Get a number from an array of digits

Question

To split a number into digits in a given base, Julia has the digits() function:

julia> digits(36, base = 4)
3-element Array{Int64,1}:
 0
 1
 2

Answer 1

The answer seems to be written directly within the documentation of digits:

help?> digits
search: digits digits! ndigits isdigit isxdigit disable_sigint

  digits([T<:Integer], n::Integer; base::T = 10, pad::Integer = 1)

  Return an array with element type T (default Int) of the digits of n in the given base,
  optionally padded with zeros to a specified size. More significant digits are at higher
  indices, such that n == sum([digits[k]*base^(k-1) for k=1:length(digits)]).

So for your case this will work:

julia> d = digits(36, base = 4);

julia> sum([d[k]*4^(k-1) for k=1:length(d)])
36

And the above code can be shortened with the dot operator:

julia> sum(d.*4 .^(0:(length(d)-1)))
36