Having trouble with the thumbnails in iMovie

Question

I'm using iMovie on my Mac os x 10.9.4, and i upload video clips from my camera (Cannon Vixia) to i movie, but recently i noticed that iMovie has ben moving slower and slower, after that all of the thumbnails in my video library were just a blank blue thumbnail and would only show the content if i scrolled over them.
Also when i start on a new movie, it will not show me the video content of each clip, but will show me the audio waveforms. everything play's perfectly fine, but not being able to see what clips i'm using makes things exceptionally difficult.
I've considered that it may be that i have too many videos in iMovie, but shouldn't those videos take up memory on the computers hard drive itself?

Answer 1

          for (int i = 1; i < courses.length; i++)Should be
for (int i = 0; i < courses.length; i++)
Indices start at 0.
          worst = getCourseGrade(i);Should be
worst = i;
Kajok both of those make sense :)
I made those two changes:
for (int i = 0; i < courses.length; i++)
                    if(getCourseGrade(i) < getCourseGrade(worst) || worst == -1)
                         worst = i;
               }but I'm still getting -1 returned...?
Thank you all for the help!