Find a value in a list given the a value contained in the first position of the list

One way could be

num = 89.9983 ;
Cases[list,{ num , x_} :> x]

(* {0.0000176696} *)

data//Pick[#[[All,2]], #[[All,1]], 89.9983]&

{0.0000176696}

%[[1]]

0.0000176696

If the use of Select is required, maybe:

Select[list, #[[1]]==89.9983 &][[1,2]]

0.0000176696

(But IMO Pick is a very powerful command in situations like this)

I’ve been really into using Extract lately. So, here’s what I would do:

Extract[Position[list,{OrderlessPatternSequence[89.9983,___]}]][list][[-1,-1]]

(* 0.0000176696 *)

If you know the underlying list structure, that is why we use [[-1,-1]]. If we don’t know the value, or, perhaps we only want to write one finder function, we can do the following:

FirstCase[list,{OrderlessPatternSequence[89.9983,v___]}:>v]

(* 0.0000176696 *)

Further generalization gives us:

valueCorrepondingTo[list_,num_]:=FirstCase[list,{OrderlessPatternSequence[num,v___]}:>v];

valueCorrepondingTo[list, 0.0000176696]

(* 89.9983 *)

I think this is really cool, this type of functionality and it is completely thanks to OrderlessPatternSequence.