So really in advance we wouldn't know whether it is in tension or compression so early id said oh yeah its going to be in compression but that is not always the case, you have to be careful, be precise, find out what the reactions are, if they come out to be positive then there in the direction that's shown on your diagram, if they come out to be negative they are in the opposite direction but in this case this hydrualic arm is indeed in compression, 'kay? 