I have a problem with this part of my code, I'm trying to read the lines of a file and cut only the first word of each line and then save it in an array.
Example:
two roads diverged in a yellow wood
and sorry i could not travel both
and be one traveler long i stood
and looked down one as far as i could
to where it bent in the undergrowth
and as a result I expect a vector like this: "two, and, and, and, to"
but I get this: "to, to, to, to, to".
My code
dictionary *load_word(int autor, dictionary *D_first)
{
FILE *date;
char line[LONG_MAX_LINE];
char exeption[4] = " \n\t";
char *word;
int j=0;
if (autor == 1)
{
if ((date = fopen("test.txt", "r")) == NULL)
{
perror("robert_frost.txt");
}
while (fgets(line, LONG_MAX_LINE, date ) != NULL)
{
word = strtok(line, exeption); /*first word*/
add_dictionary_first(D_first, j, word);
j++;
}
fclose(date);
}
return D_first;
}
void add_dictionary_first(dictionary *D, int cont, const char *value)
{
expand_dictionary(&D, 1);
D->Distribution[D->size-1]->cont = cont;
D->Distribution[D->size-1]->value = value;
}
The problem lies within this line (as Vlad from Moscow posted in the comments):
D->Distribution[D->size-1]->value = value;
This is just pointer assignment. That's not wrong per se, but depending on the
context, it is not what you want.
while (fgets(line, LONG_MAX_LINE, date ) != NULL)
{
word = strtok(line, exeption); /*first word*/
add_dictionary_first(D_first, j, word);
...
}
Here you call add_dictionary_first always with the same variable line. It is
an array but arrays decay into pointers when passing them as arguments to
functions. That means that all your D->Distribution[D->size-1]->value point to
the same location. The last line in your input file begins with to and that's why you get only
to.
You need to copy the string with strcpy.
man strcpy
#include <string.h>
char *strcpy(char *dest, const char *src);
The strcpy() function copies the string pointed to by src, including the terminating
null byte ('\0'), to the buffer pointed to by
dest. The strings may not overlap, and the destination string dest must be
large enough to receive the copy.
Because you haven't posted the structure I can only guess that value is
declared as char* (if it were char[] the compiler would have complained).
Option 1
D->Distribution[D->size-1]->value = malloc(strlen(value) + 1); // note the +1 here
if(D->Distribution[D->size-1]->value == NULL)
{
// error handling
}
strcpy(D->Distribution[D->size-1]->value, value);
Option 2
If strdup is available in your system
D->Distribution[D->size-1]->value = strdup(value);
if(D->Distribution[D->size-1]->value == NULL)
{
// error handling
}
In either case you would have to free the memory later.
So, I have an issue. I'm trying to only get the inside of a string as given by this example:
User input: insert("someWord")
And I want to first make sure that the user spelt insert(" correctly, then I want to copy the string contained inside the " ". As of now, I have a function with a parameter that is the full user input, and inside that function, I have the following:
method header(char *string){
char insert[]="insert(";
if((strncmp(string,insert,6)==0)
{
//the first part was right up to the "
//how do I now get the string contained between " "?
}
else
{ //invalid input
}
}
I'm not even 100% positive the strncmp method is comparing the first 6 letters of the two strings correctly.
sscanf(3) to the rescue:
char insert[31];
int matched = sscanf(string, "insert(\"%[^\"]30s\")", insert);
if (matched) printf("Got %s\n", insert);
This matches a string no larger than 30 characters that doesn't contain a " and is surrounded by insert(" and ").
strncmp will compare the exact number of characters or terminate when it see the first null.
So,
strncmp("inser","insert(",6) // false -- the first string too short
strncmp("insert","insert(",6) // true, the first 6 char match
strncmp("insert(","insert(",6) // true, first 6 char match
strncmp("insertxyz","insert(",6) // true, the first 6 chars match
strncmp("insert","inse",6) // false -- the second string too short
strncmp("append","insert",6) // false -- they just don't match
Note that "insert(" is actually 7 characters long (8 if you include the null byte), so comparing with strncmp(,,6) will not take the training '(' into account -- not sure if that is your problem.
In your case there is no real reason to use strncmp -- just use strcmp instead (or the sscanf solution suggested by a3f)
strchr, strncpy and simple pointer arithmetic can also do the trick, e.g.
method header(char *string){
char insert[]="insert(";
if (strncmp (string, insert, 6) == 0) {
char text[MAXC] = "", *p = string + 7, *ep = NULL;
if ((ep = strchr (p, ')')) && ep - p < MAXC)
strncpy (text, p, ep - p);
printf ("header text: '%s'\n", text);
}
else { //invalid input
}
}
A short example that prints the wanted text between the parenthesis (text cannot contain an embedded close parenthesis).
#include <stdio.h>
#include <string.h>
#define MAXC 128
void header (char *string);
int main (void) {
char string[] = "insert(headerinfo)";
header (string);
return 0;
}
void header (char *string)
{
char insert[] = "insert(";
if (strncmp (string, insert, 6) == 0) {
char text[MAXC] = "", *p = string + 7, *ep = NULL;
if ((ep = strchr (p, ')')) && ep - p < MAXC)
strncpy (text, p, ep - p);
printf ("header text: '%s'\n", text);
}
else { //invalid input
}
}
note: the nul-terminating byte is provided by virtue of initialization. If text is reused with strncpy within the same scope, you should affirmatively nul-terminate text for each subsequent use.
Example Use/Output
$ ./bin/headertxt
header text: 'headerinfo'
I have the following string
/foo123/bar123/card45/foofoo/1.3/
And I want to parse the number that follows the word "card", which in the example above would be 45. Should I use sscanf for this and if so, how would I go about doing so?
Thanks
Should I use sscanf for [XYZ problem]
No.
But you can use strstr and strtol instead:
const char *s = "/foo123/bar123/card45/foofoo/1.3/";
const char *p = "card";
const char *t = strstr(s, p);
int i = -1; // a negative number indicates a parse failure, for example
if (t != NULL) {
t += strlen(p);
char *end;
i = strtol(t, &end, 10);
if (!end || *end != '/') {
// parsing the number failed
}
}
Using strstr() followed by sscanf() will do the job. Suppose you have your source string in the character array source_string.Then use this:
char * ptr;
ptr = strstr(source_string,"card");
sscanf (ptr,"%*s %d",&number); //Sorry this is wrong!!
sscanf(ptr,"card%d",&number); //This is right!!
sscanf(ptr,"%*4s%d",&number); //This works too
printf("The card number is %d",number);
strstr() gets you the address where "card" begins.Then you pass that address to sscanf() as the source.The %*s reads "card" but then discards it.After this the %d reads the number following "card" and stores it in the integer variable number, which you then display using the printf().
How about using simple splitting function that cuts out your path by '/'?
Here is function that I use to split random char array by specific letter. It saves each parsed part into string vector.
vector<string> split(const char *str, char c)
{
vector<string> result;
while(1)
{
const char *begin = str;
while(*str != c && *str)
str++;
result.push_back(string(begin, str));
if(0 == *str++)
break;
}
return result;
}
So, you can call this function like this
vector<string> ParsedString;
ParsedString = split("Your/Random/path", '/');
Then, you can access to each index of ParsedString to see any of them has specific word that you are looking for.
Or, if the word that you are looking for is always located on 3rd or 4th from front, then you can only pick that one for searching.
string InterestedTarget = ParsedString[4];
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 ''