Programming Forums > Application Development > C++

keweedsmo · Mar 13th, 2006, 11:51 AM

how do I combine 2 char* into another?

char* s1;
char* s2;
char* s3;
char* s4
s1 = "Hi";
s2 = "Hello";
s3 = " ";
s4 = s1 + s3 + s2;

Output:
"Hi Hello"

Errors:
editorCore.cxx:143: invalid operands `char *' and `const char[2]' to binary `operator +
editorCore.cxx:147: invalid operands `char *' and `const char[2]' to binary `operator +
editorCore.cxx:150: invalid operands `char *' and `char *' to binary `operator +

I tried & and . operators also. they didn't work either.

jayme · Mar 13th, 2006, 12:14 PM

(Toggle Plain Text)

char* s1,s2,s3;
s1="Hi ";
s2="Hello";
strcat(s3,s1);
strcat(s3,s2);

this copies s1 and s2 into s3. You could simple create a function to make this easier though.

DaWei · Mar 13th, 2006, 12:28 PM

char* are pointer to an array of char. In C and C++, arrays are not objects in the sense that they are in some other languages. They are collections of objects (ints, char, whatever). Since they are collections of essentially unknown size, one can't just assign one of them to another. One must do it piecemeal. If one wants to define an object that contains an array of some sort, then one may populate the object with methods that will copy, assign, whatever. The construct, char *cPtr = "Abcd", is not really a language operation; it is a feature of the language compiler that eases your life while possibly confusing you unduly.

char*, while equivalent to an array of char in some respects, is not a complete equivalent. You will find differences explained in the "Pointer Basics" material referred to in my signature.

C++ has a string class that is much more intuitive to use; I recommend that you research it and, possibly, use it, rather than the C-style strings represented by a char array with a terminating '\0'.

keweedsmo · Mar 13th, 2006, 1:40 PM

(Toggle Plain Text)

int spw = 0;
char * stt;
char str[72];

...

strcpy(str, cur_Ptr->data);

...

stt = strtok (str," ");
cur_Ptr->data = stt;
stt = strtok (NULL, " ");
while (stt != NULL)
{
	cout << "STT: " << stt << endl;
	cout << "5" << endl;
	for(int j =0; j <= spw; j++)
	{
                         //This Line causes stt to become null.
		strcat(cur_Ptr->data, " ");

Why is the strcat making stt null?

DaWei · Mar 13th, 2006, 3:07 PM

Check your documentation for strtok. Your example code is incomplete, too. You're pushing things at us that we can't see, but you can. My crystal is in the shop for ball joints.

keweedsmo · Mar 13th, 2006, 4:34 PM

well those are the only lines that really relate to the code at hand, other than cur_Ptr. thats just a pointer to a node on a linked list. I know I can't strtok seperate strings at the same time within while loops, but I didn't think that would affect strcat...

i can print stt before the strcat, but after the strcat i get a seg fault.

The Dark · Mar 13th, 2006, 6:22 PM

strtok returns a pointer into your original string. The strcat function that you called adds a space and a zero-byte to the end of the string that is pointed to by its parameter. This will overwrite the value that stt is pointing at.

For example. If your original string is
A B C D

the first call to strtok will return a pointer to A and replace the space after A with a zero byte. This pointer is then stored in cur_Ptr->data.
The second call to strtok will return a pointer to B and replace the space after B with a zero byte. This pointer is then stored in stt.

when you call strcat(cur_Ptr->data, " "), the zero byte that was after A gets replaced with the space from the parameter and the B gets replaced with the zero byte to null terminate the string.

This means that the value which used to be B is now a zero byte.

Clear as mud!

CodeWeasel · Mar 15th, 2006, 2:28 AM

Just a heads up, a lot of the old C string manipulation functions like strcat and strtok are depricated now, with newer, "safer" versions out.

Soulstorm · Mar 15th, 2006, 2:53 AM

Quote:

Originally Posted by CodeWeasel

Just a heads up, a lot of the old C string manipulation functions like strcat and strtok are depricated now, with newer, "safer" versions out.

This may sound like a newbie's question, but I would like a reference to them, if you may...?

CodeWeasel · Mar 15th, 2006, 9:36 PM

Quote:

Originally Posted by Soulstorm

This may sound like a newbie's question, but I would like a reference to them, if you may...?

