struggling with strings

[tuchka]

Hi guys!
I am trying to learn C programming but my java background blocks my
brains.
I have a book with a lot of functions and some of them don't work
correctly, so I have opportunity to exercise in C by correction
(strange, huh?).
For example, i want to write function that removes all occurences of
given substring from string.
I have two functions so far:

1. removes first substring from the string (i made it after
corrections of the function that i had in the horrible book:

char *strstr_rem_first(char *string, char *substring) {

int i, j, k, loc=-1;

for(i=0; string && (loc==-1); i++) {
for(j=i, k=0; string[j] ==substring[k];j++, k++)
if(!substring[k+1])
loc=i;
if(loc != -1) { //substring was found
for(k=0; substring[k];k++); {
for(j=loc, i=loc+k; string; j++, i++) {
string[j] = string;
string='\0';
}
}
}
}
return (string);
}
2. finds rightmost index of the substring (returns -1 if not found):

int substring_index(const char *string, const char *substring) {

int i, j, k;
for(i=0; string;i++)
for(j=i,k=0; string[j] == substring[k]; j++, k++)
if(!substring[k+1])
return(i);

return(-1);
}

Now, armed with these I wanted to construct function that removes all
substrings.

I though about somthing like this:

while(substring_index() > 0)
string_original becomes substring_rem_first

return string_original

Unfortunately, because strings are treated as arrays and i can't
transfer references as easily as i got used to, i can't figure out how
to accomplish it in C.
I shuffled some pointers and some strcpy() etc in vain.
Especially this 'string_original becomes substring_rem_first' bothers
me.
I always end up with 'undefined' reference to 'substring_rem_first'.

Could you help me out?

 

[Ekkehard Morgenstern]

Hi tuchka,

1. removes first substring from the string (i made it after
corrections of the function that i had in the horrible book:
2. finds rightmost index of the substring (returns -1 if not found):

Check out the functions declared in "string.h".

Here's a function to remove all occurrences of "substring" from "string":

#include <string.h>

char* strstr_rem_all( char* string, const char* substring ) {
char* occurrence; int substr_len = strlen( substring ); int move_len =
strlen( string ) - substr_len + 1; /* +1 b/c of terminating '\0' */
while ( occurrence = strstr( string, substring ) ) {
memmove( occurrence, string + substr_len, move_len );
}
return string;
}

memmove() is used instead of memcpy(), because it handles overlapping
copies.

I hope that helps!

Regards,
Ekkehard Morgenstern.

 

[tuchka]

Thank you, Ekkehard,
I see how that function could've work but i still cannot get it to
work.
The function as written by you gives the following error:

C:\Sveta\C_PROG~1\STRING~1>str_test
123fart456fart789fart

Reverse traf987traf654traf321 is traf987traf654traf321
Filled with x became xxxxxxxxxxxxxxxxxxxxx
After removing first fart became 123456fart789fart
After removing all fart became
123art456rt456rt456rt456rt456rt456rt456rt?

Exiting due to signal SIGSEGV
General Protection Fault at eip=0000772e
eax=00000000 ebx=00000000 ecx=ffffffff edx=ffffffff esi=ffffffff
edi=ffffffff
ebp=00090390 esp=0008ef6c
program=C:\SVETA\C_PROG~1\STRING~1\STR_TEST.EXE
cs: sel=01a7 base=029a0000 limit=0009ffff
ds: sel=01af base=029a0000 limit=0009ffff
es: sel=01af base=029a0000 limit=0009ffff
fs: sel=017f base=00005a50 limit=0000ffff
gs: sel=01bf base=00000000 limit=0010ffff
ss: sel=01af base=029a0000 limit=0009ffff
App stack: [00090560..00010560] Exceptn stack: [000104c0..0000e580]

Call frame traceback EIPs:
0x0000772e
0x00003946
0x00001b82
0x000033d8

I re-wrote it like this but again i've got 'undefined reference' for
str_rem_first(string,substring):

char *strstr_rem_all(char *string, const char *substring) {
//char occurence[strlen(string)];
char *ptr;
int substr_len=strlen(substring);
int move_len=strlen(string)-substr_len+1;

while(ptr=strstr(string, substring)) {
char occurence[strlen(string)];
memmove(occurence, str_rem_first(string,substring),
strlen(string)-substr_len+1);
}
return string;
}

Sigh, sigh :-(
Why the lang. is so hard?

 

[Anupam]

Hi tuchka,



Check out the functions declared in "string.h".

