Length of the ranges array in a LaserScan msg

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Hi,

I'm having this programming problem: I'm cycling through the ranges array in LaserScan messages and accessing its values. I'm using just this simple for cycle:

for (int i = 0; i < sizeof(scan->ranges)/sizeof(scan->ranges[0]); i++)

But the expression for getting the array length returns the value of 6, although the scan contains many more values (seen them through rostopic echo). What am I doing wrong? Isn't this the normal way of obtaining the length of an array in C++?