How can I create a dynamically sized array of structs?

Asked 2008-11-04T04:48:39.127

I know how to create an array of structs but with a predefined size. However is there a way to create a dynamic array of structs such that the array could get bigger?

For example:

    typedef struct
    {
        char *str;
    } words;

    main()
    {
        words x[100]; // I do not want to use this, I want to dynamic increase the size of the array as data comes in.
    }

Is this possible?

I've researched this: words* array = (words*)malloc(sizeof(words) * 100);

I want to get rid of the 100 and store the data as it comes in. Thus if 76 fields of data comes in, I want to store 76 and not 100. I'm assuming that I don't know how much data is coming into my program. In the struct I defined above I could create the first "index" as:

    words* array = (words*)malloc(sizeof(words));

However I want to dynamically add elements to the array after. I hope I described the problem area clearly enough. The major challenge is to dynamically add a second field, at least that is the challenge for the moment.

I've made a little progress however:

    typedef struct {
        char *str;
    } words;

    // Allocate first string.
    words x = (words) malloc(sizeof(words));
    x[0].str = "john";

    // Allocate second string.
    x=(words*) realloc(x, sizeof(words));
    x[1].FirstName = "bob";

    // printf second string.
    printf("%s", x[1].str); --> This is working, it's printing out bob.

    free(x); // Free up memory.

    printf("%s", x[1].str); --> Not working since its still printing out BOB even though I freed up memory. What is wrong?

I did some error checking and this is what I found. If after I free up memory for x I add the following:

    x=NULL;

then if I try to print x I get an error which is what I want. So is it that the free function is not working, at least

c dynamic memory-management

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3 Answers

Another option for you is a linked list. You'll need to analyze how your program will use the data structure, if you don't need random access it could be faster than reallocating.

answered 2008-11-04T05:08:20.927

Your code in the last update should not compile, much less run. You're passing &x to LoadData. &x has the type of **words, but LoadData expects words* . Of course it crashes when you call realloc on a pointer that's pointing into stack.

The way to fix it is to change LoadData to accept words** . Thi sway, you can actually modify the pointer in main(). For example, realloc call would look like

*x = (words*) realloc(*x, sizeof(words)*2);

It's the same principlae as in "num" being int* rather than int.

Besides this, you need to really figure out how the strings in words ere stored. Assigning a const string to char * (as in str2 = "marley\0") is permitted, but it's rarely the right solution, even in C.

Another point: non need to have "marley\0" unless you really need two 0s at the end of string. Compiler adds 0 tho the end of every string literal.

answered 2008-11-05T19:18:24.767

Here is how I would do it in C++

size_t size = 500;
char* dynamicAllocatedString = new char[ size ];

Use same principal for any struct or c++ class.

answered 2011-07-13T21:58:19.383

How can I create a dynamically sized array of structs?

3 Answers

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