Pass a 2D Array Index into a Function?

Question

I have a loop that populates a 2D array, and I want to pass the values created by this population, into a function to do other calculations with it. I'm a beginner at C++, so clear explanations would help a lot. Here is my code:

for (int car = 1; car <= 27; car++) {

    int test[27][3] = {{car, mpg[car], speed[car]}};

    float speed = speed[car];

    timeGen(speed);

    cout << car << "\t" << mpg[car] << "\t" << speed[car] << endl;
}

This is the timeGen function:

float timeGen(float x)
{
int distance = 50;
float speed = x;
float time = distance/x;
return time;
}

It seems as though everything will work fine, but what happens is that I get an error saying "subscript requires array or pointer type." I'm a little confused as to what they mean. Is it telling me to create a pointer to this index, and then call the pointer in the timeGen function? An explanation would be great!

Thank you very much. Also, the values mpg , speed , are taken from another part of my code which works fine. Instructions on how to fix the issue I'm having right now, would be great!

Answer 1

Even if you have an array named speed before defining float speed , it goes out of scope right after this definition. Try this:

float Cur_speed = speed[car];
timeGen(Cur_speed);

or

timeGen(speed[car]); // without the float speed

Another thing is, at each iteration you are creating a new instance of test . It looks like what you really want is to have a single test array:

int test[27][3];
for (int car = 0; car < 27; car++) {
 test[car][0] = car;
 test[car][1] = mpg[car];
 test[car][2] = speed[car];

Make sure that mpg is an array with 27 elements, and always use indices from 0 to 26. Same for the speed array. When we say test[car][0] , we mean the first element of test[car] , which has a total of 3 elements due to your definition. Essentially, the lines test[car][X] = Y; do the same thing you want to do with test[27][3] = {{car, mpg[car], speed[car]}} , however that syntax only works when you first declare an array. And the way you write it initializes all 27 elements with the same data.

Answer 2

You need to define float car_speed rather than float speed . It will resolve your error.