Ok, so what I need to do is split a string. Here is an example string that I need to split:
test_french_fries
and let's say I want to split the string into two parts, the part before and after:
french
thus I would get a return of something like:
match[1]=[test_]
match[2]=[_fries]
I have tried messing around with strtok() but I have had no success with that. It appears that strtok() splits the string at all delims that are given - so if I were to give it a delim of french, it would separate the string for each character matched - which ISN'T what I want.
I would like to have a delim where I can provide it a str or char* such as "french" and the string be split before and after the string provided.
Are there any other methods besides strtok() that I could possibly try to implement in order to achieve my desired outcome?
I really appreciate any and all help that I receive.
-Patrick
strtok() can only split using a single symbol as separator (1 character), you can specify multiple separators in the form of a null-terminated string, but they are delt with individualy. To split a string with another string as separator, you will have to write a custom function. The example below works just like strtok(), but uses a null-terminated C string as separator:
#include <stdio.h>
char *strstrtok(char *src, char *sep)
{
static char *str = NULL;
if (src) str = src;
else src = str;
if (str == NULL)
return NULL;
char *next = strstr(str, sep);
if (next)
{
*next = '\0';
str = next + strlen(sep);
}
else
str = NULL;
return src;
}
int main(void) {
char buf[] = "yoursepmamasepissepsosepfatsepsheseptooksepasepspoonseptosepthesepsuperbowl";
char *token = strstrtok(buf, "sep");
while (token)
{
printf("%s ", token);
token = strstrtok(NULL, "sep");
}
return 0;
}
Notice just like strtok(), this solution is not thread-safe.
See it working on Ideone: http://ideone.com/cjRGgJ
char line[] = "COPY\tSTART\t0\tCOPY";
char *tmp;
tmp = strtok(line, "\t");
printf("%s", tmp);
This code's output is COPY. And when
char line[] = "\tSTART\t0\tCOPY";
Output is START.
But! I want to check there is nothing in front of string START.
That is I think \t is first delimiter so output of strtok(line, "\t") is NULL.
But real output is START.
Is there any misunderstanding? What can I do?
As per the man page of strtok() (emphasis mine)
A sequence of two or more contiguous delimiter bytes in the parsed string is considered to be a single delimiter. Delimiter bytes at the start or end of the string are ignored. Put another way: the tokens returned by strtok() are always nonempty strings.
So, what you're experiencing is the right behaviour of strtok().
OTOH, strtok() will return NULL if there is no more tokens, so as per you have expected, returning NULL for the initial delimiter will convey wrong message and it will be confusing. So, the bottom line is,
if a token is present
the tokens returned by strtok() are always nonempty strings.
if a token is not present
strtok() will return NULL.
Note: it is useful to mention that before using the retured token, always check for NULL.
What can I do?
Build your own function, not exactly how strtok works but you can get some idea:
#include <stdio.h>
#include <string.h>
char *scan(char **pp, char c)
{
char *s, *p;
p = strchr(*pp, c);
if (p) *p++ = '\0';
s = *pp;
*pp = p;
return s;
}
int main(void)
{
char line1[] = "COPY\tSTART\t0\tCOPY";
char line2[] = "\tSTART\t0\tCOPY";
char *p;
puts("Line 1");
p = line1;
while (p) {
printf("%s\n", scan(&p, '\t'));
}
puts("Line 2");
p = line2;
while (p) {
printf("%s\n", scan(&p, '\t'));
}
return 0;
}
Output:
Line 1
COPY
START
0
COPY
Line 2
START
0
COPY
I am trying to use strtok() to parse a string deliminated by spaces. From what I can tell, I am using it correctly, but it won't run on ideone.com. Is there anything wrong with the following code? I just get Runtime error time: 0 memory: 2288 signal:11
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
static void test(char *command)
{
char* output = strtok(command, " ");
printf("%s\n", output);
}
int main(void) {
test("set_rate 200");
return 0;
}
Here it is on ideone.com if you want to try it.
