# Revision history [back]

### How to make a callback function for range topic

Hello,

I am subscribing to a range message but am unsure what to do. I need to save the range value as a double into a global array variable. I have:


void front(const sensor_msgs::Range &msg)
{
data[0] = ???;
}


I am unsure was goes in the ??? obviously. I think it is something like msg->range.size() but I am not sure. Is this correct?

Thanks