I'm a beginner at C and I'm stuck on a simple problem. Here it goes:
I have a string formatted like this: "first1:second1\nsecond2\nfirst3:second3" ... and so on.
As you can see from the the example the first field is optional ([firstx:]secondx).
I need to get a resulting string which contains only the second field. Like this: "second1\nsecond2\nsecond3".
I did some research here on stack (string splitting in C) and I found that there are two main functions in C for string splitting: strtok (obsolete) and strsep.
I tried to write the code using both functions (plus strdup) without success. Most of the time I get some unpredictable result.
Better ideas?
Thanks in advance
EDIT:
This was my first try
int main(int argc, char** argv){
char * stri = "ciao:come\nva\nquialla:grande\n";
char * strcopy = strdup(stri); // since strsep and strtok both modify the input string
char * token;
while((token = strsep(&strcopy, "\n"))){
if(token[0] != '\0'){ // I don't want the last match of '\n'
char * sub_copy = strdup(token);
char * sub_token = strtok(sub_copy, ":");
sub_token = strtok(NULL, ":");
if(sub_token[0] != '\0'){
printf("%s\n", sub_token);
}
}
free(sub_copy);
}
free(strcopy);
}
Expected output: "come", "si", "grande"
Here's a solution with strcspn:
#include <stdio.h>
#include <string.h>
int main(void) {
const char *str = "ciao:come\nva\nquialla:grande\n";
const char *p = str;
while (*p) {
size_t n = strcspn(p, ":\n");
if (p[n] == ':') {
p += n + 1;
n = strcspn(p , "\n");
}
if (p[n] == '\n') {
n++;
}
fwrite(p, 1, n, stdout);
p += n;
}
return 0;
}
We compute the size of the initial segment not containing : or \n. If it's followed by a :, we skip over it and get the next segment that doesn't contain \n.
If it's followed by \n, we include the newline character in the segment. Then we just need to output the current segment and update p to continue processing the rest of the string in the same way.
We stop when *p is '\0', i.e. when the end of the string is reached.
I am trying to split a string into two tokens using strtok() that might have spaces and tabs mixed in the string.
So I made this:
struct strstr
{
char *str,
*one,
*two;
};
typedef struct strstr *STRSTR;
void split(STRSTR);
int main()
{
STRSTR str = malloc(sizeof(struct strstr));
str->str = malloc(256);
fgets(str->str, 256, stdin);
split(str);
printf("%s, %s\n", str->one, str->two);
free(str->str);
free(str);
return 0;
}
void split(STRSTR str)
{
int i;
char *temp = str->str;
while(isspace(*(str->str)))
str->str++;
str->one = strtok(str->str, " \t");
for(i = 0; i < strlen(str->one); i++)
{
if(!isspace(str->one[i]))
str->str++;
}
str->str++;
if(str->str != NULL)
{
puts("In null if");
str->two = strtok(str->str, "");
}
str->str = temp;
}
So for example if you input Hello Earth lingss, it will print out Hello, Earth lingss, which is perfect.
However, if I input Hello only, the split function goes inside the if(str->str != NULL) statement. How do I stop it from doing that with the code that I have?
EDIT: Also another problem, if someone doesn't mind checking it out. temp will only point to the first word in str->str. How can I make it point to the whole thing?
Add this statement before the last if block in the split function
str->str = strtok(str->str," \t"); like
str->str = strtok(str->str," \t");
if(str->str != NULL)
{
puts("In null if");
str->two = strtok(str->str, "");
}
you split the string based on the "\t" as the delimiter but you never changed the str->str string, use the above snippet and it should work fine
strtok is a funny function that both modifies the string you pass it, and stores information about it internally. You should pass your string to strtok once, then pass in NULL on subsequent calls. For instance, if your goal is to simply break a string up into tokens (which is obviously what strtok is for), then something like:
#define BUFFER_SIZE 256
int main(void) {
char *buffer = malloc(BUFFER_SIZE);
if (!buffer) {
return -1;
}
fgets(buffer, BUFFER_SIZE, stdin);
char *word;
char *ptr = buffer;
printf("Tokens: [");
while ((word = strtok(ptr, " \t\n"))) {
printf("%s, ", word);
ptr = NULL;
}
printf("]\n");
free(buffer);
}
will work. When I run the code like this:
./quick
when in the fun apple orange
I get the following result:
Tokens: [when, in, the, fun, apple, orange, ]
The important thing is that I only passed the buffer pointer to strtok on the first time through the loop. After that it is passed NULL.
