Creating a fixed length output string with sprintf containing floats

Question

I'm trying to create a file which has the following structure:
- Each line has 32 bytes - Each line looks like this format string: "%10i %3.7f %3.7f\n"

My Problem is the following: When i have a negative floating point numbers the line gets longer by one or even two characters because the - sign does not count to the "%3.7f".

Is there any way to do this more nicely than this?

if( node->lng > 0 && node->lat > 0 ) { 
    sprintf( osm_node_repr, "%10i %3.7f %3.7f\n", node->id, node->lng, node->lat );
} else if (node->lng > 0 && node->lat < 0) {
    sprintf( osm_node_repr, "%10i %3.7f %3.6f\n", node->id, node->lng, node->lat );
} else if (node->lng < 0 && node->lat > 0) {
    sprintf( osm_node_repr, "%10i %3.6f %3.7f\n", node->id, node->lng, node->lat );
} else if ( node->lng < 0 && node->lat < 0 ) { 
    sprintf( osm_node_repr, "%10i %3.6f %3.6f\n", node->id, node->lng, node->lat );
}

Thanks for your Answers,
Andreas

Answer 1

You can force the sign to be printed, too with the + prefix: printf( "+3.6f", 1.0 ); will result in a fixed size printout.

(courtesy to the handiest printf documentation I ever saw).

Answer 2

Why not just write a binary file, where the sign bit isn't a concern? Added benefit is that (in general) you'll be writing 12 bytes vs 32 for each item (a green solution :-). After all, you lose precision when doing a sprintf and an atof on the other side.

If this isn't viable,

int len_lng = lng < 0 ? 6: 7;
int len_lat = lat < 0 ? 6: 7;
char fmt[128];
sprintf(fmt, "%%10i %%3.%df %%3.%df\n", len_lng, len_lat);
sprintf(osm_node_repr, fmt, node->id, node->lng, node->lat);

Answer 3

I wouldn't use sprintf() for this. It looks like something that belongs in Node::operator>>(ostream&).