It's not that big a deal, old code still works like it used to, there are just new "secure" versions out there. Usually they have the same name, with a _s suffix after them. The old versions were found to be susceptible to buffer overruns. More info: http://msdn2.microsoft.com/en-US/library/8ef0s5kh.aspx

Mar 13th, 2006, 11:51 AM	#1
keweedsmo Newbie Join Date: Feb 2006 Location: Albany, NY (UALBANY) Posts: 20 Rep Power: 0	*is it possible to combine 2 char with an operator (like in other languages)** how do I combine 2 char* into another? (Toggle Plain Text) char* s1; char* s2; char* s3; char* s4 s1 = "Hi"; s2 = "Hello"; s3 = " "; s4 = s1 + s3 + s2; Output: "Hi Hello" char* s1; char* s2; char* s3; char* s4 s1 = "Hi"; s2 = "Hello"; s3 = " "; s4 = s1 + s3 + s2; Output: "Hi Hello" Errors: editorCore.cxx:143: invalid operands `char ' and `const char[2]' to binary `operator + editorCore.cxx:147: invalid operands `char ' and `const char[2]' to binary `operator + editorCore.cxx:150: invalid operands `char ' and `char ' to binary `operator + I tried & and . operators also. they didn't work either.

Mar 13th, 2006, 12:28 PM	#3
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	char* are pointer to an array of char. In C and C++, arrays are not objects in the sense that they are in some other languages. They are collections of objects (ints, char, whatever). Since they are collections of essentially unknown size, one can't just assign one of them to another. One must do it piecemeal. If one wants to define an object that contains an array of some sort, then one may populate the object with methods that will copy, assign, whatever. The construct, char cPtr = "Abcd", is not really a language operation; it is a feature of the language compiler that eases your life while possibly confusing you unduly. char, while equivalent to an array of char in some respects, is not a complete equivalent. You will find differences explained in the "Pointer Basics" material referred to in my signature. C++ has a string class that is much more intuitive to use; I recommend that you research it and, possibly, use it, rather than the C-style strings represented by a char array with a terminating '\0'. __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

Mar 13th, 2006, 1:40 PM	#4
keweedsmo Newbie Join Date: Feb 2006 Location: Albany, NY (UALBANY) Posts: 20 Rep Power: 0	(Toggle Plain Text) int spw = 0; char * stt; char str[72]; ... strcpy(str, cur_Ptr->data); ... stt = strtok (str," "); cur_Ptr->data = stt; stt = strtok (NULL, " "); while (stt != NULL) { cout << "STT: " << stt << endl; cout << "5" << endl; for(int j =0; j <= spw; j++) { //This Line causes stt to become null. strcat(cur_Ptr->data, " "); int spw = 0; char * stt; char str[72]; ... strcpy(str, cur_Ptr->data); ... stt = strtok (str," "); cur_Ptr->data = stt; stt = strtok (NULL, " "); while (stt != NULL) { cout << "STT: " << stt << endl; cout << "5" << endl; for(int j =0; j <= spw; j++) { //This Line causes stt to become null. strcat(cur_Ptr->data, " "); Why is the strcat making stt null?

Mar 13th, 2006, 3:07 PM	#5
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	Check your documentation for strtok. Your example code is incomplete, too. You're pushing things at us that we can't see, but you can. My crystal is in the shop for ball joints. __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

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Mar 13th, 2006, 4:34 PM	#6
keweedsmo Newbie Join Date: Feb 2006 Location: Albany, NY (UALBANY) Posts: 20 Rep Power: 0	well those are the only lines that really relate to the code at hand, other than cur_Ptr. thats just a pointer to a node on a linked list. I know I can't strtok seperate strings at the same time within while loops, but I didn't think that would affect strcat... i can print stt before the strcat, but after the strcat i get a seg fault.

Mar 13th, 2006, 6:22 PM	#7
The Dark Expert Programmer Join Date: Jun 2005 Posts: 824 Rep Power: 4	strtok returns a pointer into your original string. The strcat function that you called adds a space and a zero-byte to the end of the string that is pointed to by its parameter. This will overwrite the value that stt is pointing at. For example. If your original string is A B C D the first call to strtok will return a pointer to A and replace the space after A with a zero byte. This pointer is then stored in cur_Ptr->data. The second call to strtok will return a pointer to B and replace the space after B with a zero byte. This pointer is then stored in stt. when you call strcat(cur_Ptr->data, " "), the zero byte that was after A gets replaced with the space from the parameter and the B gets replaced with the zero byte to null terminate the string. This means that the value which used to be B is now a zero byte. Clear as mud!

Mar 15th, 2006, 2:28 AM	#8
CodeWeasel Newbie Join Date: Jul 2004 Location: Blacksburg, VA Posts: 13 Rep Power: 0	Just a heads up, a lot of the old C string manipulation functions like strcat and strtok are depricated now, with newer, "safer" versions out.

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Programming Forums > Application Development > C++

is it possible to combine 2 char* with an operator (like in other languages)