CODEWITHSUNDEEP
erlang
anishjp
anishjp

Reputation: 115

Erlang: Explain a piece of code

Can someone help me to understand this piece of Erlang code

to_price_file(Line, OutputFile) ->
  Fields = csv:parse_line(Line),
  [SupplierID, ProductCode, Description, DeliveryDate, CostPrice, UnitCount] = Fields,
  Product = calculate_product(list_to_integer(SupplierID),list_to_integer(ProductCode),
                              Description, **date_utils:parse_date(DeliveryDate)**,
                              list_to_integer(CostPrice), list_to_integer(UnitCount)),
  ok = write_pricefile(Product, OutputFile),
  ok.

another sub-function parse_date (below) is called. 

parse_date(DateString) ->
  Tokens = string:tokens(DateString, "/"),
  **[Year, Month, Day] = [list_to_integer(Str) || Str <- Tokens],
  {Year, Month, Day}.**

I don't understand, what the commands in bold letters do in the sub-function.

Thanks, Anish

Upvotes: 0

Views: 109

Answers (1)

rvirding
rvirding

Reputation: 20916

The function parse_date/1 assumes that the date string has the format "Year/Month/Day":

parse_date(DateString) ->
    Tokens = string:tokens(DateString, "/"),
    [Year, Month, Day] = [list_to_integer(Str) || Str <- Tokens],
    {Year, Month, Day}.

It first calls string:tokens/2 which returns a list of the separate fields, "/" is the separator string. It then does a list comprehension which for every element in Tokens calls list_to_integer/1 and returns a list of the values. It then uses pattern matching to split the list into its separate parts and returns a tuple with the values. The values of the variables in an example run could be:

DateString = "2013/03/08"
Tokens = ["2013","03","08"]
[Year,Month,Date] = [2013,3,8]
{2013,3,8}

List comprehensions are quite common and often a very concise way of applying an operation to every element in a list, like lists:map/2, with filtering options (not used here).

Note that in ISO standard the date should be written as 2013-03-08. :-)

Upvotes: 6

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