TC++PL 6.6.13 - new operator mystery

[JasonUMD]

Hello. Newbie working self through TC++PL.

Ex: 6.6.3: Write a function cat that concatenates 2 C-style strings. Use new to find store for the result.

My code:

#include <iostream>

using namespace std;

char* cat(const char* str1, const char* str2){

	char *catstr = new char[(strlen(str1) + strlen(str2) + 1)];
	
	int i = 0;
	
	for(; *str1; i++){
		catstr[i] = *str1++;
	}

	for(;*str2;i++){
		catstr[i] = *str2++;
	}		

	return catstr;

}


int main() {
	char str1[] = &quot;Hello &quot;;
	char str2[] = &quot;world!&quot;;
	char *p = cat(str1, str2);

	cout << p << endl;
	
	return 0;
}

Why does new give an array size of 20?
What am I doing wrong, and is my approach itself correct?

 

[Zyrenthian]

Well... the idea is there... it is just hard to read. Does this complile? if so, i dont understand how (unless my order of operations is wrong) If str1 & 2 come in as const you should NOT be able to perform a ++ on them.

i would change the *str1++ and *str2++ to

*(str1+i)


Where do you actually get the 20 from? Are you determining it from the for loop? or strlen + strlen +1?

Matt

 

[rjr9999]

there is another method....

char *cat(const char *buf1, const char *buf2)
{
char *catstr;

sprintf(buf1 + strlen(buf1), "%s", buf2);

// or sprintf(catstr, "%s%s", buf1, buf2);

catstr = buf1;

return catstr;

} Rob
"Programming is like art...It makes me feel like chopping my ear off."

Http://www.thisisrisk.com/

- Currently down

 

[rjr9999]

Hey, let me know if that helped! I like to know these things ;-) Rob
"Programming is like art...It makes me feel like chopping my ear off."

Http://www.thisisrisk.com/

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[JasonUMD]

Let's change this line:

cout << p << endl;

To this:

cout << c << &quot;  :  &quot; << strlen(c) << endl;

Here is the result:

Hello world!-²²²²²²²  :  20

The statement (strlen(str1)+strlen(str2)+1) should result in 13 (and it does if I check it on a different line), which would be the correct length.

I don't understand how new is getting 20, where I think it should be getting 13.

Also, regarding ++ on str1 and str2. I am reading that as a pointer to const char, which I think means I can move the pointer (which is what I am doing with the ++), but not change the char array pointed to. Please correct me if I am wrong in this line of thought.

Rob, thank you for the alternate method. I will explore it, but I am primarily interested in correcting my misunderstanding of new, since the next exercise is also failing in a smiliar manner.

 

[JasonUMD]

Rob,

The example you give does not compile.

C:\PROGRAM FILES\MICROSOFT VISUAL STUDIO\MyProjects\Test\test.cpp(30) : error C2664: 'sprintf' : cannot convert parameter 1 from 'const char *' to 'char *'
        Conversion loses qualifiers
C:\PROGRAM FILES\MICROSOFT VISUAL STUDIO\MyProjects\Test\test.cpp(34) : error C2440: '=' : cannot convert from 'const char *' to 'char *'
        Conversion loses qualifiers

 

[rjr9999]

I'm sorry, I was at work, quick example...here this will work.

char *cat(char *Abuf1, char *Abuf2)
{
char *catstr;

sprintf(Abuf1 + strlen(Abuf1), Abuf2);

catstr = Abuf1;

return catstr;

}

Make sure you declare
char *cat(const char *buf1, const char *buf2);
in your local functions.

You're welcome
Rob
"Programming is like art...It makes me feel like chopping my ear off."

Http://www.thisisrisk.com/

- Currently down