#include<stdio.h>
#include<iostream>
#define M 20
typedef struct node{
    int weight;
    int price;
    int flag;
}Node;
typedef struct knap {
    Node a[M];
    int total;
    int size;
}*Knap;
int f(Knap k)
{
    int index=-1;
    int mp=0;
    int i;
    for(i=0;i<k->size-1;i++)
    {
        if((k->a[i].flag==0)&&(k->a[i].price>mp))
        {
            mp=k->a[i].price;
            index=i;
        }
    }
    return index;
}
void PrintResult(Knap k)
{
    int i=0;
    int total=0;
    for(i=0;i<k->size-1;i++)
    {
        if(k->a[i].flag==1)
        {
           total+=k->a[i].price;
           printf("%d\n",k->a[i].price);
        }
    }
    printf("total= %d\n",total);
}
void select(Knap k,int f(Knap k))
{
    int idx;
    int count=0;
    while((idx=f(k))!=-1)
    {
        if((count+k->a[idx].weight)<=k->total)
        {
          k->a[idx].flag=1;
          count+=k->a[idx].weight;
          k->total-count;
                printf("total= %d\n",k->total);
        }
        else{
            k->a[idx].flag=2;
        }
    }
    PrintResult(k);
}
int main()
{
    int a[20];
    int b[20];
    Knap k;
    int n,i,m;
    printf("你准备对几件物品进行操作:");
    scanf("%d",&n);
    k->size=n;
    printf("请输入你限制的总重量:");
    scanf("%d",&m);
    k->total=m;
     for(i=0;i<n;i++)
     {
       printf("请输入第%d件物品的价值",i);
       scanf("%d",&a[i]);
       k->a[i].price=a[i];
     }
     for(i=0;i<n;i++)
     {
         printf("请输入第%d件物品的重量",i);
         scanf("%d",&b[i]);
         k->a[i].weight=b[i];
      }
      select(k,f);
      return 0;
}

Knap 是指针，应该给 k 申请空间才行
Knap k = (Knap)malloc(sizeof(Knap));