Want to pass a single char pointer from a double pointer

问题

I have to write a function which takes in 2 double pointers (both to char type). The first double pointer has a string of query values and the 2nd one has stopwords. The idea is to eliminate the stopwords from the query string and return all the words without those stopwords.

For example

Input - query: “the”, “new”, “store”, “in”, “SF”

    stopwords: “the”, “in”

OUTPUT new store SF

I have written the following code while trying to use strtok which takes in only single pointers to char types. How do I access the contents of a double pointer?

Thanks

#include <stdio.h>

void remove_stopwords(char **query, int query_length, char **stopwords, int stopwords_length) {
    char *final_str;

    final_str = strtok(query[0], stopwords[0]);
    while(final_str != NULL)
    {
        printf("%s\n", final_str);
        final_str = strtok(NULL, stopwords);
    }

}

回答1:

For simplicity's sake, you can assume a double pointer to be equivalent to a 2d array (it is not!). However, this means that you can use array-convention to access contents of a double pointer.

#include <stdio.h>
#include <string.h>

char *query[5] = {"the","new","store","in","SF"};
char *stopwords[2] = {"the","in"};
char main_array[256];

void remove_stopwords(char **query,int query_length, char **stopwords, int stopwords_length);

int main()
{
    remove_stopwords(query,5,stopwords,2);
    puts(main_array);
    return 0;
}

void remove_stopwords(char **query,int query_length, char **stopwords, int stopwords_length)
{
    int i,j,found;
    for(i=0;i<query_length;i++)
    {
        found=0;
        for(j=0;j<stopwords_length;j++)
        {
            if(strcmp(query[i],stopwords[j])==0)
            {
                found=1;
                break;
            }
        }
        if(found==0)
        {
            printf("%s ",query[i]);
            strncat(main_array,query[i],strlen(query[i]));
        }
    }
}

Output: new store SF newstoreSF

回答2:

@Binayaka Chakraborty's solution solved the problem but I thought it might be useful to provide an alternative that used pointers only and showed appropriate use of strtok(), the use of which may have been misunderstood in the question.

In particular, the second parameter of strtok() is a pointer to a string that lists all the single-character delimiters to be used. One cannot use strtok() to split a string based on multi-character delimiters, as appears to have been the intention in the question.

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

void remove_stopwords(char *query, char **stopwords) {
    char *final_str = strtok(query, " ");
    while(final_str != NULL) {
        int isStop = 0;
        char **s;
        for (s = stopwords; *s; s++) {
            if (strcmp(final_str,*s) == 0) {
                isStop = 1;
            }
        }
        if (!isStop) printf("%s ", final_str);
        final_str = strtok(NULL, " ");
    }
}

int main() {
    const char *q = "the new store in SF";
    char *query = malloc(strlen(q)+1);
    /* We copy the string before calling remove_stopwords() because
       strtok must be able to modify the string given as its first
       parameter */
    strcpy(query,q);
    char *stopwords[] = {"the", "in", NULL};
    remove_stopwords(query,stopwords);
    return 0;
}

The approach shown here also avoids the need to hard code the sizes of the arrays involved, which therefore reduces potential for bugs.

来源：https://stackoverflow.com/questions/17499671/want-to-pass-a-single-char-pointer-from-a-double-pointer

标签

string

double-pointer