Multi-dimensional array in c

Question

I have a problem working with multi-dimensional array in C coming from PHP background.

I have an array msg which I am using as input for now. 1st element of array stores 'node_id', 2nd element stores 'actual message' and 3rd element holds 'controller_id'.

Task is handled by function handleSlotMessage(): This function should for each controller_id receive 3 input message (wait until it receives all 3 messages), upon receiving all 3 messages pass it further for processing.

For storing messages, I am trying to use a 'Multi-dimensional array' (which can be simply achieved in PHP): resultFromNodes but underlining my limited knowledge of 'C' I am unable to get this done.

For all the usage of resultFromNodes variable in handleSlotMessage function I am getting an error Subscribed value is not an array, pointer, or vector.

How can this be implemented in 'C' ? I tried answer from this to check if array is set or not.

#define UNINITIALIZED 0xcdcdcdcd


void handleSlotMessage();

int main(int argc, const char * argv[]) {

    int resultFromNodes[99][3] = {UNINITIALIZED};
    int i, j;
    int msg[10];

    // test data only
    for ( i = 1; i < 4; i++ ) {
        for ( j = 1; j < 4; j++ ) {
            msg[0] = j; // sender's id (node id)
            msg[1] = i*j; // message
            msg[2] = i; // controller's id (requester's id)

            handleSlotMessage(msg, *resultFromNodes);

        }
    }
    return 0;
}


void handleSlotMessage(int msg[], int resultsFromNodes[]) {

    // get controller id
    int controller_id = msg[2];
    int node_id = msg[0];
    int message = msg[1];
    int slot_count = 3;

    // check if the resultFromNodes already has controller_id set
    if (resultsFromNodes[controller_id] == UNINITIALIZED) {
        resultsFromNodes[controller_id][0] = slot_count;
    }
    if(resultsFromNodes[controller_id][0] == 0) {
        printf("Message Handled and result from Nodes sent for further processing! \n");
    } else {
        resultsFromNodes[controller_id][node_id] = message;
        printf("on node: %i  msg: %i controller: %i  \n\r", node_id, message, controller_id);
        resultsFromNodes[controller_id][0]--;
    }

    return;
}

EDIT: If this gives any one better idea, 'controller_id' ranges from 1-99 so instead of checking it is set or not it is feasible to check if it is within that range. I don't know if it helps in any way.

This int resultFromNodes[99][3] = {UNINITIALIZED}; most probably does not do what you might expect. It inits resultFromNodes[0][0] with 0xcdcdcdcd and all other elements with 0. — alk
– alk, Commented Jan 11, 2015 at 11:21
@alk all other element being assigned 0, since I could check if it is 0 or not as well. but the problem is, I cannot assign values to resultsFromNodes[controller_id][0], [1] or any other array, simple assignment shows an error. — ro ko
– ro ko, Commented Jan 11, 2015 at 11:30
In C reading an uninitialised variable invokes undefined behaviour. Do not do that. — alk
– alk, Commented Jan 11, 2015 at 11:32
Thanks, got the idea. I can intialize all array as 0 an carry on, it seems problem was with my function definition, where I used 1-D array for resultFromNodes — ro ko
– ro ko, Commented Jan 11, 2015 at 11:45
You're treating resultsFromNodes in handleSlotMessage where it is a declared parameter of type int[], as if it were the same declaration in main(). — WhozCraig
– WhozCraig, Commented Jan 11, 2015 at 11:47

Marian · Accepted Answer · 2015-01-11 11:38:02Z

3

You probably wanted to define your function as:

void handleSlotMessage(int msg[], int (*resultsFromNodes)[3]) { ... }

and to call it as:

handleSlotMessage(msg, resultFromNodes);

or equivalent:

handleSlotMessage(msg, &resultFromNodes[0]);

edited Jan 11, 2015 at 11:38

answered Jan 11, 2015 at 11:31

Marian

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Comments

Ameen · Accepted Answer · 2015-01-11 13:09:12Z

2

there is more than one problem I point some of them

Inside the function handleSlotMessage the argument resultsFromNodes should be declared like int *resultsFromNodes[4] in order to be able to send a multi-dimensional arrays of type int [any number][4] to it. the declaration would be like:
```
void handleSlotMessage(int msg[], int  resultsFromNodes[][4]){...}
```
to call the handleSlotMessage you call it without dereferencing just like:
```
handleSlotMessage(msg, resultFromNodes);
```

to use the resultFromNodes inside the function just like PHP:

resultFromNodes[i][j] = 0;
printf("%i", resultFromNodes[i][j]);

you can't compare subset array with a number or using the notation {1,2,3} like:

resultFromNodes[n] == 0xCDCDCDCD; // INVALID
resultFromNodes[n] == {0xCDCDCDCD,0xCDCDCDCD,0xCDCDCDCD}; // INVALID

Instead you have to compare each element on its own like:

resultFromNodes[n][0] == 0xCDCDCDCD && resultFromNodes[n][1] == 0xCDCDCDCD && 
resultFromNodes[n][2] == 0xCDCDCDCD && resultFromNodes[n][3] == 0xCDCDCDCD;

the last thing is that your code has an overflow when accessing the subset arrays of the variable resultFromNodes

you are storing the slot_count at resultFromNodes[controller_id][0] and the counter j represents node_id will have the values 1,2,3 hence 4 elements in total you have only 3 element:

resultFromNodes[controller_id][0] = slot_count;
resultFromNodes[controller_id][1] = node_id;    // j=1
resultFromNodes[controller_id][2] = node_id;    // j=2
resultFromNodes[controller_id][3] = node_id;    // j=3

edited Jan 11, 2015 at 13:09

answered Jan 11, 2015 at 12:31

Ameen

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2 Comments

ro ko Over a year ago

Thanks, I realized all of the error and fixed it, since I am new to c, I am real sloppy. Well thank you once again.

ro ko Over a year ago

also regarding your function declaration, I don't know why but it seems *resultsFromNodes needs to be within () int (*resultsFromNodes) [4]){...}

luser droog · Accepted Answer · 2015-01-11 11:21:40Z

1

        resultsFromNodes[controller_id][0] = slot_count;

resultsFromNodes is declared as a 1-D array, but you give it a second subscript [0].

answered Jan 11, 2015 at 11:21

luser droog

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1 Comment

ro ko Over a year ago

Thanks, it seems I had this error, resultFromNodes on function definition declared as 1-D array and few other mistake that I realized from other answer and comment. Thank you!

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