Pointers and Strings

[torels]

Hi there

I am learning C++ (or trying to do that... XD) but I have a BIG perplexity on a piece of code I wrote

how come is this not working ?

#include <iostream>

using namespace std;

int Dimension(char* Ptr)
{
    int i=0;
    do {
        i++;
  } while (Ptr[i]!='\0');
  return i;
}

int main()
{
char* String;
cout<<"Scrivi una stringa"<<endl;
cin>>String; //HERE IS THE PROBLEM... I DON'T KNOW WHY...
cout<<"Al contrario: " << strrev(String) << endl;
return 0;
}

It only worked like this:

#include <iostream>

using namespace std;

int Dimension(char* Ptr)
{
    int i=0;
    do {
        i++;
  } while (Ptr[i]!='\0');
  return i;
}

int main()
{
char* String;
char StringArray[100000];
cout<<"Scrivi una stringa"<<endl;
cin.getline(StringArray, 100000);
String=StringArray;
cout<<"Al contrario: " << strrev(String) << endl;
return 0;
}

My question is... isn't there a way to get strings (or any other input) WITHOUT using an Array before ? (not using #include <String>... I'm trying to learn pointers... that's the whole point

thanks in advance

[wraithdu]

No. Just declaring 'char *string;' only declares a POINTER to a char array (currently a pointer to VOID because you haven't declared the array). You must allocate space for the string, the array, before trying to store anything there. You also cannot use just 'cin >> var;' since it won't accept strings with spaces.

char buff[100];
cout << "type something" << endl;
cin.getline(buff, 100);
cout << "you typed:  " << buff << endl;

Edited April 22, 2009 by wraithdu

[torels]

ah ok

now everything's clearer

thanks alot

btw... I knew about the cin.getline() func

and... I have another question...

How Do I Redimension an Array... So let's say... if I declare buff[100] but the input is only 5 chars and I want to free Memory by redimensioning the array to 4... how do I do that ?

[monoceres]

How Do I Redimension an Array... So let's say... if I declare buff[100] but the input is only 5 chars and I want to free Memory by redimensioning the array to 4... how do I do that ?

You can't redimension an array. If you want to beeing able to allocate and delete memory during runtime you have to use the new and delete keywords.

http://www.cplusplus.com/doc/tutorial/dynamic/

Edited April 22, 2009 by monoceres

[wraithdu]

You cannot. Standard arrays in C++ are static once declared. You're talking about dynamic memory (new and delete operators). You'd have to create your new array, copy the old values into the new array, and delete the old array.

http://www.cplusplus.com/doc/tutorial/dynamic/

I think it's time for a C++ book for you.

[wraithdu]

LOL, that's hilarious.

[torels]

You cannot. Standard arrays in C++ are static once declared. You're talking about dynamic memory (new and delete operators). You'd have to create your new array, copy the old values into the new array, and delete the old array.

http://www.cplusplus.com/doc/tutorial/dynamic/

I think it's time for a C++ book for you.

Yeah maybe XD

I'm not quite understanding it...

It's... let's say... complicated!

[cppman]

No. Just declaring 'char *string;' only declares a POINTER to a char array (currently a pointer to VOID because you haven't declared the array).

I'm not exactly sure what you mean here. Whether you initialize the pointer or not, it is still a pointer to a character. If you mean the pointer is initialized to zero, that is not true. Variables (that includes pointers) will not be initialized to any specific value - it'll take the value of what was previously in that memory address (which sometimes happens to be 0).

[monoceres]

I'm not exactly sure what you mean here. Whether you initialize the pointer or not, it is still a pointer to a character. If you mean the pointer is initialized to zero, that is not true. Variables (that includes pointers) will not be initialized to any specific value - it'll take the value of what was previously in that memory address (which sometimes happens to be 0).

I think he just means it doesn't point to any usable data.

Edited April 25, 2009 by monoceres

[system24]

So you can't use pointer-based strings for input. Everytime I do that this happens...

[Richard Robertson]

Pointer based strings are the only way to perform input. You have to have a buffer to receive to.

[system24]

So, both char *str and char str[25] are pointer-based strings?

[Valik]

So, both char *str and char str[25] are pointer-based strings?

Sigh.

Read a book or good FAQ. The terminology being used in this thread is confusing at best.

[system24]

Yeah, confusing at best.

A pointer-based string in C++ is an array of characters ending in the null character ('\0'), which marks where the string terminates in memory.

Edited April 29, 2009 by system24

[Richard Robertson]

That's not even always true. I've had strings in C++ without a null terminator. I tracked the length of the string myself and was only using the string in my own methods.

[jvanegmond]

To avoid confusion, I think it would be better to call that a character array instead of a string. Although, it is a string ofcourse.

[dexto]

Would it be easer to work with "#include <string>"?

and declare cin variable as "string". I think later you can refer to it as a char pointer or array pointer anyway.

[Richard Robertson]

cin is an object of type istream. It's not a string.

Also I have no idea what that second part was supposed to mean.