create a statement conditional on if index is in bounds

lets say A is a matrix [4 2 3 1 5 0]. during a while loop with j=j+1 on each loop, I would like to say if A(j) is out of bounds then the program needs to break the while loop. If you have any questions let me know. Thanks for your help!

[Walter Roberson]

A(j) is out of bounds with respect to what? Some other matrix? Or are you asking about whether j is out of bounds with respect to indexing A ?