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Quick answer:

float x = scan -> range_max;

The callback function receives a 'LaserScan' message as a parameter. In order to access its fields, you need to resolve/access it correctly.

Alternatively, were you to access, say the 'seq', int i = scan -> header.seq

In the answer linked, "scan->ranges[] are laser readings" refers to the data from the message that you would process.

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