# How can I find a more efficient solution to mapping characters to digits?

Given:

strings = {"send", "more", "money"};
letterMap = Flatten[Characters[strings]] // DeleteDuplicates
(*Example: {"s", "e", "n", "d", "m", "o", "r", "y"}*)
numberMap = RandomSample[Range[0, 9], 8]
(*Example: {7, 3, 6, 9, 5, 0, 4, 1}*)


I would like to convert these strings to integers as determined by the mapping specified by letterMap and numberMap.

For example, "send" should be equal to 7369 because s->7, e->3, n->6, d->9.

I have done this three different ways, but none of my solutions are computationally efficient enough. Can you find a more efficient method?

From Fastest to Slowest:

Using Associations and Lookup (Method 2):

StringToNumber2[str_String, letters_List, numbers_List] :=
(
Clear[rules, assoc];
rules = Thread[Rule[letters, numbers]];
assoc = Association[rules];
FromDigits[Lookup[assoc, #] & /@ Characters[str]]
)


Using Rules and ReplaceAll (can be improved a little using Thread like above) (Method 0):

StringToNumber0[str_String, letterMap_List, numberMap_List] :=
(
Clear[rules];
rules = Rule @@@ Partition[Riffle[letterMap, numberMap], 2];
FromDigits[Characters[str]] /. rules
)


Using Position and Part (Method 1):

StringToNumber1[str_String, letterMap_List, numberMap_List] :=
(
FromDigits[
numberMap[[Flatten[Position[letterMap, #]]]] & /@ Characters[str] //
Flatten]
)


Expected output, given inputs above:

{7369, 5043, 50631}


Most importantly, do you have any tips in improving efficiency of Mathematica code when faced with problems like this?

(Update after receiving wonderful feedback!) Henrik's Compiled SparseArray method was definitely the quickest, although I'm wondering how much improvement of other methods could be realized by utilizing Compile!

Below was how I measured the performance of each method:

(*See how long the algorithms take to analyze 100,000 pieces of data*)
Column[{Table[StringToNumber0[strings, letters, numbers], 100000]; //
AbsoluteTiming,
Table[StringToNumber1[strings, letters, numbers], 100000]; //
AbsoluteTiming,
Table[StringToNumber2[strings, letters, numbers], 100000]; //
AbsoluteTiming,
Table[StringToNumber3[strings, letters, numbers], 100000]; //
AbsoluteTiming,
Table[StringToNumber4[strings, letters, numbers], 100000]; //
AbsoluteTiming
}]


Where Method 3 = Carl's method, and Method 4 = Henrik's method.

{
{{4.79445, Null}}, (*Method 0*)
{{8.75677, Null}}, (*Method 1*)
{{2.04422, Null}}, (*Method 2*)
{{1.75599, Null}}, (*Method 3*)
{{0.900393, Null}}, (*Method 4*)
}

• Using //AbsolutTiming suggests StringToNumber2 is the fastest. But I'm curious about 2 things: (1) Do you need to convert back - assuming this is some sort of secret decoder ring method, and (2) Does it matter if there are more than 8 (or especially more than 9) unique characters? – JimB Aug 13 '18 at 3:10
• @JimB, 1) No converting back needed, 2) The particular problem dictates that there will be at most 10 unique characters with a numeric value ranging from 0..9. – tjm167us Aug 13 '18 at 3:51
• Your test timings are about 20 times slower than mine, probably because you don't give a list of strings as the first argument of StringToNumber3 and StringToNumber4. – Carl Woll Aug 14 '18 at 4:54
• @CarlWoll, I do give a list of strings, but only a list of 3! The problem requires that the number mapping changes, but the letter map to stay constant and the input strings to be constant and a length of 3. Given that, are there any optimizations you can think of? – tjm167us Aug 14 '18 at 13:30

