(2012•密云縣一模)給出定義:若
m-<x≤m+(其中m為整數(shù)),則m叫做離實數(shù)x最近的整數(shù),記作{x}=m.在此基礎(chǔ)上給出下列關(guān)于函數(shù)f(x)=|x-{x}|的四個命題:
①函數(shù)y=f(x)的定義域為R,值域為
[0,];
②函數(shù)y=f(x)的圖象關(guān)于直線
x=(k∈Z)對稱;
③函數(shù)y=f(x)是周期函數(shù),最小正周期為1;
④函數(shù)y=f(x)在
[-,]上是增函數(shù).
其中正確的命題的個數(shù)為( �。