Always consult the man pages first.
strtok(3) says:
Be cautious when using these functions. If you do use them, note that:
These functions modify their first argument.
These functions cannot be used on constant strings.
strtok will modify the string passed as the first parameter, so you can't pass a string literal, change your main like this:
int main(void) {
char str[] = "set_rate 200";
test(str);
return 0;
}
str here is an char array, but not a string literal.
As strtok modifies the string, it requires that the string is not in read-only memory. So when you pass a string literal to your test function, it crashes.
This is better:
char s[] = "set_rate 200";
test(s);
When you pass hard coded string that was stored in read-only memory.
strtok() does not work with string which is read-only memory.
You need to use string variable rather than string literal .
you can first store string in some variable and then you can pass it to the function.
char[]="set_rate 200";
test(str);
See example:
char *str = malloc(20);
char *tok = NULL;
int len = 0;
strcpy(str, "This is a string");
len = strlen(str);
printf("string before strtok(): %s\n", str);
tok = strtok(str, " ");
while (tok) {
printf("Token: %s\n", tok);
tok = strtok(NULL, " ");
}
Edit
From #Yu Hao comment i am adding this
char *str = "set_rate 200";
test(str); // This won't work. here str is pointer to the string literal.
Hi guys I'm currently using the code below and I'm pretty sure there's a better way to do it. What the code does is look if there's the delimiter (~~~~), puts everything before ~~~~ in cmd and everything after ~~~~ in param. If anyone could let me know how I should be doing this then it would be very appreciated! I'm not used to low-level languages so strings and pointers are still confusing to me!
Thanks!
char buffer[1024], *tempCharPointer, cmd[100], param[1024];
if(strstr(buffer, "~~~~"))
{
strcpy(cmd, buffer);
tempCharPointer = strstr(buffer, "~~~~");
index = (tempCharPointer-buffer) + 4;
strcpy(param, &tempCharPointer[4]);
memmove(&cmd[index-4], "", (index-4));
}
You can simplify your code as follows:
char cmd[1024], *tempCharPointer, *param = "";
// Fill in cmd from somewhere...
...
char *delim = strstr(cmd, "~~~~");
if(delim)
{
param = delim+4;
*delim = '\0';
}
You can simplify your code and insert \0 before the delimiter (modify the first character of the delimiter and make it \0) and have command be a pointer to the beginning of the string and param a pointer to the first character after the delimiter. Saves you memory and all these moves and such.
char buffer[1024], *tempCharPointer, cmd[100], param[1024];
tempCharPointer = strstr(buffer, "~~~~");
if (tempCharPointer){
*tempCharPointer = '\0';
tempCharPointer +=4;
//now buffer points to the first half, and tempCharPointer points to second half
//do with them what you will
}
The strtok function in the C library (extract tokens from strings) can be useful here.
A small example follows. man strtok for more info. Note that strtok_r (used below) is used for reentrant support.
#include <string.h>
#include <stdio.h>
int main(const int argc, const char const** argv)
{
char buffer[1024];
sprintf(buffer, "~~~~foo~~~~bar~~~~baz");
char* saveptr = NULL;
char* token = strtok_r(buffer, "~~~~", &saveptr);
while(token != NULL)
{
printf("TOKEN: %s\n", token);
token = strtok_r(NULL, "~~~~", &saveptr);
}
}
Please explain to me the working of strtok() function. The manual says it breaks the string into tokens. I am unable to understand from the manual what it actually does.
I added watches on str and *pch to check its working when the first while loop occurred, the contents of str were only "this". How did the output shown below printed on the screen?
