I have this code:
elList = {"H", "C", "N", "O", "F", "Cl"}; els = {"Ca", "O", "C", "Si", "H"}; corr = ConstantArray[1.0, Length@els]; For[i = 1, i <= Length@els, i++, If[!MemberQ[elList, els[[i]]], corr[[i]] = 1.13]; ] What would be the best way to replace the loop, in other words, how to replace it in the Mathematica way?
My only idea was:
(MemberQ[elList, #] & /@ els) /. {True -> 1.0, False -> 1.13} Is there a better way or other way?
You can rewrite your idea using If as follows:
If[MemberQ[elList, #], 1., 1.13] & /@ els {1.13, 1., 1., 1.13, 1.}
However you may find on larger problems that repetitive use of MemberQ is not as fast as you would like, so consider a hash table in the form of an Association, or if using an older version of Mathematica a Dispatch table.
Create a list of Rule expressions using Thread:
rules = Thread[elList -> 1.0] {"H" -> 1., "C" -> 1., "N" -> 1., "O" -> 1., "F" -> 1., "Cl" -> 1.}
Convert into optimized forms with Association or Dispatch
asc = Association[rules]; disp = Append[rules, _ -> 1.13] // Dispatch; Apply the hash tables:
Lookup[asc, els, 1.13] Replace[els, disp, {1}] {1.13, 1., 1., 1.13, 1.} {1.13, 1., 1., 1.13, 1.}
Lookup the third parameter specifies the default value; in Replace the third parameter is the levelspec on which to operate.Recommended reading:
If speed is not you concern, may be this can be worth a try
common = Intersection[elList, els]; u = Map[ Position[els, # ] &, common ] ; corr[[ Flatten[u] ]] = 1.13 This can be fit in a single line, and is almost readable
corr[[ Flatten[ Position[els, # ] & /@ Intersection[elList, els] ] ]] = 1.13 and will seem even more nice if you use (elList esc inter esc els) in place of Intersection[elList,els] - the parentheses around the intersection are required, the precedence of Intersection in operator form seems to be low.
Here's a little one-liner without If, probably not fast on large lists:
corr = (! MemberQ[elList, #] & /@ els) /. {True -> 1.13, False -> 1.0} 