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improve my search and replace function

P: n/a
Hi all,
I asked this question in the C group but no one seems to be interested
in answering it. :-( Basically, I wrote a search and replace function
so I can do:

char[] source = "abcd?1234?x";
char search = '?';
char* replace = "***";

char* result = search_and_replace(source,search,replace);

result will then be "abcd***1234***x". I understand that I can
probably use string instead of char* and there's probably some API in
the C++ standard library that I can use but I want to code it as an
exercise to learn the algorithm. Can someone suggest ways to improve
the performance of my search and replace algorithm? The function lacks
some error checkings but I am more interested in the algorithm.
Thanks!

char* search_and_replace(char* source,char search,char* replace){

char* result;

size_t l = strlen(source), r = strlen(replace), i, k;

int number_of_replaces = 0;

for(i = 0; i < l; i++){
if(source[i] == search)
number_of_replaces++;
}

result = malloc((l - number_of_replaces) +
number_of_replaces * r + 1);
i = 0; k = 0;

while(k < l){
if(source[k] == search){
int j;
for(j = 0; j < r; j++){
result[i++] = replace[j];
}
}else{
result[i++] = source[k];
}
k++;
}

result[i] = 0;
return result;
}
Jul 22 '05 #1
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10 Replies


P: n/a

"pembed2003" <pe********@yahoo.com> wrote in message
news:db**************************@posting.google.c om...
Hi all,
I asked this question in the C group but no one seems to be interested
in answering it. :-( Basically, I wrote a search and replace function
so I can do:

char[] source = "abcd?1234?x";
char search = '?';
char* replace = "***";

char* result = search_and_replace(source,search,replace);

result will then be "abcd***1234***x". I understand that I can
probably use string instead of char* and there's probably some API in
the C++ standard library that I can use but I want to code it as an
exercise to learn the algorithm. Can someone suggest ways to improve
the performance of my search and replace algorithm? The function lacks
some error checkings but I am more interested in the algorithm.
Thanks!

char* search_and_replace(char* source,char search,char* replace){

char* result;

size_t l = strlen(source), r = strlen(replace), i, k;

int number_of_replaces = 0;

for(i = 0; i < l; i++){
if(source[i] == search)
number_of_replaces++;
}

result = malloc((l - number_of_replaces) +
number_of_replaces * r + 1);
i = 0; k = 0;

while(k < l){
if(source[k] == search){
int j;
for(j = 0; j < r; j++){
result[i++] = replace[j];
}
}else{
result[i++] = source[k];
}
k++;
}

result[i] = 0;
return result;
}


I can't see any obvious way to improve the efficiency, you seem to be doing
all the right things, like preallocating the result buffer. This loop

for(j = 0; j < r; j++){
result[i++] = replace[j];
}

might be a little faster as a memcpy

memcpy(result + i, replace, r);
i += r;

You could also try a pointer version instead of using ints for all your loop
variables. It might be faster but it might not, worth a try though.

john
Jul 22 '05 #2

P: n/a
"pembed2003" <pe********@yahoo.com> wrote in message
char[] source = abcd?1234?x";
char search = '?';
char* replace = "***";

char* result = search_and_replace(source,search,replace);

result will then be "abcd***1234***x". I understand that I can
Wait a sec. Did you want four **** before x?
probably use string instead of char* and there's probably some API in
the C++ standard library that I can use but I want to code it as an
exercise to learn the algorithm. Can someone suggest ways to improve
the performance of my search and replace algorithm? The function lacks
some error checkings but I am more interested in the algorithm.
Thanks!

char* search_and_replace(char* source,char search,char* replace){

char* result;

size_t l = strlen(source), r = strlen(replace), i, k;

int number_of_replaces = 0;

for(i = 0; i < l; i++){
if(source[i] == search)
number_of_replaces++;
}

result = malloc((l - number_of_replaces) +
number_of_replaces * r + 1);
i = 0; k = 0;

while(k < l){
if(source[k] == search){
int j;
for(j = 0; j < r; j++){
result[i++] = replace[j];
}
}else{
result[i++] = source[k];
}
k++;
}

result[i] = 0;
return result;
}


This looks good. It's efficient. Maybe put in a few comments.

