Something like:
dataX = -1;
dataY = -1;
while dataX<0
a=input('Please enter the X co-ordinate of the bottom left hand corner of the space: ');
dataX=a;
if dataX < 0
disp('An invalid value of x was entered');
else
disp('valid values of x co-ordinates were entered');
end
end
while dataY<0
a=input('Please enter the Y co-ordinate of the bottom left hand corner of the space: ');
dataY=a;
if dataY < 0
disp('An invalid value of Y was entered');
else
disp('valid value of Y co-ordinate was entered');
end
end