/* strtok example */
#include <stdio.h>
#include <string.h>
int main ()
{
char str[] ="- This, a sample string.";
char * pch;
printf ("Splitting string \"%s\" into tokens:\n",str);
pch = strtok (str," ,.-");
while (pch != NULL)
{
printf ("%s\n",pch);
pch = strtok (NULL, " ,.-");
}
return 0;
}
Output:
Splitting string "- This, a sample string." into tokens:
This
a
sample
string
the strtok runtime function works like this
the first time you call strtok you provide a string that you want to tokenize
char s[] = "this is a string";
in the above string space seems to be a good delimiter between words so lets use that:
char* p = strtok(s, " ");
what happens now is that 's' is searched until the space character is found, the first token is returned ('this') and p points to that token (string)
in order to get next token and to continue with the same string NULL is passed as first
argument since strtok maintains a static pointer to your previous passed string:
p = strtok(NULL," ");
p now points to 'is'
and so on until no more spaces can be found, then the last string is returned as the last token 'string'.
more conveniently you could write it like this instead to print out all tokens:
for (char *p = strtok(s," "); p != NULL; p = strtok(NULL, " "))
{
puts(p);
}
EDIT:
If you want to store the returned values from strtok you need to copy the token to another buffer e.g. strdup(p); since the original string (pointed to by the static pointer inside strtok) is modified between iterations in order to return the token.
strtok() divides the string into tokens. i.e. starting from any one of the delimiter to next one would be your one token. In your case, the starting token will be from "-" and end with next space " ". Then next token will start from " " and end with ",". Here you get "This" as output. Similarly the rest of the string gets split into tokens from space to space and finally ending the last token on "."
strtok maintains a static, internal reference pointing to the next available token in the string; if you pass it a NULL pointer, it will work from that internal reference.
This is the reason strtok isn't re-entrant; as soon as you pass it a new pointer, that old internal reference gets clobbered.
strtok doesn't change the parameter itself (str). It stores that pointer (in a local static variable). It can then change what that parameter points to in subsequent calls without having the parameter passed back. (And it can advance that pointer it has kept however it needs to perform its operations.)
From the POSIX strtok page:
This function uses static storage to keep track of the current string position between calls.
There is a thread-safe variant (strtok_r) that doesn't do this type of magic.
strtok will tokenize a string i.e. convert it into a series of substrings.
It does that by searching for delimiters that separate these tokens (or substrings). And you specify the delimiters. In your case, you want ' ' or ',' or '.' or '-' to be the delimiter.
The programming model to extract these tokens is that you hand strtok your main string and the set of delimiters. Then you call it repeatedly, and each time strtok will return the next token it finds. Till it reaches the end of the main string, when it returns a null. Another rule is that you pass the string in only the first time, and NULL for the subsequent times. This is a way to tell strtok if you are starting a new session of tokenizing with a new string, or you are retrieving tokens from a previous tokenizing session. Note that strtok remembers its state for the tokenizing session. And for this reason it is not reentrant or thread safe (you should be using strtok_r instead). Another thing to know is that it actually modifies the original string. It writes '\0' for teh delimiters that it finds.
One way to invoke strtok, succintly, is as follows:
char str[] = "this, is the string - I want to parse";
char delim[] = " ,-";
char* token;
for (token = strtok(str, delim); token; token = strtok(NULL, delim))
{
printf("token=%s\n", token);
}
Result:
this
is
the
string
I
want
to
parse
The first time you call it, you provide the string to tokenize to strtok. And then, to get the following tokens, you just give NULL to that function, as long as it returns a non NULL pointer.
The strtok function records the string you first provided when you call it. (Which is really dangerous for multi-thread applications)
strtok modifies its input string. It places null characters ('\0') in it so that it will return bits of the original string as tokens. In fact strtok does not allocate memory. You may understand it better if you draw the string as a sequence of boxes.
To understand how strtok() works, one first need to know what a static variable is. This link explains it quite well....
The key to the operation of strtok() is preserving the location of the last seperator between seccessive calls (that's why strtok() continues to parse the very original string that is passed to it when it is invoked with a null pointer in successive calls)..