Here's a function to remove all occurrences of "substring" from "string":

#include <string.h>

char* strstr_rem_all( char* string, const char* substring ) {
char* occurrence; int substr_len = strlen( substring ); int move_len =
strlen( string ) - substr_len + 1; /* +1 b/c of terminating '\0' */
while ( occurrence = strstr( string, substring ) ) {
memmove( occurrence, string + substr_len, move_len );
}
return string;
}

memmove() is used instead of memcpy(), because it handles overlapping
copies.

Isn't there something a bit weird here.
Be careful abt how much you are moving ... the length of the initial
string - that of the substring +1 . This does not change inside the
loop but each time the string gets shorter.

What about this ? It should work.

char* strstr_rem_all( char* string, const char* substring ) {
char* occurrence;
int substr_len = strlen( substring );
int move_len;
while ( occurrence = strstr( string, substring ) ) {
move_len = strlen( occurrence ) - substr_len + 1;
memmove( occurrence, occurrence + substr_len, move_len );
}
return string;
}

 

[Ekkehard Morgenstern]

Hi tuchka,

I didn't test my code before posting it, because I assumed you'd be having
some thought about it before using it, but yeah, there's an obvious bug in
it:


Instead of "memmove( occurrence, string + substr_len, move_len )", it should
read "memmove( occurrence, occurrence + substr_len, move_len )", of course.


Regards,
Ekkehard Morgenstern.

 

[Ekkehard Morgenstern]

Hi tuchka,

Sorry this didn't work either. ;-)

Instead of "memmove( occurrence, string + substr_len, move_len )", it should
read "memmove( occurrence, occurrence + substr_len, move_len )", of

course.

Anupam was right, I moved too much.

Here's a complete test program that works:

#include <stdio.h>
#include <string.h>

char* strstr_rem_all( char* string, const char* substring ) {
char* occurrence; int substr_len = strlen( substring );
while ( occurrence = strstr( string, substring ) ) {
int move_len = strlen( occurrence ) - substr_len + 1; /*
+1 b/c of terminating '\0' */
memmove( occurrence, occurrence + substr_len, move_len );
}
return string;
}

int main( int argc, char** argv ) {
if ( argc >= 3 ) {
printf( "string = '%s', substr = '%s'\n", argv[1],
argv[2] );
strstr_rem_all( argv[1], argv[2] );
printf( "result = '%s'\n", argv[1] );
}
return 0;
}

Notice that the first argument in strstr_rem_all() must point to modifyable
memory, like a string buffer like argv[1]. If you let it point to a constant
(string literal), you will get an access violation error.

I hope this helped!

Regards,
Ekkehard Morgenstern.

 

[Ekkehard Morgenstern]

Hi Anupam,

char* strstr_rem_all( char* string, const char* substring ) {
char* occurrence;
int substr_len = strlen( substring );
int move_len;
while ( occurrence = strstr( string, substring ) ) {
move_len = strlen( occurrence ) - substr_len + 1;
memmove( occurrence, occurrence + substr_len, move_len );
}
return string;
}

Yup, you're right!

I didn't test the code beforehand, and it was late! I posted a complete test
program now for tuchka in the other branch of the message tree.

Coincidentially we have the same solution, I guess there's not many ways to
do it right! ;-)

Regards,
Ekkehard Morgenstern.

 

[tuchka]

Thank you very much Ekkehard and Anupam! It works!
I see that I need to get better book. When I tried to re-write
'memmove'
function (i looked it up in internet) for some reason i did not notice
that type of
arguments were 'void'. I thought it arrays. 'void' does not make much
sense to me. I am lacking
essential information, i see that much. One last small question.

When i place the function directly into the program and call it from
'main' it works like a charm,
but when i put it into 'library' file (that is with asossiated header
file) and call it from test
program it gives the following:

////////////////////////////////////////////////////

After removing first fart became 123456fart678fart
After removing all fart became 123456678
Exiting due to signal SIGSEGV
General Protection Fault at eip=0000767e
eax=00000000 ebx=00000000 ecx=ffffffff edx=ffffffff esi=ffffffff
edi=ffffffff
ebp=00090390 esp=0008ef6c
program=C:\SVETA\C_PROG~1\STRING~1\STR_TEST.EXE
cs: sel=01a7 base=029a0000 limit=0009ffff
ds: sel=01af base=029a0000 limit=0009ffff
es: sel=01af base=029a0000 limit=0009ffff
fs: sel=017f base=00005a50 limit=0000ffff
gs: sel=01bf base=00000000 limit=0010ffff
ss: sel=01af base=029a0000 limit=0009ffff
App stack: [00090560..00010560] Exceptn stack: [000104c0..0000e580]

Call frame traceback EIPs:
0x0000767e
0x00003896
0x00001942
0x000033d8
////////////////////////////////////////////////////

I'm using gcc compiler and link like this:
gcc -c -g strutil.c

gcc -c -g str_test.c

gcc -o str_test.exe str_test.o strutil.o


What it can be? Maybe my compiler or my computer?
Do you have any ideas?

