Erlang if and case

Erlang if and case

The above find_max_and_min to find the maximum and minimum values of temperature. H ere's a new structure if. I f's syntax format is as follows:

if
    Condition 1 ->
        Action 1;
    Condition 2 ->
        Action 2;
    Condition 3 ->
        Action 3;
    Condition 4 ->
        Action 4
end

Note that there is no "before end"; ” 。 C onditions work the same way as guard, i.e. test and return success or failure. E rlang starts the test from the first condition until he finds a true branch of the test. T he actions after the condition are then performed, ignoring the other conditions and actions before end. R un-time errors occur if all conditions fail the test. O ne condition in which a test is constant is true. I t is often used as the last condition of if, i.e. when all conditions fail to test, the action after true is performed.

Here's an example of how if works:

-module(tut9).
-export([test_if/2]).

test_if(A, B) ->
    if 
        A == 5 ->
            io:format("A == 5~n", []),
            a_equals_5;
        B == 6 ->
            io:format("B == 6~n", []),
            b_equals_6;
        A == 2, B == 3 ->                      %That is A equals 2 and B equals 3
            io:format("A == 2, B == 3~n", []),
            a_equals_2_b_equals_3;
        A == 1 ; B == 7 ->                     %That is A equals 1 or B equals 7
            io:format("A == 1 ; B == 7~n", []),
            a_equals_1_or_b_equals_7
    end.

Test the program:

60> c(tut9).
{ok,tut9}
61> tut9:test_if(5,33).
A == 5
a_equals_5
62> tut9:test_if(33,6).
B == 6
b_equals_6
63> tut9:test_if(2, 3).
A == 2, B == 3
a_equals_2_b_equals_3
64> tut9:test_if(1, 33).
A == 1 ; B == 7
a_equals_1_or_b_equals_7
65> tut9:test_if(33, 7).
A == 1 ; B == 7
a_equals_1_or_b_equals_7
66> tut9:test_if(33, 33).
** exception error: no true branch found when evaluating an if expression
     in function  tut9:test_if/2 (tut9.erl, line 5)

Note that tut9:test_if(33,33) all test conditions to fail, which results in if_clause runtime error. R efer to the Guard sequence for more information about the guard test.

There is also a case structure in Erlang. T hink back to the previous convert_length function:

convert_length({centimeter, X}) ->
    {inch, X / 2.54};
convert_length({inch, Y}) ->
    {centimeter, Y * 2.54}.

The function can also be implemented with case, as follows:

-module(tut10).
-export([convert_length/1]).

convert_length(Length) ->
    case Length of
        {centimeter, X} ->
            {inch, X / 2.54};
        {inch, Y} ->
            {centimeter, Y * 2.54}
    end.

Both case and if have return values. T his means that, in the example above, the case statement either returns either s inch, X/2.54, or back to s {centimeter,Y*2.54} C ase statements can also be implemented with the guard clause. T he following examples can help you tell the two. I n this example, the input year gets the number of days for a specified month. T he year must be known because February of a leap year has 29 days, so the number of days in February must be determined based on the year.

-module(tut11).
-export([month_length/2]).

month_length(Year, Month) ->
    %% 被 400 整除的为闰年。
    %% 被 100 整除但不能被 400 整除的不是闰年。
    %% 被 4 整除但不能被 100 整除的为闰年。
    Leap = if
        trunc(Year / 400) * 400 == Year ->
            leap;
        trunc(Year / 100) * 100 == Year ->
            not_leap;
        trunc(Year / 4) * 4 == Year ->
            leap;
        true ->
            not_leap
    end,  
    case Month of
        sep -> 30;
        apr -> 30;
        jun -> 30;
        nov -> 30;
        feb when Leap == leap -> 29;
        feb -> 28;
        jan -> 31;
        mar -> 31;
        may -> 31;
        jul -> 31;
        aug -> 31;
        oct -> 31;
        dec -> 31
    end

70> c(tut11).
{ok,tut11}
71> tut11:month_length(2004, feb).
29
72> tut11:month_length(2003, feb).
28
73> tut11:month_length(1947, aug).
31