One thing I can think of is that use of standard functions like memcpy
(suitable for your case) and strcpy may use special assembly instructions
created especially for memcpy, and thus the resulting code would be faster.
But I'm not an expert on what these statements are, what platforms do what,
what compilers support what, and so on.

Another thing I can think of is when you scan the array to find the number
of elements to replace, you put the index of the element into an array, so
for example in "abcd?1234?x" the array will contain 4 and 9 (the index of
the ?). Then in the next loop you can just look up the ?. This approach
may make the algorithm faster if there are few replacements in a long
stream. Also, the resulting code is more complicated, and thus harder to
maintain.

Maybe others can see other problems.

Maybe the next challenge is to do the same in place! Note, this algorithm
is not necessarily better, just different.
Jul 22 '05 #3

P: n/a
On 19 Jun 2004 23:14:29 -0700 in comp.lang.c++, pe********@yahoo.com
(pembed2003) wrote,
Hi all,
I asked this question in the C group but no one seems to be interested


Sorry, I am not interested until I see it
use std::string::find() and std::string::replace()

Jul 22 '05 #4

P: n/a
"John Harrison" <jo*************@hotmail.com> wrote in message news:<2j*************@uni-berlin.de>...
"pembed2003" <pe********@yahoo.com> wrote in message
news:db**************************@posting.google.c om...


[snip]

char[] source = "abcd?1234?x";
char search = '?';
char* replace = "***";

char* result = search_and_replace(source,search,replace);

result will then be "abcd***1234***x".
[snip]

char* search_and_replace(char* source,char search,char* replace){

char* result;

size_t l = strlen(source), r = strlen(replace), i, k;

int number_of_replaces = 0;

for(i = 0; i < l; i++){
if(source[i] == search)
number_of_replaces++;
}

result = malloc((l - number_of_replaces) +
number_of_replaces * r + 1);
i = 0; k = 0;

while(k < l){
if(source[k] == search){
int j;
for(j = 0; j < r; j++){
result[i++] = replace[j];
}
}else{
result[i++] = source[k];
}
k++;
}

result[i] = 0;
return result;
}


I can't see any obvious way to improve the efficiency, you seem to be doing
all the right things, like preallocating the result buffer. This loop


Hi John / Siemel,
Thanks for your comment. I found another way of doing it:

char* search_and_replace2(char* source, char search, char* replace){
int i = 0;
size_t r = strlen(replace);
char* tmp = malloc(strlen(source) * r + 1), *result;
while(*source){
if(*source == search){
size_t j;
for(j = 0; j < r; j++){
tmp[i++] = replace[j];
}
}else{
tmp[i++] = *source;
}
source++;
}
tmp[i] = 0;
result = malloc(i);
strcpy(result,tmp);
free(tmp);
return result;
}

With this version, I only go through source once, but it calls malloc
twice. I will time these two version and see which one is faster. I
will also change it to use the suggestions you made to see how much
improvement I can got. Just curious, which version do you think will
be faster?

Thanks!
Jul 22 '05 #5

P: n/a

"pembed2003" <pe********@yahoo.com> wrote in message
news:db**************************@posting.google.c om...
"John Harrison" <jo*************@hotmail.com> wrote in message

news:<2j*************@uni-berlin.de>...
"pembed2003" <pe********@yahoo.com> wrote in message
news:db**************************@posting.google.c om...