Thanks once again for your help. I'm going for a new book right away.

 

[Ekkehard Morgenstern]

Hi tuchka,

When i place the function directly into the program and call it from
'main' it works like a charm,
but when i put it into 'library' file (that is with asossiated header
file) and call it from test
program it gives the following:

I don't know what you're doing, but it could be that you used a string
literal for both parameters like:

strstr_rem_all( "123123abc123abc", "abc" );

This will cause an access violation, since the function assumes that the
first parameter is a buffer, not a constant.

It will be better to declare the function such that it returns no pointer.
This way you'll have to provide a buffer to get the result:

void strstr_rem_all( char* string, const char* substring ) {
char* occurrence; int substr_len = strlen( substring );
while ( occurrence = strstr( string, substring ) ) {
int move_len = strlen( occurrence ) - substr_len + 1; /* +1 b/c
of terminating '\0' */
memmove( occurrence, occurrence + substr_len, move_len );
}
}

btw, the "void*" in memmove() and memcpy() mean that these parameters can be
assigned any pointer value. Note that "void" is not the same as "void*".

I hope this helps.

Regards,
Ekkehard Morgenstern.

 

[tuchka]

Thank you, Ekkehard,
i see that i need to read regular text book not a 'bible' that i have,
to understand many concepts clearly. All my troubles are originating
from the fact that i don't know what i'm doing I just bought two
books for beginners and going to study it.
Your input was most helpful.

Cheers,
tuchka

 

[Anupam]

Thank you very much Ekkehard and Anupam! It works!
I see that I need to get better book. When I tried to re-write
'memmove'
function (i looked it up in internet) for some reason i did not notice
that type of
arguments were 'void'. I thought it arrays. 'void' does not make much
sense to me. I am lacking
essential information, i see that much. One last small question.

When i place the function directly into the program and call it from
'main' it works like a charm,
but when i put it into 'library' file (that is with asossiated header
file) and call it from test
program it gives the following:

////////////////////////////////////////////////////

After removing first fart became 123456fart678fart
After removing all fart became 123456678
Exiting due to signal SIGSEGV
General Protection Fault at eip=0000767e
eax=00000000 ebx=00000000 ecx=ffffffff edx=ffffffff esi=ffffffff
edi=ffffffff
ebp=00090390 esp=0008ef6c
program=C:\SVETA\C_PROG~1\STRING~1\STR_TEST.EXE
cs: sel=01a7 base=029a0000 limit=0009ffff
ds: sel=01af base=029a0000 limit=0009ffff
es: sel=01af base=029a0000 limit=0009ffff
fs: sel=017f base=00005a50 limit=0000ffff
gs: sel=01bf base=00000000 limit=0010ffff
ss: sel=01af base=029a0000 limit=0009ffff
App stack: [00090560..00010560] Exceptn stack: [000104c0..0000e580]

Call frame traceback EIPs:
0x0000767e
0x00003896
0x00001942
0x000033d8
////////////////////////////////////////////////////

I'm using gcc compiler and link like this:
gcc -c -g strutil.c

gcc -c -g str_test.c

gcc -o str_test.exe str_test.o strutil.o


What it can be? Maybe my compiler or my computer?

There is *always* only one problem and that's incorrect coding. It is
tempting to think that we have discovered a major bug... but in
99.967% (there's a nice signature floating around to corroborate this)
of the cases, the fault lies with us.

Interesting . Could you give us the code for both the files. It would
be helpful if we had solid code at hand. I would not like to comment
before seeing that.

Do you have any ideas?

Thanks once again for your help. I'm going for a new book right away.

Glad to be of help. Please do use the services of this newsgroup
for clearing up your doubts. I would also like to suggest that you
could try using the Turbo C IDE. I had grown up with it and have
always loved the excellent help system. It is very simple , not hi-fi
, but it gives one a beautiful test bed to program. Of course I was
initially trapped into thinking that it was C and only later realised
that C is much bigger. No doubt there'll be many people with wider
experience who can suggest better ones. As far as books go I'll just
keep mum. I have learnt that it can be very dicey to suggest a book...
It is a matter of much sensitivity. K&R would be recommended though
at least at some point in the learning curve.

