| This diagram depicts a program containing five examples of if statements. Two packages from the predefined environment are employed. |
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[ Table of Contents ]
If statements are used to build "conditional" or "branching" algorithms. They are compound statements, always using multiple lines of code and always using, as a minimum, the reserved words or combinations: if, then, and end if. Every if statement includes at least one Boolean expression, which evaluates to True or False.
General Form |
if boolean_expression_1 then
-- statement_sequence_1;
elsif boolean_expression_2 then
-- statement_sequence_2;
elsif boolean_expression_3 then
-- statement_sequence_3;
-- etc
else
-- statement_sequence_N;
end if;
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The elsif and else parts are optional, but for each one there must be at least one line of executable code. However, that line can be a null statement, which does nothing.
| This diagram depicts a program containing five examples of if statements. Two packages from the predefined environment are employed. |
|
Note that the "nested if statement" and the "if with logical operators" provide two ways of doing the same thing. The latter is easier to read. (See also the case statement version in the next section.)
Source Code Listing
------------------------------------------------------------------ -- This procedure displays messages about the current date, such -- as which half-year, quarter-year, or season we are in. ------------------------------------------------------------------ with Ada.Text_IO, Ada.Calendar; use Ada.Text_IO; procedure Show_Time_Of_Year is -- declarative part The_Time : Ada.Calendar.Time; The_Month : Ada.Calendar.Month_Number; The_Day : Ada.Calendar.Day_Number; type Month_Abr is (Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec); Month_Abbreviation : Month_Abr; begin -- executable part The_Time := Ada.Calendar.Clock; The_Month := Ada.Calendar.Month(The_Time); The_Day := Ada.Calendar.Day(The_Time); Month_Abbreviation := Month_Abr'Val(The_Month); if The_Month < 7 then -- simple if statement Put_Line("We are in the first half year."); end if; if The_Month < 7 then -- if-else statement Put_Line("We are in the first half."); else Put_Line("We are in the second half."); end if; if The_Month < 4 then -- if-elsif-else statement Put_Line("We are in the first quarter."); elsif The_Month < 7 then Put_Line("We are in the second quarter."); elsif The_Month < 10 then Put_Line("We are in the third quarter."); else Put_Line("We are in the fourth quarter."); end if; if The_Month < 3 then -- nested if statements Put_Line("The season is Winter."); elsif The_Month = 3 then if The_Day < 21 then Put_Line("The season is Winter."); else Put_Line("The season is Spring."); end if; elsif The_Month < 6 then Put_Line("The season is Spring."); elsif The_Month = 6 then if The_Day < 21 then Put_Line("The season is Spring."); else Put_Line("The season is Summer."); end if; elsif The_Month < 9 then Put_Line("The season is Summer."); elsif The_Month = 9 then if The_Day < 21 then Put_Line("The season is Summer."); else Put_Line("The season is Fall."); end if; elsif The_Month < 12 then Put_Line("The season is Fall."); elsif The_Month = 12 then if The_Day < 21 then Put_Line("The season is Fall."); else Put_Line("The season is Winter."); end if; end if; if (The_Month = 12 and The_Day >= 21) or -- if with logical operators (The_Month = 1) or (The_Month = 2) or (The_Month = 3 and The_Day < 21) then Put_Line("The season is Winter."); elsif (The_Month = 3 and The_Day >= 21) or (The_Month = 4) or (The_Month = 5) or (The_Month = 6 and The_Day < 21) then Put_Line("The season is Spring."); elsif (The_Month = 6 and The_Day >= 21) or (The_Month = 7) or (The_Month = 8) or (The_Month = 9 and The_Day < 21) then Put_Line("The season is Summer."); else Put_Line("The season is Fall."); end if; end Show_Time_Of_Year; ------------------------------------------------------------------ |
An alternative (more readable) way of coding the if-elsif-else example is to use a case statement, as illustrated in the next section. (That is why we included the Month_Abr type and Month_Abreviation variable.)
If the above program is run in July, it produces the following output:
We are in the second half. We are in the third quarter. The season is summer. The season is summer. |
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