错误：将'char *'分配给'char [4000]'时类型不兼容

Question

I've been trying to solve an easy problem, but I can't figure why my program doesn't work. I want to concatenate a string. Can you help me? If so, can you also explain me why it doesn't work?

#include <iostream>
#include <cstring>
#include <fstream>

using namespace std;
ifstream in("sirul.in");
ofstream out("sirul.out");
char a[4000]="a",b[4000]="b",aux[4000];
int main()
{ int n,i;
 in>>n;
 if(n==1)out<<"a";
 if(n==2)out<<"b";
 for(i=3;i<=n;i++)
 {
     aux=strcpy(aux,b);
     b=strcat(b,a);
     a=strcpy(a,aux);
 }

    return 0;
}

Answer 1

strcpy and strcat work directly on the pointer you pass in as the first argument, then also return is so that you can chain calls. As such, assigning their result back to the destination pointer is redundant. In this case, it's also invalid, as you can't reassign an array.

The fix is to just not assign the return value of those calls:

strcpy(aux,b);
strcat(b,a);
strcpy(a,aux);

However, since you are using C++, you should use std::string instead, which gives you nice value semantics for your string data.

Answer 2

you can not do (see 2)

char b[4000]="b";
char aux[4000];
aux /* 2 */ = strcpy(aux /* 1 */ , b);

because aux is not a pointer, but array. you can pass it as pointer argument (see 1), but you can not "collect" the result "inside" aux (see 2).

As other suggested, just remove "collection" and it will work as you expect.

char b[4000]="b";
char aux[4000];
strcpy(aux /* 1 */ , b);
// or even:
const char *s = strcpy(aux /* 1 */ , b);

Also you are mixing C and C++ in one file.

Also probably there is possibility for buffer overflow.

Answer 3

#include <iostream>
#include <cstring>
#include <fstream>

using namespace std;
ifstream in("sirul.in");
ofstream out("sirul.out");
char a[4000]="a",b[4000]="b",aux[4000];
int main()
{
 int n,i;
 cin>>n;
 if(n==1)cout<<"a";
 if(n==2)cout<<"b";
 for(i=3;i<=n;i++)
 {
     strcpy(aux,b);
     strcat(b,a);
     strcpy(a,aux);
 }

    return 0;
}

check out definition os strcpy, in should be cin and out should be cout