How to make what function I use in c be controlled by a variable

Question

This is my first year of vex. I am taking on the role of programmer. I have had this idea for rapid autonomous creation, recording the driver. Instead of the usual array/debugger dump of raw streams of power levels, I had the idea of extracting functions from driver movement. I wont go into the details, and I can code it myself, but I need some help.

There is one thing I am unable to do simply because of my lack of coding experience.

I want to create a for loop that checks every joystick button one by one.

For example:

struct button
{
    bool pressed;
}

for(int i = 0; i>12; i++) //12 is number of buttons on the joystick
{
    struct button button<cycle through buttons>;
}

I want there to then be:

struct button button6U;
struct button button6D;
struct button button6R;
etc.

Then, I want this:

for(int i = 0; i>12; i++) // 12 is number of buttons on the joystick
{
    if(VexRT[<currentButton>])
    {
        button<currentButton>.pressed = true;
    }
}

I have no idea how to do this, with a wildcard modifing the actual variable name I am writing to.

A couple thoughts: A for statement would have no idea how to advance the order of joystick buttons. So something I think I might need is:

orderOfButtons
{
    VexRT[6U];
    VexRT[6D];
    VexRT[6R];
    // etc.
}

I just cant seem to figure out how to have a variable defining what VexRT[]button I am reading from.

Any help would be appreciated! Thanks.

If you cannot easily build an array of [whatever those things are], to index, then use a case switch, (and fix your loop control, as pointed out by @WeatherVane). — Martin James, Mar 08 '18 at 23:13

score 0 · Answer 1 · answered Mar 08 '18 at 23:16

So, what you want is to sample the user input (at some specified rate), then record it into an array, then play that back at a later time? If you have functions that drive the VEX (I'm not familiar with that), each of which are associated with an input, you can use an array of function pointers to create your output.

#define MAX_ACTION 12
#define MAX_RECORDED 200

// Declare your array of function pointers
int (*action[MAX_ACTION])(void);
// Declare your stored array of recorded actions
int recorded[MAX_RECORDED];
...

// Assign function pointers to associated functions
action[0] = go_forward;
action[1] = turn_right;
...

// Record your actions into some array
while (...)
{
    // Record the action
    recorded[i++] = get_action();
    // Sample delay
}
...

// Playback the actions
for (i=0;i<RECORDED;i++)
{
    (*action[recorded[i]])();
    // delay here
}

P.S. Your for loop is backward (i<12 not i>12).

score 0 · Accepted Answer · answered Mar 08 '18 at 23:19

Sounds like you want an array:

#define NUMBER_OF_BUTTONS 12
...
struct button VexRT[NUMBER_OF_BUTTONS];

If you want to use symbolic constants to refer to specific buttons in the array, you can use an enumeration:

enum btn_id { BTN_6U, // maps to 0
              BTN_6D, // maps to 1
              BTN_6R, // maps to 2
              ...
            }

Enumeration constants are represented as integers, and by default they start at 0 and increment by 1. You can initialize them to different values if you want, and multiple enumeration constants can map to the same value. I take advantage of this when I want to identify a "first" and "last" enumeration for looping, like so:

enum btn_id {
  BTN_6U,
  BTN_FIRST = BTN_6U, // both BTN_FIRST and BTN_6U will map to 0
  BTN_6D,
  BTN_6R,
  ...
  BTN_whatever,
  BTN_LAST        
};

Thus, VexRT[BTN_6U] maps to VexRT[0], VexRT[BTN_6D] maps to VexRT[1], etc.

Note that this way, you don't have to loop through all the buttons just to set one:

enum btn_id currentButton = BTN_6D;
...
VexRT[currentButton].pressed = true;

If you do want to loop through the whole set, you can use

for ( enum btn_id i = BTN_FIRST; i < BTN_LAST; i++ )
{
  VexRT[i].pressed = false;
}

score 0 · Answer 3 · answered Mar 09 '18 at 00:15

I think you are trying to access the events coming from the joystick. You can just loop through the array of values and record them. I think the channels on the joystick are simply accessed like: vexRT[x] where x is 1-12. If you just want to store the latest value from each channel you could do this:

int value[12];
for(i=0; i<12; i++)
{
    value[i] = vexRT[i];
}

If you want to store all of the values so that you can map them or play them back or something then you will need a more complex data structure to store them, like a list of the value arrays.

I also have found documentation that says the values are accessed by like vexRT[Chx] where x is 1-12, so you could alternatively create a string and use it to access the joystick channels inside your loop:

string *s = (char *)malloc(5*sizeof(char)); //5 is the max length of the null terminated string
for() . . . {
    sprintf(s,"Ch%d", i);
    value[i] = vertRT[s];
}

How to make what function I use in c be controlled by a variable

3 Answers3