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I'm trying to split up a string (typed in by the user at run time) into words (separated by spaces), and put each word into a different slot into an array. So, for example, if I took the string "hello world", array[0] would contain "hello" and array[1] would contain "world". And the last slot (in this case array[2]) would contain NULL. Here's what I have so far, which doesn't seem to be working properly. Any help would be appreciated. (By the way, this is part of a program which will call execvp(argv[0],argv); )
char input[100];
char* argv[20];
char* token;
scanf("%s", input);
//get the first token
token = strtok(input, " ");
int i=0;
//walk through other tokens
while( token != NULL ) {
argv[i] = token;
i++;
token = strtok(NULL, " ");
}
argv[i] = NULL; //argv ends with NULL
You need to allocate memory for each argv[i] and copy the current token to argv[i]:
token = strtok(input, " ");
int i=0;
//walk through other tokens
while( token != NULL ) {
argv[i] = malloc(strlen(token) + 1);
strncpy(argv[i], token, strlen(token));
//argv[i] = token;
i++;
token = strtok(NULL, " ");
}
argv[i] = NULL; //argv ends with NULL
I have created an example of what I think you want. I have used one malloc(3) for the whole
line of strings and another for the array of pointers you will get from the function.
Also, the second parameter of strtok(3) is passed to give more flexibility (the shell normally uses the contents of IFS environment variable to separate arguments so you can use the same algorithm as the shell does) I think you should use " \n\t" at least. It has a main() test function, so it's complete for your purpose.
#include <assert.h> /* man assert(3) */
#include <stdlib.h> /* malloc lives here */
#include <string.h> /* strtok, strdup lives here */
#include <stdio.h> /* printf lives here */
char **split(const char *str, const char *delim)
{
char *aux;
char *p;
char **res;
char *argv[200]; /* place for 200 words. */
int n = 0, i;
assert(aux = strdup(str));
for (p = strtok(aux, delim); p; p = strtok(NULL, delim))
argv[n++] = p;
argv[n++] = NULL;
/* i'll put de strdup()ed string one place past the NULL,
* so you can free(3), once finished */
argv[n++] = aux;
/* now, we need to copy the array, so we can use it outside
* this function. */
assert(res = calloc(n, sizeof (char *)));
for (i = 0; i < n; i++)
res[i] = argv[i];
return res;
} /* split */
int main()
{
char **argv =
split("Put each word of a string into array in C", " ");
int i;
for (i = 0; argv[i]; i++)
printf("[%s]", argv[i]);
puts(""); /* to end with a newline */
free(argv[i+1]);
free(argv);
} /* main */
The sample code just outputs:
$ pru
[Put][each][word][of][a][string][into][array][in][C]
I think I just figured out my problem: I need to use gets() instead of scanf(), because scanf() only gets the first word, up until a space, while I want to be able to get a string containing multiple words separated by spaces.
i need to tokenize the string in c. suppose if i have a string like this
"product=c,author=dennis,category=programming".
I want to extract only the values among these key values pairs. Like
[c,dennis,programming].
I have used strtok function which tokenizes with "=" and I get values
[product,c,author,dennis,category,programming].
Is there any built in function that can generate only the values like mentioned above.
Just a simple scanf
#include<stdio.h>
int main()
{
char token[20] = { 0 };
char c, name[20];
int i=0, offset;
while (scanf("%[a-z]%*[^a-z]", token) == 1) {
i++;
if(i%2==0)
printf("[%s]\n",token );
}
return 0;
}
./a.out
product=c,author=dennis,category=programming,
[c]
[dennis]
[programming]
Ctrl+D
Note. I have added , at the end of the string
You could simply skip every second token like that:
#include <stdio.h>
#include <string.h>
int main(void) {
char str[] = "product=c,author=dennis,category=programming";
char* p = strtok(str, ",=");
while (p != NULL) {
p = strtok(NULL, ",=");
if (p != NULL) {
printf("%s\n", p);
strtok(NULL, ",="); // skip this
}
}
return 0;
}
I can think of a couple of ways:
First tokenize on ,, then split each part on the =.
Find the first =, then the , after it, and get the word in between. Repeat.
If there are always three values, you can use sscanf to read the values.
You can use a regex library to parse the string.
You can first tokenize on ,, splitting the contents into 3 different strings, then tokenize on '=' for each of those strings:
char *kvpair[N] = {NULL}; // where N is large enough for the expected
// number of key-value pairs
char *tok = strtok(input, ",");
size_t kvcount = 0;
while (tok != NULL && kvcount < N)
{
kvpair[kvcount++] = tok;
tok = strtok(NULL, ",");
}
...
for (i = 0; i < kvcount; i++)
{
char delim = '[';
char *key = strtok(kvpair[i], "=");
char *val = strtok(NULL, "=");
printf("%c%s", delim, val);
delim = ',';
}
putchar(']');
This is just a rough sketch; it assumes that the maximum number of key-value pairs is known ahead of time, it doesn't attempt to handle empty keys or values, or really do any sort of error handling at all. But it should point you in the right direction.
Remember that strok modifies its input; if your original data is a string literal or if you need to preserve the original data, you'll need to make a copy and work on that copy.
Note that, because of how strok works, you can't "nest" calls; that is, you can't tokenize the first key-value pair, then split it into key and value tokens, then get the next key-value pair. You'll have to tokenize all the key-value pairs first, then process each one in turn.
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 ''