Have a look at my own strtok() implementation, called zStrtok(), which has a sligtly different functionality than the one provided by strtok()
char *zStrtok(char *str, const char *delim) {
static char *static_str=0; /* var to store last address */
int index=0, strlength=0; /* integers for indexes */
int found = 0; /* check if delim is found */
/* delimiter cannot be NULL
* if no more char left, return NULL as well
*/
if (delim==0 || (str == 0 && static_str == 0))
return 0;
if (str == 0)
str = static_str;
/* get length of string */
while(str[strlength])
strlength++;
/* find the first occurance of delim */
for (index=0;index<strlength;index++)
if (str[index]==delim[0]) {
found=1;
break;
}
/* if delim is not contained in str, return str */
if (!found) {
static_str = 0;
return str;
}
/* check for consecutive delimiters
*if first char is delim, return delim
*/
if (str[0]==delim[0]) {
static_str = (str + 1);
return (char *)delim;
}
/* terminate the string
* this assignmetn requires char[], so str has to
* be char[] rather than *char
*/
str[index] = '\0';
/* save the rest of the string */
if ((str + index + 1)!=0)
static_str = (str + index + 1);
else
static_str = 0;
return str;
}
And here is an example usage
Example Usage
char str[] = "A,B,,,C";
printf("1 %s\n",zStrtok(s,","));
printf("2 %s\n",zStrtok(NULL,","));
printf("3 %s\n",zStrtok(NULL,","));
printf("4 %s\n",zStrtok(NULL,","));
printf("5 %s\n",zStrtok(NULL,","));
printf("6 %s\n",zStrtok(NULL,","));
Example Output
1 A
2 B
3 ,
4 ,
5 C
6 (null)
The code is from a string processing library I maintain on Github, called zString. Have a look at the code, or even contribute :)
https://github.com/fnoyanisi/zString
This is how i implemented strtok, Not that great but after working 2 hr on it finally got it worked. It does support multiple delimiters.
#include "stdafx.h"
#include <iostream>
using namespace std;
char* mystrtok(char str[],char filter[])
{
if(filter == NULL) {
return str;
}
static char *ptr = str;
static int flag = 0;
if(flag == 1) {
return NULL;
}
char* ptrReturn = ptr;
for(int j = 0; ptr != '\0'; j++) {
for(int i=0 ; filter[i] != '\0' ; i++) {
if(ptr[j] == '\0') {
flag = 1;
return ptrReturn;
}
if( ptr[j] == filter[i]) {
ptr[j] = '\0';
ptr+=j+1;
return ptrReturn;
}
}
}
return NULL;
}
int _tmain(int argc, _TCHAR* argv[])
{
char str[200] = "This,is my,string.test";
char *ppt = mystrtok(str,", .");
while(ppt != NULL ) {
cout<< ppt << endl;
ppt = mystrtok(NULL,", .");
}
return 0;
}
For those who are still having hard time understanding this strtok() function, take a look at this pythontutor example, it is a great tool to visualize your C (or C++, Python ...) code.
In case the link got broken, paste in:
#include <stdio.h>
#include <string.h>
int main()
{
char s[] = "Hello, my name is? Matthew! Hey.";
char* p;
for (char *p = strtok(s," ,?!."); p != NULL; p = strtok(NULL, " ,?!.")) {
puts(p);
}
return 0;
}
Credits go to Anders K.
Here is my implementation which uses hash table for the delimiter, which means it O(n) instead of O(n^2) (here is a link to the code):
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#define DICT_LEN 256
int *create_delim_dict(char *delim)
{
int *d = (int*)malloc(sizeof(int)*DICT_LEN);
memset((void*)d, 0, sizeof(int)*DICT_LEN);
int i;
for(i=0; i< strlen(delim); i++) {
d[delim[i]] = 1;
}
return d;
}
char *my_strtok(char *str, char *delim)
{
static char *last, *to_free;
int *deli_dict = create_delim_dict(delim);
if(!deli_dict) {
/*this check if we allocate and fail the second time with entering this function */
if(to_free) {
free(to_free);
}
return NULL;
}
if(str) {
last = (char*)malloc(strlen(str)+1);
if(!last) {
free(deli_dict);
return NULL;
}
to_free = last;
strcpy(last, str);
}
while(deli_dict[*last] && *last != '\0') {
last++;
}
str = last;
if(*last == '\0') {
free(deli_dict);
free(to_free);
deli_dict = NULL;
to_free = NULL;
return NULL;
}
while (*last != '\0' && !deli_dict[*last]) {
last++;
}
*last = '\0';
last++;
free(deli_dict);
return str;
}
int main()
{
char * str = "- This, a sample string.";
char *del = " ,.-";
char *s = my_strtok(str, del);
while(s) {
printf("%s\n", s);
s = my_strtok(NULL, del);
}
return 0;
}
strtok() stores the pointer in static variable where did you last time left off , so on its 2nd call , when we pass the null , strtok() gets the pointer from the static variable .
