I have an array, say A = [ 3 5 6 7 ]. I know I can get the maximum value of this array with max(A) and it returns 7, but how can I get the second max (6) from this array without sorting or removing the first maximum value? 

I have an array, say A = [ 3 5 6 7 ]. I know I can get the maximum value of this array with max(A) and it returns 7, but how can I get the second max (6) from this array without sorting or removing the first maximum value? 

second_max_value = max(A(A~=max(A)))

Here A(A~=max(A)) will be temporary array that not contain maximal value of original array. Than you receive maximum of this array