#include<stdio.h>
#include<math.h>
void main()
{
    int siz,i,flage;
    unsigned long int var=0,ifzs=3,mul=2;
    siz=sizeof(unsigned long int);
    printf("unsigned width is %d\n",siz);
    i=siz*8;
    printf("i=%d\n",i);
/*    do
    {
        if(i>1)
            
            printf("%lu:",var);
        else

            printf("%lu:",var-1);
        printf("%d\t",i);
        var*=2;
        --i;
   
    }while(i>0);
*/
    printf("\n2^64=%lu\n",var-1);

    do
            {
        mul=2;
        flage=1;   
   
        do{
              if(ifzs%mul==0)
              { flage=0;
                  break;}
              else
              { ++mul;}
        }while(mul<=sqrt(ifzs));
        if(flage)
        {printf("%lu\t",ifzs);
                ++ifzs;}
        else
        {    ++ifzs;
        }
        }while(ifzs<100000);
            printf("\n");
            }

恭喜楼主在写代码的征途上一步一个脚印奋勇前进！

建议你先换一个正经的C编译器，这非常重要

void main() ------ 详见 https://en. ，虽然允许实现定义，但也不可能改变返回类型
siz=sizeof(unsigned long int); ------ sizoef 的类型是 size_t，不是 int
i=siz*8; ------ char 或者说字节，未必是8bits，应该用 CHAR_BIT
printf("\n2^64=%lu\n",var-1); ------ 这我就完全看不懂了，2^64 应该是 18446744073709551616，怎么可能是 0ul - 1 ？

我将你的代码改了一下

#include <stdio.h>
#include <limits.h>

int main( void )
{
    printf( "unsigned: %zuBytes\n", sizeof(unsigned) );
    printf( "unsigned: %zuBits\n", sizeof(unsigned)*CHAR_BIT );

    for( unsigned i=2; i!=100000; ++i )
    {
        unsigned factor = 2;
        for( ; factor<=i/factor && i%factor!=0; ++factor );
        if( factor > i/factor )
            printf( "%u\t", i );
    }
    putchar( '\n' );
}

补充一下，
试除法应该用在判断少量几个数是否素数上面；
而对于生成素数表也就是大量素数的应用，筛法的效率会高很多。
当然，还没涉及数组的话，不急。

首先感谢版主的耐心指导，令本人不甚至荣幸。关于printf("\n2^64=%lu\n",var-1); ------ 这我就完全看不懂了，2^64 应该是 18446744073709551616，怎么可能是 0ul - 1 ？
因为无符号长整型变量的数值范围为：0——2^64-1，所以当计算2^64的时候，变量var为变成0，只好用0-1来表示了（这可能涉及到补码）。上面部分的代码确实有问题，更正如下：


#include<stdio.h>
#include<math.h>
void main()
{
    int i;
    size_t siz;
    unsigned long int var=2;
    siz=sizeof(unsigned long int);
    printf("unsigned width is %d\n",siz);
    i=siz*8; /*计算无符号长整型变量所占用的位数，如为4Byte,则字节数为4*8=32Bits。
    printf("i=%d\n",i);
    do
    {
        if(i>1)
            
            printf("%lu:",var);
        else

            printf("%lu:",0-1);
        printf("%d\t",i);
        var*=2;
        --i;
   
    }while(i>0);

    printf("\n2^64=%lu\n",0-1);

            }

[此贴子已经被作者于2023-2-26 09:13编辑过]

感谢鼓励和指点！

又修改了一下，这样显示的结果格式似乎更好看一些：(以下代码在DEV-C++5.11下编译通过）
#include<stdio.h>
#include<math.h>
void main()
{
    int i,flage=1;
    size_t siz;
    unsigned long int var=2;
    siz=sizeof(unsigned long int);
    printf("unsigned width is %d\n",siz);
    i=siz*8;
    printf("siz width is %d\n",i);
    do
    {
        if(flage>(i-1))
            
            printf("2^%d=%lu\t",flage,var-1);
        else

            {printf("2^%d=%lu\t",flage,var);}
        var*=2;
        ++flage;
   
    }while(flage<=i);

    printf("\n2^32=%lu\n",var-1);


}

[此贴子已经被作者于2023-2-26 09:37编辑过]

#include <stdio.h>
#include <limits.h>

int main( void )
{
    size_t nBytes = sizeof(unsigned long int);
    printf( "The \"unsigned long\" type has %zu bytes\n", nBytes );

    size_t nBits = nBytes * CHAR_BIT;
    printf( "The \"unsigned long\" type has %zu bits\n", nBits );

    for( size_t i=0; i!=nBits; ++i )
        printf( "2^%zu = %lu\n", i, 1ul<<i );
    printf( "2^%zu - 1 = %lu\n", nBits, ~0ul );
}

一种「可能的」输出

#include<stdio.h>
#include<math.h>
void main()
{
    int flage;
    unsigned long int ifzs=2,mul=2;
    printf("%-10lu",ifzs);
    do
           {
          mul=2;
          flage=1;   
   
        do{
              if(ifzs%mul==0)
                   {
                flage=0;
                    break;
               }
              else
                   ++mul;
          } while(mul<=sqrt(ifzs));
        
        if(flage)
            {
             printf("%-10lu",ifzs);
                 ++ifzs;
                }
        else
            {   
                ++ifzs;
            }
        }while(ifzs<10000000);
            printf("\n");
            
   }

[此贴子已经被作者于2023-3-1 08:10编辑过]

#include <stdio.h>
#include <math.h>

int main( void )
{
    for( unsigned long ifzs=2; ifzs!=10000000; ++ifzs )
    {
        unsigned long mul = 2;
        const unsigned long sr = (unsigned long)sqrt(ifzs);
        for( ; mul<=sr && ifzs%mul!=0; ++mul );
        
        if( mul > sr )
            printf( "%-10lu", ifzs );
    }
}