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i have connecter ping sensors to arduino uno, and want to know if after i Uploaded the schetch for ping sensors in my arduino, can he not run the program until the sensors have read a specific distance ? like the program is uploaded and arduino will not run it until 1 specific sensor read 3cm ? Thank you

As soon as the sketch is uploaded, the Arduino starts running. If it didn't, it could not know whether the sensor had detected the required value.

As for doing something specific only when the output from the sensor has been determined to exceed some value, yes that is possible. You need to define what is to happen when the sensor output exceeds/drops below some value, and what that value is.