Regards,
Anupam

 

[Anupam]

[snip]
Here's a great list of books mainatined by Richard Heathfield.
http://users.powernet.co.uk/eton/clc/cbooks.html
While you are at it, one of the good places to look for information is
right at your fingertips...literally... look at the signatures on clc.
You are invariably led to the *good* places.
Best of luck,
Anupam

 

[tuchka]

Hi Anupam:
Please take a look at my files. I wonder if you find the cause of this
exception. It would be very interesting. I checked once again
everything (i even found out that i forgot to include strstr_rem_all()
into header!) but the message still appears after execution of
str_test.exe that i am getting by the following commands:
C:\C_PROG~1\STRING~1>gcc -c -g strutil.c

C:\C_PROG~1\STRING~1>gcc -c -g str_test.c

C:\C_PROG~1\STRING~1>gcc -o str_test.exe str_test.o strutil.o

C:\C_PROG~1\STRING~1>str_test
123fart456fart789fart

The following are my files:
--------------------------------------------------------
str_test2.c (as you guys suggested is working very well)
--------------------------------------------------------

#include <stdio.h>
#include <string.h>

char* strstr_rem_all( char* string, const char* substring ) {
char* occurrence; int substr_len = strlen( substring );
while ( occurrence = strstr( string, substring ) ) {
int move_len = strlen( occurrence ) - substr_len + 1;
/*
+1 b/c of terminating '\0' */
memmove( occurrence, occurrence + substr_len, move_len
);
}
return string;
}

int main( int argc, char** argv ) {
if ( argc >= 3 ) {
printf( "string = '%s', substr = '%s'\n", argv[1],
argv[2] );
strstr_rem_all( argv[1], argv[2] );
printf( "result = '%s'\n", argv[1] );
}
return 0;
}
------------------------------------------------------------------------
strutil.h (i am making to create my library)
----------------------------------------------------------------------
/*strutil.h*/

extern char *strrev(char *string); //return reversed string
extern char *strset(char *string, int letter);
extern char *strstr_rem_first(char *string, char *substring);
extern int substring_index(const char *string, const char *substring);
extern char *strstr_rem_all( char *string, const char *substring );
-----------------------------------------------------------------------
strutil.c (contains my functions)
-----------------------------------------------------------------------
#include <string.h>
#include "strutil.h"

//reverse
char *strrev(char *string) {
char *backward = string;
char *forward = string;
char temp;
//set 'backward' to the null byte terminating the string
while(*backward){
backward++;
}
while(forward<backward) {
--backward;
temp=*backward;
*backward=*forward;
*forward=temp;
++forward;
}
return (string);
}
//setting to char
char *strset(char *string, int letter) {
char *original=string;

while(*string) {
*string++ = letter;
}
return (original);
}
//finds rightmost index of substr
int substring_index(const char *string, const char *substring) {
int i, j, k;

for(i=0; string;i++)
for(j=i,k=0; string[j] == substring[k]; j++, k++)
if(!substring[k+1])
return(i);
return(-1);
}
//removes first sbstr
char *strstr_rem_first(char *string, char *substring) {
int i, j, k, loc=-1;

for(i=0; string && (loc==-1); i++) {
for(j=i, k=0; string[j] ==substring[k];j++, k++)
if(!substring[k+1])
loc=i;
if(loc != -1) { //substring was found
for(k=0; substring[k];k++); {
for(j=loc, i=loc+k; string; j++, i++) {
string[j] = string;
string='\0';
}
}
}
}
return (string);
}
//removes all substr
char *strstr_rem_all( char *string, const char *substring ) {
char *occurrence; int substr_len = strlen( substring );
while (occurrence = strstr( string, substring ) ) {
int move_len = strlen( occurrence ) - substr_len + 1;
memmove( occurrence, occurrence + substr_len, move_len
);
}
return string;
}
-----------------------------------------------------------------------
str_test.c (which after compiling and linking with strutil.c gives the
above message)
-------------------------------------------------------------------------
#include <stdio.h>
#include <string.h>
#include "strutil.h"

int main() {

char *str_ptr;
char *substr_ptr;

char letter ='x';
char s[100];
char s_cache1[100];
char s_cache2[100];
char substr[5];

gets(s);

str_ptr=s;
strcpy(substr, "fart");

strcpy(s_cache1, s);
strcpy(s_cache2, s);

printf("\nReverse %s is %s", strdup(s), strrev(s));

printf("\nFilled with %c became %s", letter, strset(s, letter));

printf("\nAfter removing first %s became %s", substr,
strstr_rem_first(s_cache1, "fart"));
printf("\nAfter removing all %s became %s", substr,
strstr_rem_all(s_cache2, "fart"));

printf("\nIndex of first substring in %s is %d",
substring_index("hello", "fart"));

return 0;

