Replacing a word in a string

[jacob navia]

Hi guys!

I like C because is fun. So, I wrote this function for the lcc-win32
standard library: strrepl.

I thought that with so many "C heads" around, maybe we could improve it
in a collective brainstorming session.

Let's discuss some C here, for a change :-)

Specs:
-----
Function: strrepl

Synopsis

#include <string.h>
char *strrepl(char *InputString, char *StringToFind,
char *Replacement, char *Output);
Description

The strrepl function replaces in InputString all occurrences of
StringToFind by Replacement, writing the modified contents into the
Output string. The original input string is not modified.

If the Output argument is NULL, strrepl will return the space that would
be needed (including the terminating zero) for the replacement.

If Replacement is NULL and Output is not NULL, all occurrences of
StringToFind will be erased.

Returns

The strrepl function returns the needed length for the replacements if
its Output argument is NULL. If not, it returns the number of
replacements done.

Code:
-----
#include <string.h>
int strrepl(char *InputString,ch ar *StringToFind,
char *StringToReplac e,char *output)
{
char *offset = NULL, *CurrentPointer = NULL;
int insertlen;
int findlen = strlen(StringTo Find);
int result = 0;

if (StringToReplac e)
insertlen = strlen(StringTo Replace);
else
insertlen = 0;
if (output) {
if (output != InputString)
memmove(output, InputString,str len(InputString )+1);
InputString = output;
}
else
result = strlen(InputStr ing)+1;

while (*InputString) {
offset = strstr(!offset? InputString:Cur rentPointer,Str ingToFind);
if (offset == NULL)
break;
CurrentPointer = (offset + (output ? insertlen : findlen));
if (output) {
strcpy (offset, (offset + findlen));
memmove (offset + insertlen,
offset, strlen (offset) + 1);
if (insertlen)
memcpy (offset, StringToReplace , insertlen);
result++;
}
else {
result -= findlen;
result += insertlen;
}
}
return result;
}

All kinds of comments are welcome, regarding the code, the interface
design, the documentation, etc etc.

jacob

[Eric Sosman]

jacob navia wrote On 12/06/05 15:41,:

Synopsis

#include <string.h>
char *strrepl(char *InputString, char *StringToFind,
char *Replacement, char *Output);
[...]

Code:
-----
#include <string.h>
int strrepl(char *InputString,ch ar *StringToFind,
char *StringToReplac e,char *output)

A diagnostic is required if both the declaration
and the definition appear in the same translation unit.
(In C, anyhow. That other language you're fond of may
have different rules.)

--
Er*********@sun .com

[Michael Mair]

jacob navia wrote:

Hi guys!

I like C because is fun. So, I wrote this function for the lcc-win32
standard library: strrepl.

I thought that with so many "C heads" around, maybe we could improve it
in a collective brainstorming session.

Let's discuss some C here, for a change :-)

Specs:
-----
Function: strrepl

Synopsis

#include <string.h>
char *strrepl(char *InputString, char *StringToFind, ^^^^^^
ITYM: int or size_t char *Replacement, char *Output);
In order to avoid name clashes, I'd rather call the function
str_repl or strRepl or replaceSubstrin g.
Description

The strrepl function replaces in InputString all occurrences of
StringToFind by Replacement, writing the modified contents into the
Output string. The original input string is not modified.
Then your prototype is wrong: You want
size_t str_repl(const char* restrict InputString,
const char* StringToFind,
const char* Replacement,
char* restrict Output);
to avoid this. Or do away with the restrict if you want
to express that the original string is not modified via
InputString.
If the Output argument is NULL, strrepl will return the space that would
be needed (including the terminating zero) for the replacement.

If Replacement is NULL and Output is not NULL, all occurrences of
StringToFind will be erased.

Returns

The strrepl function returns the needed length for the replacements if
its Output argument is NULL. If not, it returns the number of
replacements done.
I would rather like a size parameter as in snprintf() so you
can give the size of the storage output points to.
<snip: Code>
All kinds of comments are welcome, regarding the code, the interface
design, the documentation, etc etc.

