"generate and export the data of a semicircle with chosen radius"
SetDirectory[NotebookDirectory[]];
R=0.45;
x[t_]:=R*Cos[t];y[t_]:=R*Sin[t];
datasemicircle=Table[{x[t],y[t]},{t,0,Pi,0.05Pi}];
lp=ListPlot[datasemicircle];
pp=ParametricPlot[{x[t],y[t]},{t,0,Pi}];
Show[lp,pp]
Export["datasemicircle.dat",datasemicircle];

"Can you make the cirle to look really circular? To this end, you have to assure that the axes are of the same scale. Use AspectRatio->1 in ListPlot"
lp=ListPlot[datasemicircle,AspectRatio->1];
Show[pp,lp]

"Check whether the files is in your current Directory[]"
Directory[]
FileNames[]
FileExistsQ["datasemicircle.dat"]

"Now, import the file and name it 'rawdata' "
rawdata=Import["datasemicircle.dat"];
rawdata

ListPlot[rawdata]