}

 

[Anupam]

Hi Anupam:
Please take a look at my files. I wonder if you find the cause of this
exception. It would be very interesting. I checked once again
everything (i even found out that i forgot to include strstr_rem_all()
into header!) but the message still appears after execution of
str_test.exe that i am getting by the following commands:
C:\C_PROG~1\STRING~1>gcc -c -g strutil.c

C:\C_PROG~1\STRING~1>gcc -c -g str_test.c

C:\C_PROG~1\STRING~1>gcc -o str_test.exe str_test.o strutil.o

C:\C_PROG~1\STRING~1>str_test
123fart456fart789fart

Wow didn't you supply the command line parameters here? This I cant
get. And still it is outputting stuff. Check it once . Maybe
something's wrong in what u gave us.

The following are my files:

?? str_test.c I think from the compilation. Is this the code u
compiled?

int main( int argc, char** argv ) {
if ( argc >= 3 ) {
printf( "string = '%s', substr = '%s'\n", argv[1],
argv[2] );
strstr_rem_all( argv[1], argv[2] );
printf( "result = '%s'\n", argv[1] );
}
return 0;

Consider return EXIT_SUCCESS (or FAILURE) by including stdlib.h

}
------------------------------------------------------------------------
strutil.h (i am making to create my library)
----------------------------------------------------------------------
/*strutil.h*/

extern char *strrev(char *string); //return reversed string
extern char *strset(char *string, int letter);

Whoa, check out your names, they are common to functions in string.h
which also u r including.

extern char *strstr_rem_first(char *string, char *substring);
extern int substring_index(const char *string, const char *substring);
extern char *strstr_rem_all( char *string, const char *substring );
-----------------------------------------------------------------------
strutil.c (contains my functions)
-----------------------------------------------------------------------
#include <string.h>
#include "strutil.h"

//reverse
char *strrev(char *string) {
char *backward = string;
char *forward = string;
char temp;
//set 'backward' to the null byte terminating the string
while(*backward){
backward++;
}
while(forward<backward) {
--backward;
temp=*backward;
*backward=*forward;
*forward=temp;
++forward;
}
return (string);
}

Compiler is thinking : Is this the real strrev or is the one in the
standard library the one u want to use? BIG problem.

//setting to char
char *strset(char *string, int letter) {
char *original=string;

while(*string) {
*string++ = letter;
}
return (original);
}
//finds rightmost index of substr
int substring_index(const char *string, const char *substring) {
int i, j, k;

for(i=0; string;i++)
for(j=i,k=0; string[j] == substring[k]; j++, k++)
if(!substring[k+1])
return(i);
return(-1);
}
//removes first sbstr
char *strstr_rem_first(char *string, char *substring) {
int i, j, k, loc=-1;

for(i=0; string && (loc==-1); i++) {
for(j=i, k=0; string[j] ==substring[k];j++, k++)
if(!substring[k+1])
loc=i;
if(loc != -1) { //substring was found
for(k=0; substring[k];k++); {
for(j=loc, i=loc+k; string; j++, i++) {
string[j] = string;
string='\0';
}
}
}
}
return (string);
}
//removes all substr
char *strstr_rem_all( char *string, const char *substring ) {
char *occurrence; int substr_len = strlen( substring );
while (occurrence = strstr( string, substring ) ) {
int move_len = strlen( occurrence ) - substr_len + 1;
memmove( occurrence, occurrence + substr_len, move_len
);
}
return string;
}
-----------------------------------------------------------------------
str_test.c (which after compiling and linking with strutil.c gives the
above message)
-------------------------------------------------------------------------
#include <stdio.h>
#include <string.h>
#include "strutil.h"

int main() {

char *str_ptr;
char *substr_ptr;

char letter ='x';
char s[100];
char s_cache1[100];
char s_cache2[100];
char substr[5];

gets(s);

Warning ! Warning ! gets is dangerous . Refer to Richard Heathfield'
s page for more info...(The URL was already given earlier)

str_ptr=s;
strcpy(substr, "fart");

strcpy(s_cache1, s);
strcpy(s_cache2, s);

printf("\nReverse %s is %s", strdup(s), strrev(s));

printf("\nFilled with %c became %s", letter, strset(s, letter));

printf("\nAfter removing first %s became %s", substr,
strstr_rem_first(s_cache1, "fart"));
printf("\nAfter removing all %s became %s", substr,
strstr_rem_all(s_cache2, "fart"));

printf("\nIndex of first substring in %s is %d",
substring_index("hello", "fart"));

return 0;

}

Try changing the names of those functions. Your function names are
colliding with the standard ones in the global namespace.