- I would rather aim for a standard order of the parameters,
say leading
destination, size,
followed by either
1) replacestr, source, findstr
or
2) source, replacestr, findstr

1) has the advantage that all parameters which can have special
values are gathered to the start.

- I would rather return size_t as strlen() returns size_t

- I would not "overload" the return value semantics depending
on destination -- the user cannot detect errors this way.
I'd rather settle for returning the number of bytes needed for
the "fully replaced" destination string and returning 0 on
error.
If the number of replacements is needed, use an additional
size_t* parameter.

- give a str_n_repl variant allowing to restrict the number of
replacements.
Cheers
Michael
--
E-Mail: Mine is an /at/ gmx /dot/ de address.

[Ben Pfaff]

Eric Sosman <er*********@su n.com> writes:

jacob navia wrote On 12/06/05 15:41,:

#include <string.h>
char *strrepl(char *InputString, char *StringToFind,
char *Replacement, char *Output);

[...]

int strrepl(char *InputString,ch ar *StringToFind,
char *StringToReplac e,char *output)

A diagnostic is required if both the declaration
and the definition appear in the same translation unit.

What? Have you never heard of a "function prototype"? They've
been all the rage since 1989 or so. Even before that, function
declarations before definitions were, I believe, allowed.

The real problem here is the differing return types.
--
char a[]="\n .CJacehknorstu" ;int putchar(int);in t main(void){unsi gned long b[]
={0x67dffdff,0x 9aa9aa6a,0xa77f fda9,0x7da6aa6a ,0xa67f6aaa,0xa a9aa9f6,0x11f6} ,*p
=b,i=24;for(;p+ =!*p;*p/=4)switch(0[p]&3)case 0:{return 0;for(p--;i--;i--)case+
2:{i++;if(i)bre ak;else default:continu e;if(0)case 1:putchar(a[i&15]);break;}}}

[Robert Gamble]

Ben Pfaff wrote:

Eric Sosman <er*********@su n.com> writes:

jacob navia wrote On 12/06/05 15:41,:

#include <string.h>
char *strrepl(char *InputString, char *StringToFind,
char *Replacement, char *Output);

[...]

int strrepl(char *InputString,ch ar *StringToFind,
char *StringToReplac e,char *output)

A diagnostic is required if both the declaration
and the definition appear in the same translation unit.

What? Have you never heard of a "function prototype"? They've
been all the rage since 1989 or so. Even before that, function
declarations before definitions were, I believe, allowed.

The real problem here is the differing return types.

I think it is pretty safe to assume that he was referring to the
declaration and definition that the OP presented.

Robert Gamble

[buda]

"Michael Mair" <Mi**********@i nvalid.invalid> wrote in message
news:3v******** *****@individua l.net...

jacob navia wrote:

If the Output argument is NULL, strrepl will return the space that would
be needed (including the terminating zero) for the replacement.

If Replacement is NULL and Output is not NULL, all occurrences of
StringToFind will be erased.

Returns

The strrepl function returns the needed length for the replacements if
its Output argument is NULL. If not, it returns the number of
replacements done.

I would rather like a size parameter as in snprintf() so you
can give the size of the storage output points to.

I think that wouldn't necessarily make things any better. As I imagine, you
would rarely know how much storage could be needed by the function in
advance without calling it first with NULL as the Output argument. Also, if
you knew that, for example, strlen(StringTo Find)>=strlen(R eplacement) then
passing the size of storage that Output points to is pointless (since you
know how much you need, you just allocate it and that's it). While I like
the overall idea, I find this size issue very inpractical and error prone (I
imagine this is one of the reasons something like this isn't in the standard
library in the first place - but there are probably many reasons far more
important than this). It might be better for str_repl to allocate the needed
space and return a pointer to it (with the proper output written to it
ofcourse).

[jacob navia]

jacob navia wrote:

char *strrepl(char *InputString, char *StringToFind,
char *Replacement, char *Output);

OOOOPS
That should have been:

int strrepl (...)
^^^
of course!

