concat two char * variables

santhosh padamatinti

Hi to all, I have a small doubt.Is it possible to use arithmetic operators between two char* variables, like bellow

char\* conc(const char\* c1,const char\* c2)
{
 char\* c3=c1+c2     // error" adding two pointer variables is invalid   
 return c3
}

Above code is giving error "adding two pointer variables is invalid". Please tell me how to concat(add) two char*.

To invent something, you need a mountain of junk in your mind. ---------------------Thomas alva edison

Rajesh R Subramanian

You could use strcat()[^] to concatenate strings. Alternatively, you could use one of those string classes (such as CString) which will allow you to just use + (the addition operator) to add two strings.

“Follow your bliss.” – Joseph Campbell

santhosh padamatinti

Rajesh R Subramanian wrote:

You could use strcat()[^] to concatenate strings.

Actually my requirement is "not use the strcat() function".

To invent something, you need a mountain of junk in your mind. ---------------------Thomas alva edison

Fatbuddha 1

If you have to stick to the two pointer of char in order to concat them. You have to creat a new char * with the length of both. Then copy both to the one. I would suggest using stings. You could also create a string and use += to do it. I guess that is your homework, isn't it? Sounds like. Cheers

You have the thought that modern physics just relay on assumptions, that somehow depends on a smile of a cat, which isn’t there.( Albert Einstein)

Avi Berger

Note that in what you are doing, you are dealing with 6 variables. You are talking about the 3 char * variables, but as you said, those are pointers. What you must remember is that pointers are pointers - they are used to point to things, in this case arrays of char. These arrays are not automatically provided. You, the programmer, are responsible for providing them for the pointers to point to. You have to keep in mind the distinction between pointer and "pointee". Once you do that, you would not say that you want to add or concatenate 2 pointers. Instead, you want to concatenate the 2 strings contained in 2 char arrays and put the result in a third char array. You use the pointers to access the char arrays. Good luck

David Crow

sampath-padamatinti wrote:

return c3

Are you really wanting to return a variable that will go out of scope when conc() ends?

sampath-padamatinti wrote:

Please tell me how to concat(add) two char*.

Something like:

char *c3 = c1;

while (*c3)
c3++;

while (*c3++ = *c2++)
;

While not exact, this is roughly what strcat() does.

"One man's wage rise is another man's price increase." - Harold Wilson

"Fireproof doesn't mean the fire will never come. It means when the fire comes that you will be able to withstand it." - Michael Simmons

CPallini

sampath-padamatinti wrote:

Actually my requirement is "not use the strcat() function".

Why? :)

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler. -- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong. -- Iain Clarke
[My articles]

Luc Pattyn

Are you new around here? It probably is one of these: - I let others do my homework - I am troll - strcat() isn't safe enough; I prefer strcat_s() :)

Luc Pattyn [Forum Guidelines] [Why QA sucks] [My Articles]

I only read code that is properly formatted, adding PRE tags is the easiest way to obtain that.
[The QA section does it automatically now, I hope we soon get it on regular forums as well]

CPallini

Which one, sir? I suppose: strcat isn't safe enough, I prefer mess up with pointers myself. :-D

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler. -- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong. -- Iain Clarke
[My articles]

Lost User

DavidCrow wrote:

Something like: char *c3 = c1; while (*c3) c3++; while (*c3++ = *c2++) ;

:wtf:

MVP 2010 - are they mad?

santhosh padamatinti

My interviewer thrown a question like: He wants to implement a function where he needs to takes two char* parameters as input at the end it should return a char* by concatenating these two char* values(without using strcat(), strcmp() functions). required function prototype is

char\* concat(const char\* c1, const char\* c2)

I am unable to solve this one. Please give me the approach Thanx in advance.....

To invent something, you need a mountain of junk in your mind. ---------------------Thomas alva edison

modified on Thursday, January 21, 2010 2:21 AM

Fatbuddha 1

Yes, I was wondering too

You have the thought that modern physics just relay on assumptions, that somehow depends on a smile of a cat, which isn’t there.( Albert Einstein)

CPallini

Why wondering? It is concise and elegant, functionally equivalent to strcat, here the complete function

char * concat( char * c1, const char * c2)
{
char *c3 = c1;

while (*c3)
c3++;

while (*c3++ = *c2++)
;
return c1;
}

here a test program:

#include <stdio.h>
void main()
{
char buf[100];
sprintf(buf, "hello");
char * str = " folks!";
printf("%s\n", concat(buf, str));
}

He just missed the const reuqirement for the first argument. But this is an OP fault (i.e. David's prototype is better). :)

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler. -- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong. -- Iain Clarke
[My articles]

CPallini · modified on Thursday, January 21, 2010 2:21 AM

sampath-padamatinti wrote:

I am unable to solve this one. Please give me the approach

David did, here. However, if you're stuck with the const requirement on the first argument, then:

char * concat(const char * c1, const char * c2)
{
size_t size[2] = {strlen(c1), strlen(c2)};
char * c = new char[size[0]+size[1]+1];
strcpy(c, c1);
strcpy(c+size[0],c2);
return c;
}

if you can't use strlen and/or strcpy:

char * concat(const char * c1, const char * c2)
{
size_t len=0;
char * c;
const char *p;
p = c1;
while (*p++) len++;
p = c2;
while (*p++) len++;
c = new char[len+1];
p=c1;
while (*c=*p++) c++;
p=c2;
while (*c++=*p++) ;
return (c-len-1);
}

Test program:

void main()
{
const char * str1 = "hello, ";
const char * str2 = "folks!";
char * result = concat(str1, str2);
printf("%s\n", result);
delete [] result;
}

:)

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler. -- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong. -- Iain Clarke
[My articles]

Lost User

:laugh: :laugh:

MVP 2010 - are they mad?

CPallini

What's that funny, Rick? :)

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler. -- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong. -- Iain Clarke
[My articles]

Lost User

That code is too dangerous for words. As long as the length of buf is greater than the combined length of c1 and c2 plus a null terminator it will work. But as soon as the result overflows all hell breaks loose. This maybe OK for a skilled developer such as yourself, but I would not suggest it as a solution for a newbie.

MVP 2010 - are they mad?

CPallini

The code provide what provides strcat, no less no more. I know strcat is a very dangerous function... :rolleyes: God once said: "The newbies should learn C pointers or go to Hell managed". :-D Moreover, since that was an interview question (see [^]), the OP was expected to have such a skill. :) BTW My 5 for the 'skilled developer'. :laugh:

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler. -- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong. -- Iain Clarke
[My articles]

Lost User

:thumbsup:

MVP 2010 - are they mad?

Saravanan Sundaresan · modified on Thursday, January 21, 2010 2:21 AM

Here is the function as per your requirement:

char* concat(const char* c1, const char* c2)
{
int size1 = 0, size2 = 0;

for (int i = 0; c1\[i\] != '\\0'; i++)
	size1++;
for (int j = 0; c2\[j\] != '\\0'; j++)
	size2++;

char\* result = new char\[size1+size2\];

for(int i=0; i<size1; i++)
{
	result\[i\] = c1\[i\];
}

for(int j = 0; j <= size2; j++)
{
	result\[size1+j\] = c2\[j\];
}

result\[size1+size2\] = 0;

return result;

}