#include <stdio.h>
main()
{
float a,b,c,d;
int i;
char P;
  scanf("%f",&a);
  for(i=0;i>=0;i++)
  {   sconf("%c",&P);
      if (P=="\n")
      printf("%f",a);
  switch(P)
   {case +:  scanf("%f",&b);a=a+b;break;
    case *:  scanf("%f",&b);a=a*b;break;
    case /:  scanf("%f",&b);a=a/b;break;
    case -:  sconf("%f",&b);a=a-b;break;
    default:  printf("从输");break;
     } }


    getch();
}

#include <stdio.h>
main()                //多看看书把
{
float a,b,c,d;
int i;
char P;
  scanf("%f",&a);
  for(i=0;i<=5;i++)
  {   scanf("%c",&P);
      
  switch(P)
   {case '+':  scanf("%f",&b);a=a+b;break;
    case '*':  scanf("%f",&b);a=a*b;break;
    case '/':  scanf("%f",&b);a=a/b;break;
    case '-':  scanf("%f",&b);a=a-b;break;
    default:  printf("从输");
     }
     }



}

if (P=="\n")
      printf("%f",a);   没了判断打印结果啊

if (P=="\n")
      printf("%f",a); 打印判断要怎么加下去吖！！555555晕

#include <stdio.h>
main()
{
    float a,b;
    int i;
    char P;
    scanf("%f", &a);
    for (i = 0;i >= 0;i++)
    {   
        scanf("%c", &P);
        if (P=="\n")
            printf("%f", a);
        switch(P)
        {
            case '+':
                scanf("%f", &b);
                a = a + b;
                break;
            case '*':
                scanf("%f", &b);
                a = a * b;
                break;
            case '/':
                scanf("%f", &b);
                a = a / b;
                break;
            case '-':
                scanf("%f", &b);
                a = a - b;
                break;
            default:
                printf("从输");
                break;
        }
    }
    getch();
}

#include <stdio.h>
void main()
{
    float a,b;
    int i;
    char P;
    scanf("%f", &a);
    for (i = 0;i >= 0;i++)
    {   
        scanf("%c", &P);
        if (P=="\n")
            printf("%f", a);
        switch(P)
        {
            case '+':
                scanf("%f", &b);
                a = a + b;
                break;
            case '*':
                scanf("%f", &b);
                a = a * b;
                break;
            case '/':
                scanf("%f", &b);
                a = a / b;
                break;
            case '-':
                scanf("%f", &b);
                a = a - b;
                break;
            default:
                printf("从输");
                break;
        }
    }
    return;
}
用这个试试

#include <stdio.h>
void main()
{
    float a,b;
    int i;
    char P;
    scanf("%f", &a);
    for (i = 0;i >= 0;i++)
    {   
        scanf("%c", &P);
        if (P == '\n')
            printf("%f", a);
        switch(P)
        {
            case '+':
                scanf("%f", &b);
                a = a + b;
                break;
            case '*':
                scanf("%f", &b);
                a = a * b;
                break;
            case '/':
                scanf("%f", &b);
                a = a / b;
                break;
            case '-':
                scanf("%f", &b);
                a = a - b;
                break;
            default:
                printf("从输");
                break;
        }
    }
    return;
}
错误太多，没有改权，试试。