[snip]

char[] source = "abcd?1234?x";
char search = '?';
char* replace = "***";

char* result = search_and_replace(source,search,replace);

result will then be "abcd***1234***x".
[snip]

char* search_and_replace(char* source,char search,char* replace){

char* result;

size_t l = strlen(source), r = strlen(replace), i, k;

int number_of_replaces = 0;

for(i = 0; i < l; i++){
if(source[i] == search)
number_of_replaces++;
}

result = malloc((l - number_of_replaces) +
number_of_replaces * r + 1);
i = 0; k = 0;

while(k < l){
if(source[k] == search){
int j;
for(j = 0; j < r; j++){
result[i++] = replace[j];
}
}else{
result[i++] = source[k];
}
k++;
}

result[i] = 0;
return result;
}


I can't see any obvious way to improve the efficiency, you seem to be doing all the right things, like preallocating the result buffer. This loop


Hi John / Siemel,
Thanks for your comment. I found another way of doing it:

char* search_and_replace2(char* source, char search, char* replace){
int i = 0;
size_t r = strlen(replace);
char* tmp = malloc(strlen(source) * r + 1), *result;
while(*source){
if(*source == search){
size_t j;
for(j = 0; j < r; j++){
tmp[i++] = replace[j];
}
}else{
tmp[i++] = *source;
}
source++;
}
tmp[i] = 0;
result = malloc(i);
strcpy(result,tmp);
free(tmp);
return result;
}

With this version, I only go through source once, but it calls malloc
twice. I will time these two version and see which one is faster. I
will also change it to use the suggestions you made to see how much
improvement I can got. Just curious, which version do you think will
be faster?

Thanks!


I would expect the first to be faster. Timing malloc can be tricky however,
it could be that the second is faster at first but if your program runs for
a while the extra malloc starts to slow it down.

A third possibility would be to use a fixed size temporary buffer and only
call malloc if the temporary space needed exceeds the size of the temporary
buffer. Like this

char* search_and_replace3(char* source, char search, char* replace){
int i = 0;
size_t r = strlen(replace);
char tmp_buffer[1000], *tmp, *result;
if (strlen(source) * r + 1 > 1000)
tmp = malloc(strlen(source) * r + 1);
else
tmp = tmp_buffer;

// code as before

if (tmp != tmp_buffer)
free(tmp);
return result;
}

This way you avoid the cost of the extra malloc most of the time.

john
Jul 22 '05 #6

P: n/a
"John Harrison" <jo*************@hotmail.com> wrote in message
for(i = 0; i < l; i++){
if(source[i] == search)
number_of_replaces++;
}
You could also try a pointer version instead of using ints for all your loop variables. It might be faster but it might not, worth a try though.


This is a good point. The expression p[i] implies an arithmetic operation
as in *(p+sizeof(*p)*i). I imagine this would be slower on all platforms
than just *(p2), but could be wrong. Also, some compilers may realize
you're using the index variables as pointers to an array, and replace them
with pointer version. Or you could just do it explicitly:

const char * scan = source;
while (true) {
const char c = *scan;
if (!c) break;
if (c == search) ++number_of_replaces;
++scan;
}

An advantage of the iterator style is that now it's easy to generalize to
any container, say a list of chars or any other object. Though it does take
some getting used to. When I do interviews or talk with my friends from
work, and ask them to write a function to find the first occurrence of a
certain character in a string (i.e.. like the std::find algorithm), they
almost always use the p[i] style like so:

const char * find(const char * s, char c) {
int N = strlen(s);
for (int i=0; i<N; i++) {
if (p[i] == c) return &p[i];
}
return NULL;
}
Jul 22 '05 #7

P: n/a
"pembed2003" <pe********@yahoo.com> wrote in message
char* search_and_replace2(char* source, char search, char* replace){
int i = 0;
size_t r = strlen(replace);
char* tmp = malloc(strlen(source) * r + 1), *result;
while(*source){
if(*source == search){
size_t j;
for(j = 0; j < r; j++){
tmp[i++] = replace[j];
}
}else{
tmp[i++] = *source;
}
source++;
}
tmp[i] = 0;
result = malloc(i);
strcpy(result,tmp);
free(tmp);
return result;
}

With this version, I only go through source once, but it calls malloc
twice. I will time these two version and see which one is faster. I
will also change it to use the suggestions you made to see how much
improvement I can got. Just curious, which version do you think will
be faster?


