=1; } for(int i=4;i<20000;i++){ for(int j=2;j<i;j++){ if(i%j==0){ p1[i]=0; break; } } } for(int n=3;n<20000;n++){ if(p1[n]){ for(int s1=30;s1<1000;s1++){ for(int s2=1;s2<=10;s2++){ if(p1[n+s1*s2]=0){ break; } } if(s2=10){ cout<<s1<<endl; break; } } } } return 0; }

The output is in a mess...

#include <bits/stdc++.h> using namespace std; int p[100010]; int prim[100010]; int len=0; void isp() { memset(p,0,sizeof(p)); p[0]=1;p[1]=1;p[2]=0; for(int i=0;i<10000;i++) { if(p[i]) continue; for(int j=i;j*i<10000;j++) { p[i*j]=1; } prim[len++]=i; } } int main() { isp(); for(int i=0;i<len;i++) { int ss=prim[i]; for(int c=1;c<1000;c++) { int j; for(j=1;j<10;j++) { if(p[ss+c*j]) break; } if(j>=10) { cout<<c<<' '<<ss<<endl; return 0; } } } }