Currently I'm reading each character from the user and storing it into a char array called str. From there I'm trying to use a pointer to loop through the string until it sees a space, once a space is seen I want to take the characters already and create an array of strings. Is that possible? Reasons why I'm doing this is because I later want to use an execlp function to execute a process after my initial program was executed.
If you want to split the string into tokens separated by delimiters you could use the strtok function.
An example would be:
#include <stdio.h>
#include <string.h>
int main(void)
{
int i, n;
char str[] = "Hello World";
char *token[4], *act_token;
token[0] = strtok(str, " ");
n=1;
while(n<4 && (act_token=strtok(NULL, " ")))
{
token[n] = act_token;
n++;
}
for(i=0;i<n;i++)
{
printf("%d: %s\n", i, token[i]);
}
return 0;
}
Related
#include <string.h>
#include <stdio.h>
#include <unistd.h>
#define MAX_LIMIT 20
int main () {
printf("Shell> ");
char str1[MAX_LIMIT];
fgets(str1, MAX_LIMIT, stdin);
char delim[] =" ";
char *parsed;
int index = 0;
char *cmd[index];
parsed = strtok(str1,delim);
while( parsed != NULL) {
cmd[index] = parsed;
index++;
parsed = strtok(NULL, delim);
}
cmd[index] = NULL;
int wow = fork();
if(wow == 0){
execvp((char*)cmd[0],cmd);
}
return(0);
}
Everything above is working fine except execvp()
cmd[0] receives string 'ls', technically execvp() should display the list but it doesn't.
If I replace cmd[0] with 'ls', it works.
You have two main issues here.
First when you declare your array as char *cmd[index];, the size of the array is not tied to current value of index as index changes. It sets the size to the current value of index which is 0. Creating an array of size 0 invokes undefined behavior. You need to set a fixed size for the array that will be big enough for your needs.
char *cmd[MAX_LIMIT];
The other problem is your choice of delimiters. The fgets function reads a line of text including the newline at the end of the input. So whichever parameter is read last will have a \n at the end of it. To fix this, add \n to the delimiter list.
char delim[] =" \n";
so this is a code that reads 3 strings (orig // test1 // orig_copy) from 2 different files (firstline // secondline)**and calls divide_string to use strtok and take tokens and store them in **(token_orig // token_test // token_orig_copy),
--> this is the problem :
- when i put the three lines in main it does compile and take token from all 3 strings and "Done ." in the end.
-but when i try the next three lines (notice how i changed "HAHAHAH" to "HAHAHAHA", that little changing changes everything and make the program stops at printf("for the string number two :"); .
i hope i cleared the problem
PS : you can past copy the program so you can compile yourself easily
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
const char s[4] = " ,.";
int divide_string(char* thestring,char** destination)
{
int i=0;
char* token=strtok(thestring,s);
destination[i]=malloc(sizeof(token)+1);
strcpy(destination[i],token);
i++;
printf("the word %d is 'tokened' \n",i);
while(token!=NULL)
{
token =strtok(NULL,s);
if (token != NULL)
{
destination[i]=malloc(sizeof(token)+1);
strcpy(destination[i],token);
printf("the word %d is 'tokened' \n",i);
++i;
}
}
return i;
}
void main ()
{ //TRY THESE THREE LINES THAT WORKS<-----------------------------
char orig[]= "does work HAHAHAH";
char orig_copy[] = "does work HAHAHAH";
char test1[]="does work HAHAHAH";
// char orig[]= "doesnt work HAHAHAHA";
// char orig_copy[] = "doesnt work HAHAHAHA";
// char test1[]="doesnt work HAHAHAHA";
char *token_orig[81];
char *token_test[81];
char *token_orig_copy[81];
strcpy(orig_copy,orig);
printf("for string number one : \n");
int max_orig = divide_string(orig,token_orig);
printf("for string number two : \n");
int a = divide_string(orig_copy,token_orig_copy);
printf("for string number three : \n");
int max_test = divide_string(test1,token_test);
printf("%s-",token_orig[0]);
printf("%s-",token_orig[1]);
printf("%s-\n",token_orig[2]);
printf("%s-",token_orig_copy[0]);
printf("%s-",token_orig_copy[1]);
printf("%s-\n",token_orig_copy[2]);
printf("%s-",token_test[0]);
printf("%s-",token_test[1]);
printf("%s-\n",token_test[2]);
printf("done .");
return 0;
}
Since token is a pointer, sizeof(token) gives you the size of the pointer variable (4 or 8 bytes probably), NOT the number of chars in the string it points to! You want:
strlen(token) + 1
instead (+1 for the \0).
