'Ui' was not declared in this scope

[anon73444976]

@shanren

Serial.print(sonar[i].ping_cm());
    Serial.print("cm ");

    Ui = sonar[i].ping_cm();

You really should check the return type of ping_cm - 255xm.isnt that far, and a sensor could return a valid, bigger value.
And you really should ping just once, and store the result.