How to select based on a partial string match in Mathematica - string

Question

Say I have a matrix that looks something like this:
{{foobar, 77},{faabar, 81},{foobur, 22},{faabaa, 8},
{faabian, 88},{foobar, 27}, {fiijii, 52}}
and a list like this:
{foo, faa}
Now I would like to add up the numbers for each line in the matrix based on the partial match of the strings in the list so that I end up with this:
{{foo, 126},{faa, 177}}
I assume I need to map a Select command, but I am not quite sure how to do that and match only the partial string. Can anybody help me? Now my real matrix is around 1.5 million lines so something that isn't too slow would be of added value.

Answer 1

Here is a starting point:
data={{"foobar",77},{"faabar",81},{"foobur",22},{"faabaa",8},{"faabian",88},{"foobar",27},{"fiijii",52}};
{str,vals}=Transpose[data];
vals=Developer`ToPackedArray[vals];
findValPos[str_List,strPat_String]:=
Flatten[Developer`ToPackedArray[
Position[StringPosition[str,strPat],Except[{}],{1},Heads->False]]]
Total[vals[[findValPos[str,"faa"]]]]

Answer 2

Here is yet another approach. It is reasonably fast, and also concise.
data =
{{"foobar", 77},
{"faabar", 81},
{"foobur", 22},
{"faabaa", 8},
{"faabian", 88},
{"foobar", 27},
{"fiijii", 52}};
match = {"foo", "faa"};
f = {#2, Tr # Pick[#[[All, 2]], StringMatchQ[#[[All, 1]], #2 <> "*"]]} &;
f[data, #]& /# match
{{"foo", 126}, {"faa", 177}}
You can use ruebenko's pre-processing for greater speed.
This is about twice as fast as his method on my system:
{str, vals} = Transpose[data];
vals = Developer`ToPackedArray[vals];
f2 = {#, Tr # Pick[vals, StringMatchQ[str, "*" <> # <> "*"]]} &;
f2 /# match
Notice that in this version I test substrings that are not at the beginning, to match ruebenko's output. If you want to only match at the beginning of strings, which is what I assumed in the first function, it will be faster still.

Answer 3

make data
mat = {{"foobar", 77},
{"faabar", 81},
{"foobur", 22},
{"faabaa", 8},
{"faabian", 88},
{"foobar", 27},
{"fiijii", 52}};
lst = {"foo", "faa"};
now select
r1 = Select[mat, StringMatchQ[lst[[1]], StringTake[#[[1]], 3]] &];
r2 = Select[mat, StringMatchQ[lst[[2]], StringTake[#[[1]], 3]] &];
{{lst[[1]], Total#r1[[All, 2]]}, {lst[[2]], Total#r2[[All, 2]]}}
gives
{{"foo", 126}, {"faa", 177}}
I'll try to make it more functional/general if I can...
edit(1)
This below makes it more general. (using same data as above):
foo[mat_, lst_] := Select[mat, StringMatchQ[lst, StringTake[#[[1]], 3]] &]
r = Map[foo[mat, #] &, lst];
MapThread[ {#1, Total[#2[[All, 2]]]} &, {lst, r}]
gives
{{"foo", 126}, {"faa", 177}}
So now same code above will work if lst was changed to 3 items instead of 2:
lst = {"foo", "faa", "fii"};

Answer 4

How about:
list = {{"foobar", 77}, {"faabar", 81}, {"foobur", 22}, {"faabaa",
8}, {"faabian", 88}, {"foobar", 27}, {"fiijii", 52}};
t = StringTake[#[[1]], 3] &;
{t[#[[1]]], Total[#[[All, 2]]]} & /# SplitBy[SortBy[list, t], t]
{{"faa", 177}, {"fii", 52}, {"foo", 126}}
I am sure I have read a post, maybe here, in which someone described a function that effectively combined sorting and splitting but I cannot remember it. Maybe someone else can add a comment if they know of it.
Edit
ok must be bedtime -- how could I forget Gatherby
{t[#[[1]]], Total[#[[All, 2]]]} & /# GatherBy[list, t]
{{"foo", 126}, {"faa", 177}, {"fii", 52}}
Note that for a dummy list of 1.4 million pairs this took a couple of seconds so not exactly a super fast method.