How would I find the element number of a certain point x along a bar broken into y # of elements? (Shape Functions FEA)

I am performing FEA on a 205mm bar broken 1-205 equal length elements (code must work for any number of elements). I need to find the displacement at the point x=100. I plan to use a shape function to interpolate using the nodal displacements of the element that this point lies on. However, I am not sure how I would calculate which element the point lies on (or which nodes it is in between).