vector Entry lookup in matlab

Given a vector U n x 1 which contains the entries from 0,1,2,3 , I would like to create another vector V in char type such that the entry

0 in U will be '0' in V开发者_如何学编程

1 in U will be '1' in V

2 in U will be '12' in V

3 in U will be '123' in V

What is the optimal way to do in matlab rather scanning each individual entry in the vector and then uses switch case?

You can easily define a set of rules and index into it.

rules={'0','1','12','123'};
out=rules(A+1)

In the above, A is the vector you have.

Create an anonymous function to convert one element to the desired output and then use ARRAYFUN to apply this function to all of the inputs:

>> f = @(x) sprintf('%u', sum(10.^((x-1):-1:0) .* (1:x)));
>> x = 0:3

x =

     0     1     2     3

>> c = arrayfun(f, x, 'UniformOutput', 0)

c = 

    '0'    '1'    '12'    '123'

I think the simplest approach is to create a cell array 'C' containing your 4 string values, then index the array with U+1:

>> C = {'0' '1' '12' '123'};  %# Cell array with 4 strings corresponding to 0..3
>> U = [0 1 2 3 2 1 0];       %# Sample U vector
>> V = C(U+1)                 %# Index C with U+1

V = 

    '0'    '1'    '12'    '123'    '12'    '1'    '0'

And if you want V to be a single character string instead of a cell array of strings, you can do this instead:

>> V = [C{U+1}]

V =

01121231210

I'd say create a hashmap with the set of pairs you gave there. Every time you want to insert into V based on U, insert the value paired with the key that is the value of the entry of U into V. I.e. if U[0] = 2, then do V[0] = myMap.get(2), or whatever the MATLAB syntax is.

Here's a vectorized version which is idiomatic Matlab:

Nevertheless, this in effect does a linear scan 4 times. If you truly want more efficiency, write a C mex function.

V = cell(size(U));
V{U==0} = '0';
V{U==1} = '1';
V{U==2} = '12';
V{U==3} = '123';

Edit:

gnovice's solution is far superior. See above.