Conditionals

Operators

Conditional statements evaluate to true or false only. The most common conditional operators are == (equal), != (not equal), > (greater than), >= (greater than or equal to), < (less than), and <= (less than or equal to). You can also define the operators as abbreviations: EQ, NEQ, GT, GTE, LT, and LTE.

a = 1;
if( a == 1 )
if( a > 2 )
if( a < 2 )
if( a != 2 )
if( a >= 1 )
if( a <= 1 )

Some instructions return a true or false, so they're used in conditional statements, for example, IsArray which is true only when the variable is an "array". Structures have an instruction named structKeyExists() or keyExists() which returns true if a key is present in a structure. Strings can also be used for conditional operations by checking the .length() member function.

a = [1,3];

if( isArray( a ) ){
    // work on the array
}

produce = {
    grapes     = 2,
    lemons     = 1,
    eggplants  = 6
};

if( produce.keyExists( "grapes" ) ){
    // eat a grape
    produce.grapes--;
}

Also integers can be evaluated as true or false. In BoxLang, 0 (zero) is false and any other integers are true.

<bx:if 1>I am true so will show</bx:if>

<bx:if -2>I am true so will show</bx:if>

<bx:if 0>I am false so will not show</bx:if>

If, Else If, & Else

Why do we have conditional statements? Most often it's to control conditional instructions, especially if / else if / else expressions. Let's write an example by adding a method to our PersonalChef.bx class:

class accessors=true{

    property name="status";

    function init(){
        status = "The water is not boiling yet.";

        return this;
    }

    function water_boiling( numeric minutes ){
        if( arguments.minutes < 7 ){
            status = "The water is not boiling yet.";
        } else if ( arguments.minutes == 7 ){
            status = "It's just barely boiling.";
        } else if ( arguments.minutes == 8 ){
            status = "It's boiling!";
        } else {
            status = "Hot! Hot! Hot!";
        }

        return this;
    }

}

Try this example using 5, 7, 8 and 9 for the values of minutes.

chef = new PersonalChef();

for( i in [ 5, 7, 8, 9 ] ){
    chef.water_boiling( i );
    systemOutput( chef.getStatus() );
}

• When the minutes is 5, here is how the execution goes: Is it true that 5 is less than 7? Yes, it is, so print out the line The water is not boiling yet..

• When the minutes is 7, it goes like this: Is it true that 7 is less than 7? No. Next, is it true that 7 is equal to 7? Yes, it is, so print out the line It's just barely boiling.

• When the minutes is 8, it goes like this: Is it true that 8 is less than 7? No. Next, is it true that 8 is equal to 7? No. Next, is it true that 8 is equal to 8? Yes, it is, so print out the line It's boiling!.

Lastly, when total is 9, it goes:" Is it "true" that 9 is less than 7?

No. Next, is it true that 9 is equal to 7? No. Next, is it true that 9 is equal to 8? No. Since none of those are true, execute the else and print the line Hot! Hot! Hot!.

An if block has:

• One if statement whose instructions are executed only if the statement is true

• Zero or more else if statements whose instructions are executed only if the statement is true

• Zero or one else statement whose instructions are executed if no if nor else if statements were true

Only one section of the if / else if / else structure can have its instructions run. If the if is true, for instance, BoxLang will never look at the else if. Once one block executes, that’s it.

Ternary Operator

The ternary operator is a compact way to do an if, else, else if expression statements. It is very common in other languages and can be used for a more fluent expressive conditional expression.

( condition ) ? trueStatement : falseStatement

The way it works is that the condition is evaluated. If it is true, then the true statement executed; if it is false, then the false statement executes.

Please note that you can chain the trueStatement and the falseStatement into more tenrary operations. However, don't abuse it as they will look ugly and just be very complex to debug.

( 1 == 1 ) ? systemOutput( "true" ) : systemOutput( "false" );

The output of the above statement will be..... true of course!

Elvis Operator

Before Elvis we had isDefined(), structKeyExists() and IF statements to do these kind of evaluations. They work, but not very expressive or concise.

The Elvis operator is primarily used to assign the right default for a variable or an expression Or it is a short-hand way to do parameterization. It will allow us to set a value if the variable is Null or does not exist.

For instance,

myName = userName ?: "Anonymous";

If userName does not exist or evaluates to null then the default value of the myName will be assigned the right part of the ?: elvis operator -> Anonymous

Safe Navigation Operator

The safe navigation operator allows for you to navigate structures by not throwing the dreaded key not exists exception but returning an undefined or null value. You can then combine that with the elvis operator and create nice chainable struct navigation. For example instead of doing things like:

result = "";
if( structKeyExists( var, "key" ) ){
    if( structKeyExists( var.key, "otherkey" ){
        result = var.key.otherkey;
    }
}

You can do things like this:

result = var?.key?.otherKey ?: "";

The hook operator (?) along with the dot operator (.) is known as safe navigation operator(?.). The safe navigation operator makes sure that if the variable used before the operator is not defined or java null, then instead of throwing an error, the operator returns undefined for that particular access.

Switch, Case, & Default

Another situation that involves conditional logic is when a single variable or expression that can have a variety of values and different statements or functions needed to be executed depending on what that value is. One way of handling this situation is with a switch / case / default block.

switch( expression ){
    case value : [ case otherValue ] : {
        // operations
        break;
    }

    default : {
        // Default operations
    }
}

Much like how the if statement marks the start of an if block and contains one or more else if statements and perhaps one (and only one) else statement, the switch statement marks the start of a switch block and can contain multiple case statements and perhaps one (and only one) default statement.

The main difference is that switch / case / default can only evaluate the resulting value of a single variable or expression, while the if / else if / else block lets you evaluate the true or false result of different variables or expressions throughout the block.

switch( city ){

    case "New York":
         region= "East Coast";
         break;

    case "Los Angeles":
          region= "West Coast";
         break;

     case "Phoenix":
          region= "Phoenix";
         break;

     case "Cleveland" : case "Cincinnati" : {
          region= "Midwest";
        break;
    }
     default:
          region="Unknown";
}

Please note that you can create a body for the case statements with curly braces. As best practice, do so for all case and/or default blocks

While Loops

The while( conditional ) expression allows you to execute a code block as many times as the conditional expression evaluates to true. This is a great way to work with queues, stacks or just simple evaluations.

testCondition = true;
count = 0;
while( testCondition ){
    count++;
    if( count == 5) {
        testCondition = false;
    }
}
systemOutput( count );

The == and = Common Mistake

The #1 mistake people encounter when writing conditional statements is the difference between = and ==.

• = is an assignment. It means "take what's on the right side and stick it into whatever is on the left side" (or its telling not asking.)

• == is a question. It means "is the thing on the right equal to the thing on the left" (or its asking not telling.)