About the only time sizeof is useful for character strings is literals like:
sizeof("Hello World")
Ok, so I'm going to explain my program.
It takes a text file that's setup as such: in pairs, first line being the title of an experiment, and the second line being 10 numbers separated by spaces. It saves the first lines of pairs in *experiments and the second lines of pairs in data. The last line is *** END *** which is what it's supposed to end with.
For some reason *** END *** doesn't end the program. Any ways I can fix this? I'm assuming it's because fgets gives str blank spaces (99 chars total) so that the string in quotes will never be equal to str?
Thanks.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main()
{
int var;
int i=0,j,k;
char seps[] = " ";
char *experiments[20];
int data[10][20];
char str[100]; // make sure that this size is enough to hold the single line
char *ptr, *token;
int no_line=1;
while(fgets(str,100,stdin) != NULL && strcmp(str,"*** END ***"))
{
if(no_line % 2 == 0)
{
k=0;
token = strtok (str, seps);
while (token != NULL)
{
sscanf (token, "%d", &var);
data[i][k++] = var;
token = strtok (NULL, seps);
}
i++;
/*read integer values from the string "str" using sscanf, sscanf can be called in a loop with %d untill it fails */
}
else
{
ptr = strdup(str);
experiments[i] = ptr;
/*strore string in your variable "experiments" , before copying allocate a memory for the each entry */
}
no_line++;
}
for(j=0;j<i;j++)
{
printf("%s",experiments[j]);
for(k=0;k<10;k++)
{
printf("%d ",data[j][k]);
}
printf("\n");
}
}
You're declaring i here ...
int i,j,k;
... and using it here ...
data[i][k++] = var;
Nowhere do you initialize i. Also, why does data need to be a 2D array? Can't it just be a 1D array?
int data[10];
...
data[k++] = var;
From this code, int i seems to be declared, but not initialized?
data[i][k++] = var;
It may be helpful to use Eclipse or Code Block IDE to try small testable codes because it has all sorts of syntax and error checking features.
I tried using strncmp but it only works if I give it a specific number of bytes I want to extract.
char line[256] = This "is" an example. //I want to extract "is"
char line[256] = This is "also" an example. // I want to extract "also"
char line[256] = This is the final "example". // I want to extract "example"
char substring[256]
How would I extract all the elements in between the ""? and put it in the variable substring?
Note: I edited this answer after I realized that as written the code would cause a problem as strtok doesn't like to operate on const char* variables. This was more an artifact of how I wrote the example than a problem with the underlying principle - but apparently it deserved a double downvote. So I fixed it.
The following works (tested on Mac OS 10.7 using gcc):
#include <stdio.h>
#include <string.h>
int main(void) {
const char* lineConst = "This \"is\" an example"; // the "input string"
char line[256]; // where we will put a copy of the input
char *subString; // the "result"
strcpy(line, lineConst);
subString = strtok(line,"\""); // find the first double quote
subString=strtok(NULL,"\""); // find the second double quote
printf("the thing in between quotes is '%s'\n", subString);
}
Here is how it works: strtok looks for "delimiters" (second argument) - in this case, the first ". Internally, it knows "how far it got", and if you call it again with NULL as the first argument (instead of a char*), it will start again from there. Thus, on the second call it returns "exactly the string between the first and second double quote". Which is what you wanted.
Warning: strtok typically replaces delimiters with '\0' as it "eats" the input. You must therefore count on your input string getting modified by this approach. If that is not acceptable you have to make a local copy first. In essence I do that in the above when I copy the string constant to a variable. It would be cleaner to do this with a call to line=malloc(strlen(lineConst)+1); and a free(line); afterwards - but if you intend to wrap this inside a function you have to consider that the return value has to remain valid after the function returns... Because strtok returns a pointer to the right place inside the string, it doesn't make a copy of the token. Passing a pointer to the space where you want the result to end up, and creating that space inside the function (with the correct size), then copying the result into it, would be the right thing to do. All this is quite subtle. Let me know if this is not clear!
if you want to do it with no library support...
void extract_between_quotes(char* s, char* dest)
{
int in_quotes = 0;
*dest = 0;
while(*s != 0)
{
if(in_quotes)
{
if(*s == '"') return;
dest[0]=*s;
dest[1]=0;
dest++;
}
else if(*s == '"') in_quotes=1;
s++;
}
}
then call it
extract_between_quotes(line, substring);
#include <string.h>
...
substring[0] = '\0';
const char *start = strchr(line, '"') + 1;
strncat(substring, start, strcspn(start, "\""));
Bounds and error checking omitted. Avoid strtok because it has side effects.
