Task to create flexible (dynamic memory allocation) array

Question

We've been given a task to write a dynamic data structure in C. I'm still incredibly new to C and I would really appreciate it if someone could give me some pointers (lol) on what I'm doing wrong / what I should read more about.

Here's what I have so far:

flexarray.h

#ifndef FLEXARRAY_H
#define FLEXARRAY_H

typedef struct flexarrayrec flexarray;

flexarray *flexarray_new();
extern void flexarray_add(flexarray *a, char item);
extern void flexarray_set(flexarray *a, char item, int index);
extern char flexarray_get(flexarray *a, int index);
extern void flexarray_trim(flexarray *a);
extern void flexarray_print(flexarray *a);

#endif  /* FLEXARRAY_H */

flexarray.c

#include "flexarray.h"
#include "mylib.h"

#define DEFAULT_CAPACITY 10

typedef struct flexarray{
    int size;
    int capacity;
    char *array;
};

flexarray *flexarray_new(){
    flexarray a;
    a->capacity = DEFAULT_CAPACITY;
    a->size = 0;
    a->array = (char*) erealloc (a ,a->capacity * sizeof(char)); 
    return a;
}

void grow(flexarray a){
    a->capacity = a->capacity * 2;
    a->array = (int*) erealloc (a ,a->capacity * sizeof(int)); 
}

void flexarray_add(flexarray a, char item){
    if(a->size == a->capacity) grow(a);
    a->array[a->size - 1] = item;
    a->size++;    
}

void set(flexarray a, char c, int index){
    if(index < 0 || index  > a->size) return;
    a->array[index] = c;
}

char get(flexarray a, int index){
    if(index < 0 || index  > a->size) return;
    return a->array[index];
}


void flexarray_trim(flexarray a){
    if(a->size == a->capacity) return;
    a->capacity = a->size;
    a->array = (char*) erealloc (a, a->capacity * sizeof(char)); 
}

void flexarray_print(flexarray a){
    int i;
    printf("[");
    for(i = 0; i < a->size - 1; i++){
        printf("%c,", a->array[i]);
    }
    printf("%c]\n", a->array[i + 1]);
}

Thank you

EDIT Current compile log:

flexarray.c:10:1: warning: useless storage class specifier in empty declaration [enabled by default]
flexarray.c: In function 'flexarray_new':
flexarray.c:13:15: error: storage size of 'a' isn't known
flexarray.c: At top level:
flexarray.c:20:21: error: parameter 1 ('a') has incomplete type
flexarray.c:25:30: error: parameter 1 ('a') has incomplete type
flexarray.c:31:20: error: parameter 1 ('a') has incomplete type
make[2]: *** [build/Debug/MinGW_Actual-Windows/flexarray.o] Error 1
make[1]: *** [.build-conf] Error 2
make: *** [.build-impl] Error 2


BUILD FAILED (exit value 2, total time: 1s)

What is the question? What doesn't work? What doesn't compile? — unexpectedvalue
– unexpectedvalue, Commented May 3, 2012 at 15:48
You probably have to write a small main() function that will test your data structure. With that main you can test compilation and debug... — eyalm
– eyalm, Commented May 3, 2012 at 15:52
@eyalm I've excluded the main function, as it's not relevant to the question. — rtheunissen
– rtheunissen, Commented May 3, 2012 at 15:57
@ssteinberg I'm getting a variety of compiling errors (which I'll add to the question now). — rtheunissen
– rtheunissen, Commented May 3, 2012 at 16:00

wildplasser · Accepted Answer · 2012-05-03 16:25:34Z

void flexarray_add(flexarray a, char item){
    if(a->size == a->capacity) grow(a);
    a->array[a->size - 1] = item;
    a->size++;    
}

Should be:

void flexarray_add(flexarray *a, char item){
    if(a->size == a->capacity) grow(a);
    a->array[a->size] = item;
    a->size++;    
}

And:

void flexarray_print(flexarray a){
    int i;
    printf("[");
    for(i = 0; i < a->size - 1; i++){
        printf("%c,", a->array[i]);
    }
    printf("%c]\n", a->array[i + 1]);
}

Could be:

void flexarray_print(flexarray *a){
    int i;
    printf("[");
    for(i = 0; i < a->size; i++){
        printf("%c,", a->array[i]);
    }
    printf("]\n" );
}

Additional stylistic hints: for sizes and indexes try to use unsigned types as much as you can.

struct buff {
     size_t size;
     size_t used;
     char *data;
     };

The advantage of this is that attempts to use negative indices ( b->data[b->used - 1] = item; ) will fail misarably and fast (instead of corrupting your memory slowly).

Also: you should test for the success/failure of realloc()

UPDATE2: it also appears there are some asterixes missing.

Martin James · Accepted Answer · 2012-05-03 16:11:01Z

1

flexarray *flexarray_new(){
    flexarray a;
    return a; // not a flexarray *
}

..and

a->array = (char*) erealloc (a ,a->capacity * sizeof(char));

What does 'erealloc()' do?

answered May 3, 2012 at 16:11

Martin James

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1 Comment

Martin James Over a year ago

@wildplasser - there are quite a few to go at :)

Yefim Dinitz · Accepted Answer · 2012-05-03 16:43:50Z

0

your typedefs are wrong -- you either want to write

typedef struct flexarray { ... } flexarray;

and remove the typedef from the header, or

 struct flexarrayrec { ... }; // no typedef

and keep the tyepdef struct flexarrayrec flexarray; in the header.

answered May 3, 2012 at 16:43

Yefim Dinitz

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1 Comment

rtheunissen Over a year ago

Brilliant, this was one of the main things that confused me.

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