zscript-doc/lang-Statements.md

Statements

• Compound Statements
• Expression Statements
  • Example: Expression statements
• Conditional Statements
  • Example: Conditional statements
• Switch Statements
  • Example: Switch statements
• Loop Statements
• Control Flow Statements
  • Example: Control flow statements
• Local Variable Statements
• Multi-assignment Statements
  • Example: Multi-assignment statements
• Static Array Statements
• Null Statements

All functions are made up of a list of statements enclosed with left and right braces, which in and of itself is a statement called a compound statement, or block.

Compound Statements

A compound statement is formed as:

{
   $[Statement...]$
}

Note that the statement list is optional, so an empty compound statement {} is entirely valid.

Expression Statements

An expression statement is the single most common type of statement in just about any programming language. In ZScript, exactly like C and C++, an expression statement is simply formed with any expression followed by a semicolon. Function calls and variable assignments are expressions, for instance, so it is quite clear why they are common.

Their syntax is:

Expression ;

Example: Expression statements

Some basic expressions.

MyCoolFunction(5, 4);
m_MyCoolMember = 500;
5 * 5; // does nothing of course, but valid

Conditional Statements

A conditional statement will, conditionally, choose a statement (or none) to execute. They work the same as in C and ACS:

if ( Expression ) Statement $[ else Statement]$

Example: Conditional statements

Simple conditional.

if(a)
   B();

Simple conditional, with else statement and a block.

if(a)
{
   B();
   c = d;
}
else
   e = f;

Switch Statements

A switch statement takes an expression of integer or name type and selects a labeled statement to run. They work the same as in C and ACS:

switch ( Expression ) Statement

Example: Switch statements

A switch demonstrating fall-through and default cases.

switch(a)
{
case 500: Console.Printf("a is 500"); break;
case 501: Console.Printf("a is 501"); // falls through to next case
case 502: Console.Printf("a is 501 or 502"); break;
default:
   Console.Printf("not sure what a is!");
   // break is implied here
}

Loop Statements

ZScript has five loop statements, for, while, until, do while and do until. for, while and do while work the same as in C, C++ and ACS, while until and do until do the inverse of while and do while.

The for loop takes a limited statement and two optional expressions: The statement for when the loop begins (which is scoped to the loop,) one expression for checking if the loop should break, and one which is executed every time the loop iterates. Its syntax is:

for ( $[Expression-or-Local-variable-statement]$ ; $[Expression]$ ; $[Expression]$ ) Statement

The while loop simply takes one expression for checking if the loop should break, equivalent to for(; a;).

The until loop is equivalent to while(!a). Their syntax are:

while ( Expression ) Statement
until ( Expression ) Statement

do while and do until will only check the expression after the first iteration is complete. The do while and do until loops are formed as such:

do
   Statement
while ( Expression ) // unlike C, you don't need a semicolon here

do
   Statement
until ( Expression )

Control Flow Statements

As in C, there are three control flow statements that manipulate where the program will execute statements next, which are available contextually. They are continue, break and return.

continue is available in loop statements and will continue to the next iteration immediately:

continue ;

break is available in loop statements and switch statements, and will break out of the containing statement early:

break ;

return is available in functions. If the function does not return any values, it may only be spelled return; and will simply exit the function early. If the function does return values, it takes a comma-separated list for each value returned:

return $[Expression $[ , Expression]$...]$ ;

Example: Control flow statements

Use of continue.

for(int i = 0; i < 50; i++)
{
   if(i == 25) continue; // don't do anything on 25!

   DoThing(i);
}

Use of break.

for(int i = 0; i < 50; i++)
{
   if(i == 25) break; // exit the loop at 25!

   DoThing(i);
}

Use of return in various contexts.

void ReturnsNothing()
{
   if(m_Thing != 50) return; // exit early if m_Thing isn't 50.

   DoThing(m_Thing);
}

int ReturnsInt()
{
   if(m_Thing == 50)
      return 50; // m_thing is 50, so return 50.

   return 0; // must have a return eventually
}

int, int ReturnsTwoInts()
{
   return 1, 2; // returns 1 and 2.
}

Local Variable Statements

Local variable statements are formed in one of 2 ways. The let keyword can be used to automatically determine the type of the variable from the initializer, while the regular syntax uses an explicit type, and initialization is optional.

Variables' syntax are one of:

Variable-name
Variable-name = Expression

And local variable statements have the syntax of either:

let Identifier = Expression ;
Type Variable $[ , Variable]$... ;

Multi-assignment Statements

Expressions or functions that return multiple values can be assigned into multiple variables with the syntax:

[ Expression $[ , Expression]$... ] = Expression ;

Note that the surrounding brackets are literal and not an optional element.

Example: Multi-assignment statements

Getting the actor out of A_SpawnItemEx.

Actor mo;
bool spawned;
[spawned, mo] = A_SpawnItemEx("MyCoolActor");

Static Array Statements

Static arrays can be defined normally as a statement.

Null Statements

A null statement does nothing, and is formed ;. It is similar to an empty compound statement.