Accessing Array Elements: i vs. .at(i)

[ineedhelpnow]

ok I am a little confused.
can both and .at(i) be used to same way to access an array element?

 

[MarkFL]

The [m].at(n)[/m] function automatically checks whether $n$ is within the bounds of valid elements in the vector, throwing an out_of_range exception if it is not (i.e., if $n$ is greater or equal than its size). This is in contrast with member operator[], that does not check against bounds.