这个程序中的for循环如何工作? [英] how for loop works in this program ?

问题描述

我想对此程序及其工作原理进行一些解释.

i want some explanation of this program and how it works.

#include <stdio.h>

int main(void)
{
  int num, i, is_prime;

  printf("Enter a number: ");
  scanf("%d", &num);


  is_prime = 1;

  for(i = 2; i <= num / 2; i = i + 1)
    if((num%i)==0)
         is_prime = 0;

  if(is_prime==1)
      printf(" is prime.");
  else
      printf("not prime.");

  return 0;
}

推荐答案

它基本上是一个简单的试验部门主要发现者.参见此处: http://en.wikipedia.org/wiki/Trial_division [

It''s basically a simple trial division prime finder. See here: http://en.wikipedia.org/wiki/Trial_division[^] which explains it very well.

int num, i, is_prime; // declare three variables, num for user input, i for loop and is_prime used as boolean for the result

//get number from user
 printf("Enter a number: ");
 scanf("%d", &num);

//set is_prime to true
 is_prime = 1;

 for(i = 2; i <= num / 2; i = i + 1) //start from 2 and reach until the half the number
   if((num%i)==0)// check if any of the number in for loop range successfully divides the number, if so set is_prime to false, as dividable no are not prime
        is_prime = 0;

 if(is_prime==1) // check if Is_prime was never changed in loop, if s
     printf(" is prime.");
 else // if the is_prime was changed
     printf("not prime.");

大多数时候我们使用

most of time we use

for(i = 2; i <= num / 2; i = i + 1)
    if((num%i)==0){
         is_prime = 0;
         break; }

一旦发现no不为质数，这将停止for循环，保存循环的剩余迭代次数

this stops the for loop as soon as the no is found not to be prime, save remaining iterations of loop

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