C ++将char数组传递给void函数

Question

Task is to change every uppercase letter to lowercase and vice versa, and increase every number by 1, and change 9 for 0.

#include <iostream>
using namespace std;

void modif ( char*& arr){
    for (int i=0;arr[i]!=0;i++)
        if ('a'<=arr[i] && arr[i]<='z'){
            arr[i]=char(((int)arr[i])-32);
        }
        else if ('A'<=arr[i] && arr[i]<='Z'){
            arr[i]=char(((int)arr[i])+32);
        }
        else if ('0'<=arr[i] && arr[i]<='8'){
            arr[i]=char(((int)arr[i])+1);
        }
        else if (arr[i]== '9'){
            arr[i]=char(((int)arr[i])-8);
        }
}

int main() {
    char * s=new char [100];
    cin.getline (s, 100);
    void modif (char * s);

    cout << s  << endl;
    delete [] s;
}

I can get it to work without void function, but can't with it

Answer 1

Try this so that you actually call the function:

int main() {
    char * s=new char [100];
    cin.getline (s, 100);
    // void modif (char * s); // removed line
    modif(s);                 // new line

    cout << s  << endl;
    delete [] s;

}

Answer 2

This line:

void modif (char * s);

is a function declaration, you want to call it with:

modif(s);

also you dont need char*& in your function, char* will suffice.

Answer 3

void modif (char * s); is a forward declaration . It is unnecessary in your example, because modif is defined above its point of invocation. You would need it if you wanted to move modif to a place in the file below main , or to a different file altogether:

#include <iostream>
using namespace std;

int main() {
    char * s=new char [100];
    cin.getline (s, 100);
    void modif (char*&); // Forward declaration
    modif(s);            // Call
    cout << s  << endl;
    delete [] s;
}

void modif ( char*& arr){
    for (int i=0;arr[i]!=0;i++)
    if ('a'<=arr[i] && arr[i]<='z'){
       arr[i]=char(((int)arr[i])-32);
    }

   else if ('A'<=arr[i] && arr[i]<='Z'){
       arr[i]=char(((int)arr[i])+32);
    }
    else if ('0'<=arr[i] && arr[i]<='8'){
       arr[i]=char(((int)arr[i])+1);
    }
    else if (arr[i]== '9'){
       arr[i]=char(((int)arr[i])-8);
    }
}

Forward declaration is a promise to the compiler that there is going to be a definition of your function somewhere else. This way the compiler knows how to make a call if you choose to call your function. Note that the signatures in the declaration and in the definition must match.

The call, on the other hand, looks like this:

modif(s);

Here, you tell the compiler that you want to invoke function modif , and pass the current value of s for its formal parameter called arr .