Here is a long way to do this: Assuming string to be extracted will be in quotation marks
(Fixed for error check suggested by kieth in comments below)
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(){
char input[100];
char extract[100];
int i=0,j=0,k=0,endFlag=0;
printf("Input string: ");
fgets(input,sizeof(input),stdin);
input[strlen(input)-1] = '\0';
for(i=0;i<strlen(input);i++){
if(input[i] == '"'){
j =i+1;
while(input[j]!='"'){
if(input[j] == '\0'){
endFlag++;
break;
}
extract[k] = input[j];
k++;
j++;
}
}
}
extract[k] = '\0';
if(endFlag==1){
printf("1.Your code only had one quotation mark.\n");
printf("2.So the code extracted everything after that quotation mark\n");
printf("3.To make sure buffer overflow doesn't happen in this case:\n");
printf("4.Modify the extract buffer size to be the same as input buffer size\n");
printf("\nextracted string: %s\n",extract);
}else{
printf("Extract = %s\n",extract);
}
return 0;
}
Output(1):
$ ./test
Input string: extract "this" from this string
Extract = this
Output(2):
$ ./test
Input string: Another example to extract "this gibberish" from this string
Extract = this gibberish
Output(3):(Error check suggested by Kieth)
$ ./test
Input string: are you "happy now Kieth ?
1.Your code only had one quotation mark.
2.So the code extracted everything after that quotation mark
3.To make sure buffer overflow doesn't happen in this case:
4.Modify the extract buffer size to be the same as input buffer size
extracted string: happy now Kieth ?
--------------------------------------------------------------------------------------------------------------------------------
Although not asked for it -- The following code extracts multiple words from input string as long as they are in quotation marks:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(){
char input[100];
char extract[50];
int i=0,j=0,k=0,endFlag=0;
printf("Input string: ");
fgets(input,sizeof(input),stdin);
input[strlen(input)-1] = '\0';
for(i=0;i<strlen(input);i++){
if(input[i] == '"'){
if(endFlag==0){
j =i+1;
while(input[j]!='"'){
extract[k] = input[j];
k++;
j++;
}
endFlag = 1;
}else{
endFlag =0;
}
//break;
}
}
extract[k] = '\0';
printf("Extract = %s\n",extract);
return 0;
}
Output:
$ ./test
Input string: extract "multiple" words "from" this "string"
Extract = multiplefromstring
Have you tried looking at the strchr function? You should be able to call that function twice to get pointers to the first and second instances of the " character and use a combination of memcpy and pointer arithmetic to get what you want.
Well, I declared a global array of chars like this char * strarr[];
in a method I am tokenising a line and try to put everything into that array like this
*line = strtok(s, " ");
while (line != NULL) {
*line = strtok(NULL, " ");
}
seems like this is not working.. How can I fix it?
Thanks
Any number of things could be going wrong with the code you haven't shown us, such as undefined behaviour by strtoking a string constatnt, or getting your parameters wrong when calling the function.
But the most likely problem from the code we can see is the use of *line instead of line, assuming that line is of type char *.
Use the following code as a baseline:
#include <stdio.h>
#include <string.h>
int main (void) {
char str[] = "My name is paxdiablo";
// Start tokenising words.
char *line = strtok (str, " ");
while (line != NULL) {
// Print current token and get next word.
printf ("[%s]\n", line);
line = strtok(NULL, " ");
}
return 0;
}
This outputs:
[My]
[name]
[is]
[paxdiablo]
and should be easily modifiable into something you can use.
Be aware that, if you're trying to save the character pointers returned from strtok (which would make sense for using *line), they are transitory and will not be what you expect after you're done. That's because modifications are made in-place within the source string. You can do it with something like:
#include <stdio.h>
#include <string.h>
int main (void) {
char *word[4]; // The array of words.
size_t i; // General counter.
size_t nextword = 0; // For preventing array overflow.
char str[] = "My name is paxdiablo";
// Start tokenising.
char *line = strtok (str, " ");
while (line != NULL) {
// If array not full, duplicate string to array and advance index.
if (nextword < sizeof(word) / sizeof(*word))
word[nextword++] = strdup (line);
// Get next word.
line = strtok(NULL, " ");
}
// Print out all stored words.
for (i = 0; i < nextword; i++)
printf ("[%s]\n", word[i]);
return 0;
}
Note the specific size of the word array in that code above. The use of char * strarr[] in your code, along with the message tentative array definition assumed to have one element is almost certainly where the problem lies.
If your implementation doesn't come with a strdup, you can get a reasonably-priced one here :-)