ros::Time doesn't update in variable

rostime

asked 2013-09-08 09:40:14 -0600

benjaminabruzzo
45 ●3 ●4 ●8

I have trimmed most of the irrelevant code from this post.

int main(int argc, char **argv)
{
 float f_grabtime;
 do
 {
  ROS_INFO("\n###############################################");
  f_grabtime = ros::Time::now().toSec();
  ROS_INFO("f_grabtime = : %6.4f", f_grabtime);
  DummyFunction();
  ROS_INFO("ros::Time::now().toSec(); %6.4f", ros::Time::now().toSec());
  ROS_INFO("delta time; %6.4f", ros::Time::now().toSec() - f_grabtime);
 } 
 while (ros::ok());
return 0;
} //end main

When run, I get the output:

###############################################
[ INFO] [1378668866.709873391]: f_grabtime = : 1378668928.0000
[ INFO] [1378668866.709900160]: ros::Time::now().toSec(); 1378668866.7099
[ INFO] [1378668866.709920681]: delta time; -61.2901
[ INFO] [1378668866.881583005]: 
###############################################
[ INFO] [1378668866.881641471]: f_grabtime = : 1378668928.0000
[ INFO] [1378668866.881672380]: ros::Time::now().toSec(); 1378668866.8817
[ INFO] [1378668866.881696694]: delta time; -61.1183
[ INFO] [1378668867.037738183]: 
###############################################
[ INFO] [1378668867.037806140]: f_grabtime = : 1378668928.0000
[ INFO] [1378668867.037836070]: ros::Time::now().toSec(); 1378668867.0378
[ INFO] [1378668867.037860362]: delta time; -60.9622
[ INFO] [1378668867.201626074]: 
###############################################
[ INFO] [1378668867.201681767]: f_grabtime = : 1378668928.0000
[ INFO] [1378668867.201705114]: ros::Time::now().toSec(); 1378668867.2017
[ INFO] [1378668867.201727003]: delta time; -60.7983

A) why does f_grabtime have a value in the future?

B) why doesn't it update at every loop (as the raw ros::Time callout does)?

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answered 2013-09-08 21:10:29 -0600

tfoote

58457 ●128 ●550 ●526 http://www.ros.org/

You are just observing floating point precision errors. That is why we use 64 bit precision to store ROS time values.

I added this line to your program:

   ROS_INFO("delta between float and double %f %f float cast of time %f", f_grabtime, d_grabtime, (float) d_grabtime);

Where d_grabtime is a double instead of a float. And you get this output:

[ INFO] [1378710468.378363642]: delta between float and double 1378710528.000000 1378710468.378310 float cast of time 1378710528.000000

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Thanks. I'll make that change tomorrow and ensure that fixes the problem (I assume it will). I'll check this as the correct answer when I'm done.

benjaminabruzzo ( 2013-09-10 10:41:55 -0600 )edit

That fixed it, thanks!

benjaminabruzzo ( 2013-09-11 03:22:20 -0600 )edit

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answered 2013-09-08 19:32:20 -0600

Martin Peris
5625 ●34 ●83 ●98 http://www.linkedin.co...

If you are using ros::Time functions outside of a ROS node, as it seems to be the case, first you need to call:

ros:Time::init();

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Sorry, trimming the code seems to have made that less obvious. The code IS in a ROS node, though its good to know that it doesn't need to be, thanks.

benjaminabruzzo ( 2013-09-10 10:40:03 -0600 )edit

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