memory access error in some simple code :)

[Pavel Sorokin]

Hello,

This is probably a very dumb question, but I don't understand what's
the problem with the following code. It gives a memory access
violation on the marked line. I'm using VC++ 7.1.

void reverse(char* str, int start, int end)
{
for(int i=start; i<(start+end)/2; i++)
{
char buf = str;
int endIndex = end-i;
str = str[endIndex]; // <-- this causes an error
str[endIndex] = buf;
}
}

Thanks,
Pavel

 

[Artie Gold]

Hello,

This is probably a very dumb question, but I don't understand what's
the problem with the following code. It gives a memory access
violation on the marked line. I'm using VC++ 7.1.

void reverse(char* str, int start, int end)
{
for(int i=start; i<(start+end)/2; i++)
{
char buf = str;
int endIndex = end-i;
str = str[endIndex]; // <-- this causes an error
str[endIndex] = buf;
}
}

The code itself -- while it could be improved -- is fine. The problem
(he said, turning up the sensitivity of his crystal ball) is the way
you're calling the function.

No doubt you're doing something like:

char * something = "reverse me";
reverse(something);

In this case you're trying to alter the value of a string literal (which
should more properly be defined as a `const char *') which is not
permissable.

HTH,
--ag

 

[John Harrison]

Hello,

This is probably a very dumb question, but I don't understand what's
the problem with the following code. It gives a memory access
violation on the marked line. I'm using VC++ 7.1.

void reverse(char* str, int start, int end)
{
for(int i=start; i<(start+end)/2; i++)
{
char buf = str;
int endIndex = end-i;
str = str[endIndex]; // <-- this causes an error
str[endIndex] = buf;
}
}

Thanks,
Pavel

Could be anything, str could be NULL, str could be garbage, end could be
too big, end could be negative, start could be too big, start could be
negative.

Either check the values for yourself, or post a complete program. There
isn't enough information to solve your problem in what you have posted.

john

 

[Anil Mamede]

Hello,

This is probably a very dumb question, but I don't understand what's
the problem with the following code. It gives a memory access
violation on the marked line. I'm using VC++ 7.1.

void reverse(char* str, int start, int end)
{
for(int i=start; i<(start+end)/2; i++)
{
char buf = str;
int endIndex = end-i;
str = str[endIndex]; // <-- this causes an error
str[endIndex] = buf;
}
}

Thanks,
Pavel

Maybe endIndex is bigger that the char* length. Try to use the debugger
step by step and watch the endIndex value.


BTW,

char string[40] = "Hello World";

Suppose reserve(string, 4, 7);

In the first iteration:
i = start = 4
end = 7
endIndex = 7 - 4 = 3

buf = str[4] = 'o'
char[3] = 'l'
str = "Helol World";

Next iteration:

i = start = 5
end = 7
endIndex = 7 - 5 = 2

buf = str[5] = ' '
char[2] = 'l'
str = "He ollWorld";

You want that result above or this:

"HelloW oorld"?


Anil Mamede

 

[Pavel Sorokin]

Hi Artie,

You were right I already found the problem. It works however, when
I replace the string definition with
char something[] = "reverse me";
Then the string can be modified. Here's a description of this issue:
http://www.eskimo.com/~scs/C-faq/q1.32.html

Pavel

 

[Pavel Sorokin]

Hi Artie,

You were right I already found the problem. It works however, when
I replace the string definition with
char something[] = "reverse me";
Then the string can be modified. Here's a description of this issue:
http://www.eskimo.com/~scs/C-faq/q1.32.html

Pavel