Look up a value in an incomplete list

I have keys and values like this:

keys = ["a", "b", "c", "d"];
vals = [3,4,2,1];

and I need to look up the values for x, but not all keys exist, e.g.:

x = ["c", "d", "e"];

which should result in:

y = [2, 1, NaN];

The problem I have is that strfind(), matches() yield empty arrays for "e", which leads to errors when looking up in vals.

Also, all arrays are >100k so I would like to avoid a for-loop.

Answer 1

[Stephen23]

Simpler and more efficient:

keys = ["a","b","c","d"];
vals = [3,4,2,1];
x = ["c","d","e"];
[idx,idy] = ismember(x,keys);
out = nan(size(x));
out(idx) = vals(idy(idx))

out = 1×3

2 1 NaN

Answer 2

[DGM]

How about:

keys = ["a", "b", "c", "d"];
vals = [1,2,3,4];

x = ["c", "d", "e"];

[~,y] = ismember(x,keys);
y(y==0) = NaN

y = 1×3

3 4 NaN

[clauper]

This looks promising. However, I need the values in vals, not the indices. I changed vals in the question such that the two can be distinguished.

[DGM]

Oof. I forgot about that.

keys = ["a", "b", "c", "d"];
vals = [11,22,33,44];

x = ["c", "d", "e"];

[m idx] = ismember(x,keys);
y = NaN(size(m));
y(m) = vals(idx(m))

y = 1×3

33 44 NaN

Answer 3

[clauper]

I figured it out:

keys = ["a", "b", "c", "d"];
vals = [3,4,2,1];
x = ["c", "d", "e"];


[~, idx] = ismember(x, keys);
keyExists = matches(x, keys);

y = nan(length(x),1);
y(keyExists) = vals(idx(idx~=0))

y = 3×1

2 1 NaN

I don't know if there is a more efficient way but this works.