Allocating memory for an array of structs

Question

This is my code for a program that counts the words in the standard input and orangizes them into a histogram. There is an array of structs called wordArray, and I don't know exactly how to allocate memory for it. I understand there are probably other problems and variables that I haven't used yet, but I just want to know how to fix the error I keep getting at compile time:

countwords.c: In function 'main':  
countwords.c:70:22: error: incompatible types when assigning to type 'WordInfo' 
from type 'void *'  
    wordArray[nWords] = malloc(sizeof(WordInfo));
                      ^

Source:

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

struct WordInfo {
    char * word;
    int count;
};

typedef struct WordInfo WordInfo;

int maxWords;
int nWords = 0;
WordInfo*  wordArray;

#define MAXWORD 100
int wordLength;
char word[MAXWORD];
FILE * fd;
int charCount;
int wordPos;

void toLower(char *s) {
    int slen = 0;
    while (*(s + slen) != '\0') {
        if (*(s + slen) < 'a') *(s + slen) += 'a' - 'A';
        slen++;
    }
}

// It returns the next word from stdin.
// If there are no more more words it returns NULL.
static char * nextword() {
    char * word = (char*)malloc(1000*sizeof(char));
    char c = getchar();
    int wordlen = 0;
    while (c >= 'a' && c <= 'z') {
        *(word + wordlen) = c;
        wordlen++;
        c = getchar();
    }
    if (wordlen == 0) return NULL;
    return word;
}

int main(int argc, char **argv) {
    if (argc < 2) {
        printf("Usage: countwords filename\n");
        exit(1);
    }

    char * filename = argv[1];
    int wordfound = 0;
    fd = fopen(filename, "r");
    char * next = nextword();
    while (next != NULL) {
        int i;
        for (i = 0; i < nWords; i++) {
            if (strcmp((wordArray[i]).word, next)) {
                wordArray[i].count++;
                wordfound = 1;
                break;
            }
        }
        if (!wordfound) {
            wordArray[nWords] = malloc(sizeof(WordInfo));
            strcpy(next, wordArray[nWords].word);
            wordArray[nWords].count++;
            nWords++;
        }
    }

}

You like global variables, huh? – Fiddling Bits Mar 31 '15 at 00:56 — Fiddling Bits, Mar 31 '15 at 00:56

score 1 · Answer 1 · answered Mar 31 '15 at 05:54

1

To allocate space for an array of nWords elements, use

wordArray = malloc(nWords * sizeof(*WordInfo));

answered Mar 31 '15 at 05:54

mlp

809
7
21

clearlight · Answer 2 · 2015-03-31T00:55:49.113

Cast malloc as the type you're returning.
Remove subscript for the pointer.

Your objective is to allocate memory to hold the array and then set the pointer to it. One problem you'd have doing what you were doing is, if you you try to save the address malloc returns in a specific 'slot' of an array you haven't even created yet you will get an access error because wordArray[n] doesn't refer to any memory yet.

wordArray is pointer type variable you're trying to make point to the memory you allocate.

 wordArray = (WordInfo *)malloc(sizeof(WordInfo));

Then you can access wordArray with subscripts.

E.g. wordArray[n]

In C pointers can be accessed via subscripts and arrays can be referenced with pointers. They are different representations and syntaxes for the pretty much the same thing.

No, you don't cast the `malloc`(unless you're writing c++, in which case you should avoid `malloc` entirely), see: http://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc — Alex Díaz, Mar 31 '15 at 01:13

Allocating memory for an array of structs

2 Answers2