tempest

It seems like this shoud be very basic, what i'm trying to do is this...

if i have str:
char *str = "Hello, world!";

and i want to change the first H to an h for example, how could that be done?

I've come up with this solution after lots of frustration, but it seems like there should be an easier way to accomplish this.

I have tried memset, etc, always gives a seg fault...

[php]
/*
Output:

*ello, world!
H*llo, world!
He*lo, world!
Hel*o, world!
Hell*, world!
Hello* world!
Hello,*world!
Hello, *orld!
Hello, w*rld!
Hello, wo*ld!
Hello, wor*d!
Hello, worl*!
Hello, world*

*/
#include <stdio.h>
#include <stdlib.h>

char *strchr_rep(char *buf, char ch, int pos) {
int i;
char tmp[2] = " ",
*nbuf = (char *)malloc(strlen(buf));

if(pos >= strlen(buf) || pos < 0)
return NULL;

for(i=0;i<pos;i++) {
tmp[0] = buf[i];
strcat(nbuf, tmp);
}

tmp[0] = ch;

if(pos == 0)
strcpy(nbuf, tmp);
else
strcat(nbuf, tmp);

for(i=pos+1;i<strlen(buf);i++) {
tmp[0] = buf[i];
strcat(nbuf, tmp);
}

return nbuf;
}

int main(int argc, char* argv) {
char *str = "Hello, world!", i;

for(i=0;i<strlen(str);i++)
printf("%s\n", strchr_rep(str, '*', i));

return EXIT_SUCCESS;
}
[/php]

__________________

OpenLoop

why not use a char array instead of a char pointer? It makes it easier to manipulate single letters.

tempest

flexibility...

__________________

stevengs

I believe "Hello World!" is a string constant in this sense.

__________________
-Steven
"Is this a piece of your brain?" - Basil Fawlty

jim mcnamara

Back to basics.
Some parts of memory allocated to a program allow read/write. Othere segments allow read-only.

Constant value like live in read only memory.
Segfaults occurs when you try to write to read only memory.

therefore:

DaWei

You can let the compiler handle it for you, in this way:

char * aString = "Hello, World"; // will put the string in write protected (usually) memory, seg faultitis

char bString [] = "Hello, World"; // will put the string in local memory, writes fine.

The problem with the second method is the compiler will allot just enough to hold the initializing string, plus the terminator. Overwriting it with a longer string will lead to trouble (destroy something else, possibly with drastic consequences).

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tempest

read-only data would explain it, guess i read over that part..

__________________

aznluvsmc

Sorry to sound like a newbie but why is the string read-only? I thought that you had to use the const modifier to make a variable read-only.


Can someone explain this to me?

Polyphemus_

"Hello World" is contained in the read-only part of the program in memory. It doesn't matter you use const or not, since you will only make the pointer to the string constant, but not the string itself.

DaWei

The const modifier only tells the compiler that it shouldn't allow you to write code that modifies that value. It's a convention, an agreement between you and the compiler that you are a screw-up, and the compiler is allowed to warn you when you do it. Declaring something const does NOT make it actually write protected (normally; one could build a system that did that, in cooperation with the compiler).

In Tempest's case, he is on an OS that has actually flagged that memory as read-only. Trying to write to it causes a system fault. Had he realized that many modern systems would do that, and realized that it WAS write protected, he could have declared it const so that the compiler would remind him if he forgot. That isn't germane. It isn't that he forgot, it's merely that he was unaware of the restrictions imposed on him by his particular implementation. A declaration like that is not, defacto, write protected. It just is in many implementations.

__________________
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