Java, 130 117 114 107107 105 bytes
This is the obvious solution. I don't usually golf in Java, but I was curious to see if Java could beat the Brainfuck version. Doesn't seem like I did a good job then.. Maybe one could use the new Map/Reduce from Java 8 to save some bytes.
Thanks to @flawr (13 bytes), @KevinCruijssen (79 bytes) and @DarrelHoffman (3 bytes)!
Golfed:
double d(float[]a,float[]b){float x=0,s;for(int i=-1;++i<ai=0;i<a.length;s=a[i]-b[i],x+=s*slength;x+=s*s);returns=a[i]-b[i++];return Math.sqrt(x);} Ungolfed:
public static double d(float[]afloat[] a,float[]b float[] b) { float x=0,s;fors; for(int i=-1;++i<ai=0; i<a.length;s=a[i]-b[i],length; x+=s*s); s = a[i] - b[i++]; return Math.sqrt(x); }