Excuse me for this error.

[jacob navia]

Robert Gamble wrote:

I think it is pretty safe to assume that he was referring to the
declaration and definition that the OP presented.

Robert Gamble

Yes, I think Eric was misled by a confusion in the declaration:
it is int strrepl, not char *strrepl, as I mistakenly wrote.

Excuse me for this error.
jacob

[jacob navia]

Michael Mair wrote:

jacob navia wrote:

Hi guys!

I like C because is fun. So, I wrote this function for the lcc-win32
standard library: strrepl.

I thought that with so many "C heads" around, maybe we could improve it
in a collective brainstorming session.

Let's discuss some C here, for a change :-)

Specs:
-----
Function: strrepl

Synopsis

#include <string.h>
char *strrepl(char *InputString, char *StringToFind,
^^^^^^
ITYM: int or size_t

char *Replacement, char *Output);

In order to avoid name clashes, I'd rather call the function
str_repl or strRepl or replaceSubstrin g.

Description

The strrepl function replaces in InputString all occurrences of
StringToFind by Replacement, writing the modified contents into the
Output string. The original input string is not modified.

Then your prototype is wrong: You want
size_t str_repl(const char* restrict InputString,
const char* StringToFind,
const char* Replacement,
char* restrict Output);
to avoid this. Or do away with the restrict if you want
to express that the original string is not modified via
InputString.

If the Output argument is NULL, strrepl will return the space that
would be needed (including the terminating zero) for the replacement.

If Replacement is NULL and Output is not NULL, all occurrences of
StringToFind will be erased.

Returns

The strrepl function returns the needed length for the replacements if
its Output argument is NULL. If not, it returns the number of
replacements done.

I would rather like a size parameter as in snprintf() so you
can give the size of the storage output points to.

<snip: Code>

All kinds of comments are welcome, regarding the code, the interface
design, the documentation, etc etc.

- I would rather aim for a standard order of the parameters,
say leading
destination, size,
followed by either
1) replacestr, source, findstr
or
2) source, replacestr, findstr

1) has the advantage that all parameters which can have special
values are gathered to the start.

- I would rather return size_t as strlen() returns size_t

Yes, this is a good remark.

- I would not "overload" the return value semantics depending
on destination -- the user cannot detect errors this way.
I'd rather settle for returning the number of bytes needed for
the "fully replaced" destination string and returning 0 on
error.
Yes, but how would the user know how much to allocate?
The problem is that the user can't know how long the result string
will be.
If the number of replacements is needed, use an additional
size_t* parameter.
I think the number of replacements is only useful for knowing
if there was any replaceements at all, i.e; != 0

- give a str_n_repl variant allowing to restrict the number of
replacements.
Cheers
Michael

Thanks for your input.

[Flash Gordon]

jacob navia wrote:

Michael Mair wrote:

jacob navia wrote:
<snip string replacement function>
- I would not "overload" the return value semantics depending
on destination -- the user cannot detect errors this way.
I'd rather settle for returning the number of bytes needed for
the "fully replaced" destination string and returning 0 on
error.

Yes, but how would the user know how much to allocate?
The problem is that the user can't know how long the result string
will be.

The user can always know the upper bound by knowing the length of the
input string, length of search string and length of replacement string
then assuming if the replacement is longer than the search that the
input string consists entirely of strings to be replaced.

If the number of replacements is needed, use an additional
size_t* parameter.

I think the number of replacements is only useful for knowing
if there was any replaceements at all, i.e; != 0

A counter example: implementing a text editor doing a search and replace
that tells the user how many instances have been replaced. I've used
such text editors and it is a nice thing to see.

I agree with Michael that the string length and count of items replaced
should be seperate returned values.

- give a str_n_repl variant allowing to restrict the number of
replacements.

The main use I see for that is to replace only the first instance.

I think being able to specify the buffer size would be good.
--
Flash Gordon
Living in interesting times.
Although my email address says spam, it is real and I read it.