If you provide the same string name , it again starts from beginning.
Moreover strtok() is destructive i.e. it make changes to the orignal string. so make sure you always have a copy of orignal one.
One more problem of using strtok() is that as it stores the address in static variables , in multithreaded programming calling strtok() more than once will cause an error. For this use strtok_r().
strtok replaces the characters in the second argument with a NULL and a NULL character is also the end of a string.
http://www.cplusplus.com/reference/clibrary/cstring/strtok/
you can scan the char array looking for the token if you found it just print new line else print the char.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main()
{
char *s;
s = malloc(1024 * sizeof(char));
scanf("%[^\n]", s);
s = realloc(s, strlen(s) + 1);
int len = strlen(s);
char delim =' ';
for(int i = 0; i < len; i++) {
if(s[i] == delim) {
printf("\n");
}
else {
printf("%c", s[i]);
}
}
free(s);
return 0;
}
So, this is a code snippet to help better understand this topic.
Printing Tokens
Task: Given a sentence, s, print each word of the sentence in a new line.
char *s;
s = malloc(1024 * sizeof(char));
scanf("%[^\n]", s);
s = realloc(s, strlen(s) + 1);
//logic to print the tokens of the sentence.
for (char *p = strtok(s," "); p != NULL; p = strtok(NULL, " "))
{
printf("%s\n",p);
}
Input: How is that
Result:
How
is
that
Explanation: So here, "strtok()" function is used and it's iterated using for loop to print the tokens in separate lines.
The function will take parameters as 'string' and 'break-point' and break the string at those break-points and form tokens. Now, those tokens are stored in 'p' and are used further for printing.
strtok is replacing delimiter with'\0' NULL character in given string
CODE
#include<iostream>
#include<cstring>
int main()
{
char s[]="30/4/2021";
std::cout<<(void*)s<<"\n"; // 0x70fdf0
char *p1=(char*)0x70fdf0;
std::cout<<p1<<"\n";
char *p2=strtok(s,"/");
std::cout<<(void*)p2<<"\n";
std::cout<<p2<<"\n";
char *p3=(char*)0x70fdf0;
std::cout<<p3<<"\n";
for(int i=0;i<=9;i++)
{
std::cout<<*p1;
p1++;
}
}
OUTPUT
0x70fdf0 // 1. address of string s
30/4/2021 // 2. print string s through ptr p1
0x70fdf0 // 3. this address is return by strtok to ptr p2
30 // 4. print string which pointed by p2
30 // 5. again assign address of string s to ptr p3 try to print string
30 4/2021 // 6. print characters of string s one by one using loop
Before tokenizing the string
I assigned address of string s to some ptr(p1) and try to print string through that ptr and whole string is printed.
after tokenized
strtok return the address of string s to ptr(p2) but when I try to print string through ptr it only print "30" it did not print whole string. so it's sure that strtok is not just returning adress but it is placing '\0' character where delimiter is present.
cross check
1.
again I assign the address of string s to some ptr (p3) and try to print string it prints "30" as while tokenizing the string is updated with '\0' at delimiter.
2.
see printing string s character by character via loop the 1st delimiter is replaced by '\0' so it is printing blank space rather than ''