I'm having some trouble when parsing a text file. Each line of the text has a name followed after three float values. All of them are separated by a blankspace. What I want is to store the name in a string and the numbers in an array. I know I have to read each line using fgets and then strtok but the thing is I don't understand how strtok works. Do I have to call strtok four times? How do I assign each "piece" to my variables ?
Thank you for your time!
The strtok will search for the given tokens in a string. You must call it until the it returns NULL.
char *strtok(char *str, const char *delim)
The first call is done passing the string (char*) as the argument str and the remaining times are done passing NULL, as this will define that it should keep looking for the next token from that point onwards.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main()
{
char line[] = "name 1.45 2.55 3.65";
char* name;
double values[3] = { 0 };
char* ptr = NULL;
int i = 0;
ptr = strtok(line, " "); // Search for the first whitespace
if (ptr != NULL) // Whitespace found
{
/* 'line' is now a string with all the text until the whitespace,
with terminating null character */
name = calloc(1, strlen(line));
strcpy(name, line);
while ((i < 3) && (ptr != NULL))
{
ptr = strtok(NULL, " "); // Call 'strtok' with NULL to continue until the next whitespace
if (ptr != NULL) // Whitespace found
{
/* 'ptr' has now the text from the previous token to the found whitespace,
with terminating null character */
values[i] = atof(ptr); // Convert to double
}
i++;
}
}
printf("%s %lf %lf %lf\n", name, values[0], values[1], values[2]);
}
To understand the behavior of strtok() in C ANSI, I worte two code.
#include <stdio.h>
#include <string.h>
int main()
{
char str[101] = "This is";
char *pch;
printf("Splitting string %s into tokens : \n",str);
pch = strtok(str," ");`enter code here`
while(pch != NULL)
{
printf("%s\n",pch);
pch = strtok(NULL, " ");
}
return 0;
}
The result of This program is
Splitting string "This is " into tokens:
This
is
Next, I changed it a little bit.
#include <stdio.h>
#include <string.h>
int main()
{
char str[101] = ;
char *pch;
scanf("%s",str); //After launch program, I typed "This is "
str[strcspn(str,"\n")] = '\0'
printf("Splitting string %s into tokens : \n",str);
pch = strtok(str," ");`enter code here`
while(pch != NULL)
{
printf("%s\n",pch);
pch = strtok(NULL, " ");
}
return 0;
}
It prints
Splitting string "This" into tokens:
This
I can't understand why the second word is gone when I use stdin.
The problem isn't with strtok, but with your use of scanf and the "%s" format specifier. That format specifier reads space delimited strings, i.e you can not use "%s" to read anything with a space in it.
The natural solution is to use fgets instead, which you have already prepared for by "removing the newline" (which scanf would not usually read anyway).
It should have been pretty obvious that the strtok can't be involved, since you print the input string before even calling strtok.
I'm new to C.
I'm trying to eliminate ".,;?!" from a string using strtok and then to create a simple string without any punctuation marks, but it gives me a 'Segmentation fault' after compilation. Why and how to fix it ?
char simple_s[100];
char delim[20];
memset(simple_s,0,100);
memset(delim,0,100);
strcpy(delim,strtok(s,",.;:!? "));
while(delim != NULL) {
strcat(simple_s,delim);
strcpy(delim,strtok(NULL,",.;:!? "));
}
printf("%s",simple_s);
There are several errors in the code. First you zero too many bytes with
char delim[20];
memset(delim,0,100);
To avoid this error, you should use
char simple_s[100];
char delim[20];
memset(simple_s,0,sizeof(simple_s));
memset(delim,0,sizeof(delim));
Next, you have used the return value of strtok() before checking if it is NULL
strcpy(delim,strtok(s,",.;:!? "));
and from there you go on to test delim for being NULL instead of checking for a NULL pointer from strtok()
while(delim != NULL) {
strcat(simple_s,delim);
strcpy(delim,strtok(NULL,",.;:!? ")); // <--- copying from NULL pointer
}
but delim is not even necessary, you need to work with the pointer returned by strtok(). Putting this back together, I would have
char simple_s[100] = ""; // initialise string
char seps[] = ",.;:!? "; // added separators, so not to duplicate
char *tok; // added to receive value from strtok()
tok = strtok(s, seps);
while(tok) { // until `NULL` returned
strcat(simple_s, tok);
tok = strtok(NULL, seps);
}
printf("%s",simple_s);
Additionally, I have skipped over the string length checking. When you have it working, check that the new length of simple_s[] won't break, before you strcat() the next substring.
delim must be defined as a char *. Here is your code with a couple of bugs fixed:
#include <stdio.h>
#include <string.h>
#include <strings.h>
int main(int argc, char **argv)
{
char s[100];
char simple_s[100];
char *delim;
strcpy(s, "abc,def");
memset(simple_s,0,sizeof(simple_s));
delim = strtok(s,",.;:!? ");
while(delim != NULL) {
strcat(simple_s,delim);
delim = strtok(NULL,",.;:!? ");
}
printf("%s",simple_s);
return 0;
}
This outputs abcdef
I want to add string "ay" to each word by using both strtok and strncat. But there seemed to be a conflict somewhere that I cannot find. It only gives me the first word "Computeray" for an output. Help?
#include <stdio.h>
#include <string.h>
int main(void)
{
char str[] = "Computer science is hard";
char* Token;
char* work = "ay";
Token = strtok(str, " ");
while (Token != NULL)
{
strncat(Token, work, 2);
printf("%s", Token);
Token = strtok(NULL, " ");
}
return 0;
}
You're modifying the string (with strcat) and expecting strtok to still behave properly - that's not going to work. Instead of using strcat, just print the "ay" separately:
while (Token != NULL)
{
printf("%say ", Token);
Token = strtok(NULL, " ");
}
Even if it were working the way you'd like, you'd be overwriting a bunch of your input along the way. Probably not what you were going for - if you need to build up a whole new string, you should do it into a new buffer, instead of overwriting the input.
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 ''