4.2 If Statements

In the following example we define a function sign which determines the sign of a number. 

gap> sign:= function(n)
>        if n < 0 then
>           return -1;
>        elif n = 0 then
>           return 0;
>        else
>           return 1;
>        fi;
>    end;
function ( n ) ... end
gap> sign(0); sign(-99); sign(11);
0
-1
1

This example also introduces the if statement which is used to execute statements depending on a condition. The if statement has the following syntax.

  • if condition then statements elif condition then statements else statements fi

    There may be several elif parts. The elif part as well as the else part of the if statement may be omitted. An if statement is no expression and can therefore not be assigned to a variable. Furthermore an if statement does not return a value.

    Fibonacci numbers are defined recursively by f(1) = f(2) = 1 and f(n) = f(n-1) + f(n-2) for n ³ 3. Since functions in GAP may call themselves, a function fib that computes Fibonacci numbers can be implemented basically by typing the above equations. (Note however that this is a very inefficient way to compute f(n).) 

    gap> fib:= function(n)
>       if n in [1, 2] then
>          return 1;
>       else
>          return fib(n-1) + fib(n-2);
>       fi;
>    end;
function ( n ) ... end
gap> fib(15);
610

    There should be additional tests for the argument n being a positive integer. This function fib might lead to strange results if called with other arguments. Try inserting the necessary tests into this example.

    [Top] [Previous] [Up] [Next] [Index]

    GAP 4 manual
    February 2000