pointer arithmetic with characters

Hello,

I am attempting to use pointer arithmetic to extract a certain portion of a string but have met nothing but errors.

I want to extract the characters from the original string (origStr) from the third character to the '.' character or the '_' character (whatever comes first).

I would like to output it to the monitor.

My code is below. Could someone tell me what is wrong?

Code:

#include <iostream.h>

//begin main
int main()
{
	char origStr[] = "alHello_0.shp";
	char newStr[25];

	//strip the name down to the component
	*newStr = *(origStr + 2);	//set the pointer in a to the third character of b
	
	//eliminate the rest of the characters from either the _ or the . char
	for( ; (*origStr != '_') && (*origStr != '.'); origStr++, newStr++) 
	{ *newStr = *origStr; }

	++newStr;

	//place a null at the present character
	*newStr = '\0';

	cout << newStr << endl;

	return 0;

}	//end main

Here are my compliation errors:


c:\temp\c++\pec\pec\test.cpp(13): error C2105: '++' needs l-value
c:\temp\c++\pec\pec\test.cpp(13): error C2105: '++' needs l-value
c:\temp\c++\pec\pec\test.cpp(16): error C2105: '++' needs l-value

What kind of errors? Post them please.

Note that 'origStr' and 'newStr' are const(arrays).
You cannot make them point elsewhere.

They will always point to the first element of the array.

I think the problem lies in the fact that your strings are declared as character arrays. This means the array identifier is a constant pointer to the first element which cannot be modified. This can be removed by using dynamic memory or declaring pointers to the arrays. eg

Code:

void main( int argc, char* argv )
{
	char* origStr = "alHello_0.shp";
	char newStr[25];
	char* ptr = newStr;

	//strip the name down to the component
	*newStr = *(origStr + 2);	//set the pointer in a to the third character of b
	
	//eliminate the rest of the characters from either the _ or the . char
	for( ; (*origStr != '_') && (*origStr != '.'); origStr++, ptr++) 
	{ *ptr = *origStr; }

	++ptr;

	//place a null at the present character
	*ptr = '\0';

	cout << newStr << endl;
}

>Could someone tell me what is wrong?
You have been tricked into thinking that arrays and pointers are always the same. This is not so and is often the cause for confusion among even seasoned programmers.

-Prelude

Originally posted by Prelude
>Could someone tell me what is wrong?
You have been tricked into thinking that arrays and pointers are always the same. This is not so and is often the cause for confusion among even seasoned programmers.

-Prelude

I always thought of them as similar but not quite the same.

What is the technical difference? Does anybody have any good links or explanations with a compare/contrast?

Is an array name a pointer?

Thanks for the help

This might have what you're looking for.

>> Is an array name a pointer?
Yes, it is. The only thing is that it is a constant pointer, so it will always point to the array.