# Conditional structures/ALGOL 68

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Conditional structures/ALGOL 68 is part of Conditional Structures. You may find other members of Conditional Structures at Category:Conditional Structures.

### IF ~ THEN ~ ELSE ~ FI clause

<lang algol68>main: (

``` INT input := 2;
```
``` IF  3 = input THEN
# Do something #
FI;
```
``` IF  3 = input THEN
# Do something #
ELSE
# Do something ELSE #
FI
```

)</lang> One line predicates do not any braces: <lang algol68>IF cond THEN

``` ~
```

FI;

IF cond THEN

``` ~
```

ELSE

``` ~
```

FI;</lang> And these may be mixed: <lang algol68>IF cond THEN

``` ~
IF cond THEN
# multiple expressions #
ELSE
~
FI;
~
```

FI</lang> Or merged: <lang algol68>IF cond1 THEN

``` ~
```

ELIF cond2 THEN

``` # multiple expressions #
```

ELSE

``` ~
```

FI # etc #</lang>

### Short-circuit conditionals

The short-circuit evaluation of the ANDIF and ORELSE boolean operators are also often used for control flow. <lang algol68>IF node /= NULL ANDIF next OF node /= NULL ANDIF guarded_test(node) THEN

```  next OF node := next OF next OF node
```

FI</lang> Note that these are not part of the standard library and had to be defined by the user.

Neither the assignment nor guarded_test() will be called IF the previous two conditions aren't met. Other languages such as Pascal don't make that guarantee, and they certainly could not be user defined.

### CASE ~ IN ~ OUT ~ ESAC clause

<lang algol68>main: (

``` INT input := 42;
```
``` CASE input IN
~ # Do something, because input = 1 #
,
~ # Do something, because input = 2 #
,
~ # Do something, because input = 3 #
,
# fall through to the next statement IF there is no "OUT" #
OUT
~ # Do something ELSE. #
CASE
```

)</lang> The "CASE ~ IN ~ OUT ~ ESAC" is directly interchangeable with the "( ~ | ~ | ~ )", hence the above may be written as follows: <lang algol68>main: (

``` INT input := 42;
```
``` ( input |
~ # Do something, because input = 1 #
,
~ # Do something, because input = 2 #
,
~ # Do something, because input = 3 #
# fall through to the next statement IF there is no "OUT" #
|
~ # Do something ELSE. #
)
```

)</lang> Case statements may also be merged: <lang algol68>main:(

``` INT a1, a2, b1, b2, c99;
CASE 99 IN
a1,
a2
OUSE 99 IN
b1,
b2
OUT
c99
ESAC := 333
```

)</lang> In this case a1, a2, b1 and b2 remain undefined, but c99 ends up being half-evil.

Finally labels, and an EXIT "completion symbol" can be used to engineer the more familiar "switch" statement. <lang algol68>STRING food := (

``` CASE 666 IN a,b,c OUT else ESAC;
a:
"Apple"
EXIT
b:
"Banana"
EXIT
c:
"Carrot"
EXIT
else:
"Baked beans"
```

);

print((food))</lang>

The EXIT "completion symbol" here serves to yield the found value.

### Ternary ( ~ | ~ | ~ )

Conditionals in C can also be done with the ternary operator, ?:. ALGOL 68 has the "brief" equivalents ( ~ | ~ | ~ ) <lang algol68>main : (

``` INT input := 2;
INT output := (input = 2 | 42 | 4711);  # sets output to 42 #
INT output2 := (input = 3 | 42 | 4711); # sets output2 to 4711 #
```
``` PROC do_something = INT: random;
PROC do_something_else = INT: random;
input := (TRUE | do_something | do_something_else); # only calls do_something() #
```

)</lang> The "( ~ | ~ | ~ )" is directly interchangeable with the "IF ~ THEN ~ ELSE ~ FI" syntax. Hence the above could be written: <lang algol68>main : (

``` INT input := 2;
INT output := IF input = 2 THEN 42 ELSE 4711 FI;  # sets output to 42 #
INT output2 := IF input = 3 THEN 42 ELSE 4711 FI; # sets output2 to 4711 #
```
``` PROC do_something = INT: random;
PROC do_something_else = INT: random;
input := IF TRUE THEN do_something ELSE do_something_else FI; # only calls do_something() #
```

)</lang> A more complex example created by combining the above: <lang algol68>main: (

``` INT x,y,z;
CASE 2 IN x,y OUT z ESAC := IF 1+2=2 THEN 333 ELSE 666 FI
```

)</lang> In the result y becomes fully-evil, with x and z remaining undefined.