## 2 Answers

Update

The following approach is much faster when the number of words is small and the number of mappings is large:

stringsToNumberMappings[words_List, letterMap_, samples_]:=Module[
{
len = Length @ letterMap, lrule, wvector
},

lrule = Thread @ Rule[letterMap, Range@len];

wvector = Transpose @ Normal @ Table[
SparseArray[
Thread @ Rule[Characters[w] /. lrule, 10^Range[StringLength[w]-1, 0, -1]],
len
],
{w, words}
];

samples . wvector
]


Comparison with previous answer:

words = {"send", "more", "money"};
letterMap = Flatten[Characters[words]] // DeleteDuplicates;
samples = Table[RandomSample[0 ;; 9, 8], 10^5];

r1 = stringsToNumberMappings[words, letterMap, samples]; //AbsoluteTiming
r2 = StringsToNumbers[words, letterMap, #]& /@ samples; //AbsoluteTiming

r1===r2


{0.010555, Null}

{2.2923, Null}

True

Original answer

I assume the computational inefficiency you're encountering is performing your replacement on lots of words? In that case, you could make use of the Listable attribute of StringReplace:

StringsToNumbers[str_List, letters_List, numbers_List] := With[
{d = Thread[letters -> ToString /@ numbers]},

FromDigits /@ StringReplace[str, d]
]


Simple example:

words = {"send", "more", "money"};
StringsToNumbers[words, letterMap, {7, 3, 6, 9, 5, 0, 4, 1}]


{7369, 5043, 50631}

Bigger example with a million words:

big = RandomChoice[words, 10^6];
StringsToNumbers[big, letterMap, {7, 3, 6, 9, 5, 0, 4, 1}]; //AbsoluteTiming


{0.908794, Null}

• Curiously enough, stringsToNumberMappings yielded worse results (5.7s for 100,000 iterations). The problem requires a stringList of three words, a letterMap, and a numberMap. The letterMap are the unique characters in the stringList (must be 10 or less), and the numberMap is a list of n numbers, were n is the number of unique characters in stringList (Length[letterMap]). This mapping is performed with the stringList and letterMap constant, with the number map varying through all permutations of n numbers. To approximate this, I used Table to perform 100,000 calculations. – tjm167us Aug 16 '18 at 0:43
• @tjm167us You're comparing the wrong thing. If permutations is your list of 100000 permutations, then you should use stringsToNumberMappings[words, letterMap, permutations] and not Table[stringsToNumberMappings[words, letterMap, p], {p, permutations}]. I did not optimize the speed of creating wvector because there are only 3 words, and I only need to create wvector once. If you use Table, though, wvector gets created 100000 times, which is slow. – Carl Woll Aug 16 '18 at 1:09

Recently, I found out for myself that ToCharacterCode is very efficient in transforming strings to numbers. In particular, ToCharacterCode turns strings into a packed arrays which is very good for performance.

Here the preparation (stealing a bit from Carl Woll).

SeedRandom[123];
words = {"send", "more", "money"};
letterMap = DeleteDuplicates@Flatten[Characters[strings]];
numberMap = RandomSample[Range[0, 9], 8];
big = RandomChoice[words, 10^6];


We create a packed array as lookup table via SparseArray and perform the actual lookup together with conversion to numbers in one go with the following compiled function

lookup = Compile[{{lookuptable, _Integer, 1}, {idx, _Integer, 1}},
Sum[10^(Length[idx] - i) CompileGetElement[lookuptable,
CompileGetElement[idx, i]], {i, 1, Length[idx]}],
CompilationTarget -> "C",
RuntimeAttributes -> {Listable},
Parallelization -> True,
RuntimeOptions -> "Speed"
];

StringsToNumbers2[strings_, letterMap_, numberMap_] := lookup[
Normal[SparseArray[ToCharacterCode[letterMap] -> numberMap]],
ToCharacterCode[strings]
];


On my machine, method is approximately twice as fast as using StringReplace and FromDigits:

result1 = StringsToNumbers[big, letterMap, numberMap]; // RepeatedTiming // First
result2 = StringsToNumbers2[big, letterMap, numberMap]; // RepeatedTiming // First
result1 === result2


0.810

0.40

True