My guess is the first way will be faster because malloc and free are
generally expensive calls (though see John's excellent reply on this issue
too). Also note that the strcpy at the end implies the 2nd pass through the
loop, but if it translates to a special assembler function, it might be
faster than an explicit byte by byte scan.

The 2nd way also uses a lot of space.
Jul 22 '05 #8

P: n/a
"pembed2003" <pe********@yahoo.com> wrote in message
size_t l = strlen(source), r = strlen(replace), i, k;

int number_of_replaces = 0;

for(i = 0; i < l; i++){
if(source[i] == search)
number_of_replaces++;
}


Also, strlen(source) implies one pass through the string, although it might
be very fast if it translates to an assembler instruction, though it's
probably still O(N). Assuming there's no special assembler instruction,
this way would be faster

const char * scan = source;
for ( ; ; ++scan) {
const char c = *scan;
if (!c) break;
if (c == search)
number_of_replaces++;
}
l = scan - source; // same as strlen(source)

Though it's possible you don't need to know l as John pointed out.
Jul 22 '05 #9

P: n/a
pe********@yahoo.com (pembed2003) wrote in message
[snip]

char[] source = "abcd?1234?x";
char search = '?';
char* replace = "***";

char* result = search_and_replace(source,search,replace);

result will then be "abcd***1234***x".
[snip]

char* search_and_replace(char* source,char search,char* replace){

char* result;

size_t l = strlen(source), r = strlen(replace), i, k;

int number_of_replaces = 0;

for(i = 0; i < l; i++){
if(source[i] == search)
number_of_replaces++;
}

result = malloc((l - number_of_replaces) +
number_of_replaces * r + 1);
i = 0; k = 0;

while(k < l){
if(source[k] == search){
int j;
for(j = 0; j < r; j++){
result[i++] = replace[j];
}
}else{
result[i++] = source[k];
}
k++;
}

result[i] = 0;
return result;
}


char* search_and_replace2(char* source, char search, char* replace){
int i = 0;
size_t r = strlen(replace);
char* tmp = malloc(strlen(source) * r + 1), *result;
while(*source){
if(*source == search){
size_t j;
for(j = 0; j < r; j++){
tmp[i++] = replace[j];
}
}else{
tmp[i++] = *source;
}
source++;
}
tmp[i] = 0;
result = malloc(i);
strcpy(result,tmp);
free(tmp);
return result;
}

With this version, I only go through source once, but it calls malloc
twice. I will time these two version and see which one is faster. I
will also change it to use the suggestions you made to see how much
improvement I can got. Just curious, which version do you think will
be faster?


I time these 2 version and found out that the first version is faster.
I time the 2 functions with 10 million iterations and here are the
numbers:

time test
17.0u 0.0s 0:17.01 99.9% 0+0K 0+0io 2pf+0w

time test2
28.2u 0.0s 0:28.29 99.8% 0+0K 0+0io 2pf+0w

test = first version (walks the source twice with one malloc)
test2 = second version (walks the srouce onec with two malloc)

Thanks!
Jul 22 '05 #10

P: n/a
"pembed2003" <pe********@yahoo.com> wrote in message
time test
17.0u 0.0s 0:17.01 99.9% 0+0K 0+0io 2pf+0w

time test2
28.2u 0.0s 0:28.29 99.8% 0+0K 0+0io 2pf+0w

test = first version (walks the source twice with one malloc)
test2 = second version (walks the srouce onec with two malloc)


Did John's suggestion of using pointers rather than integers make a
difference?
Jul 22 '05 #11

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