Message sent to turtlesim does not execute.

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I am sending a velocity command message to turtlesim from a C++ program. If I put the message in my main(), it does not appear to execute even though it shows up as a rostopic. If I put the the message in a 'for' or 'while' loop for some number of iterations, it will execute. I thought that (according to the tutorials) that turtlesim executed each velocity command for one second. Can anyone tell me why a single command does not execute for the one second interval?

Running lucid, electric.

Thanks.