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Difference between revisions of "Legacy:Operators"

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|True iff one or both of the operands are true

Revision as of 19:35, 6 May 2016

Basics

Preoperators / Postoperators

If any operator is available as pre- and postoperator both versions basically have the same effect on the variable. The only difference is what they return: Preoperators return the variable's new value (after the operator's effect) while postoperators return the old value (the value before the operator did something).

Precedence of operators

The precedence of operators is used to group expressions.

local int a,b,c,d,e;
 
a=2; b=3; c=4; d=2; e=5;
 
                    // log output:
log(a@b@c@d@e);     // 2 3 4 2 5
log(a*b+++c**d*e);  // 86.000000
log(a@b@c@d@e);     // 2 4 4 2 5

UnrealScript actually evaluates the expression like this:

(a * (b++)) + ((c ** d) * e)

You should use a form that uses some spaces and only few (or no) parentheses. This helps understanding the expression a lot when you look at it again after some time:

a * b++ + c**d * e

Boolean Operators

iff (if and only if) 
The word "iff" means "if and only if," implying that if the given condition isn't met, the operation will yield a different (the opposite) result. (This is maths and logic jargon, not an UnrealScript operator.)
short-circuit 
Some binary boolean operators only evaluate their right operand if the result of the entire expression isn't already defined by the result of the first operand. In this case, the right operand is skipped, which is a particularly handy feature if evaluation of that right operand has non-trivial side effects (like a function call) or relies on the left operand's result.
if (bAlreadyConfirmed || ShowMessageBox("Are you sure?")) ...  // show message only when needed

if (ThisPawn != None && ThisPawn.bIsPlayer) ... // prevent "Accessed None"s

Operation Operator Precedence Description Short-circuit
Negation ! Unary Negates a boolean expression
And && 30 True iff both operands are true First operand false -> expression false
Exclusive Or ^^ 30 True iff one of the operands is true (but not both)
Inclusive Or || 32 True iff one or both of the operands are true First operand true -> expression true

Comparison Operators

Comparison operators apply to values of all types. Only compatible types can be compared (numbers with numbers, strings with strings and so on). Object (classes, actors, textures, etc.) and Name properties can only be compared using Equal or Unequal.

Operation Operator Precedence Remarks
Equal == 24
Approximately Equal ~= 24 Equality within 0.0001 (numbers)
Case-insensitve equality (strings)
Less < 24
Less or Equal <= 24
Greater > 24
Greater or Equal >= 24
Not Equal != 26

Conditional Operator

The conditional operator is a ternary operator in Devastation and UnrealEngine 3 games. In UnrealEngine 3 it has the following syntax:

condition ? expression1 : expression2

In Devastation the syntax is as follows:

condition ?? expression1 : expression2

In both cases it works similar to the If/Else statement, but returns the value of one of the two expressions. If the condition evaluates to True, then the first expression is evaluated, otherwise the second expression is evaluated.

Let's consider an UnrealScript implementation of the global function Max(). In UnrealEngine 1 or 2 you would have to implement it as something like the following:

static final function int Max(int A, int B)
{
  if (A > B) {
    return A;
  }
  else {
    return B;
  }
}

In UnrealEngine 3 you can now express the same feature in a much more compact form:

static final function int Max(int A, int B)
{
  return A > B ? A : B;
}

Very pleasing to the eye and exactly what you mean: Depending on a condition, this function is supposed to return one of two values, not execute one of two statements.

Note that the conditional operator is no general replacement for the If statement! The If statement selects statements to be executed, while the conditional operator selects values to be returned. In fact, you get a compiler error if any of the three operands is a statement that doesn't return a value.

Numeric Operators

Numeric operators apply to values of type int, float and byte.

Operation Operator Precedence Description
Negation - Preoperator Returns the negative value of the operand
Increment ++ Pre-/Postoperator Increments the variable operand by 1
Decrement -- Pre-/Postoperator Decrements the variable operand by 1
Exponentiation ** 12 Puts the first operand to the power of the second operand
Multiplication * 16 Multiplies both operands
Division / 16 Divides the first operand by the second operand
Modulo % 18 Divides the first operand by the second and returns the remainder
Addition + 20 Adds both operands
Subtraction - 20 Subtracts the second operand from the first operand

The modulo operator follows the computer science definition of modulus rather than the mathematical one::

-8 % 10 = -8 (and not 2)

This results from differing interpretations of whether % should be a "modulo" or "remainder" operator (which is not the same thing), as well as differing results of what the integer division operation returns for negative numbers in different programming languages. UnrealScript's definition is consistent with what (most) C/C++ does, which is probably appropriate considering that the language is similar in many other ways. See Useful Maths Functions for the alternative.

The assignment operator = can be combined with +, -, * and /. These combined operators +=, -=, *= and /= assign the result of the operation to the first operand (which must be a variable). These combined assignment operators also return the new value of the variable, so

b += c;      // equivalent to  b = b + c;
a = b += c;  // equivalent to  b = b + c;  a = b;  or  a = (b = b + c);

will add b and c and assign the result to a and b.

Bitwise Operators

Operation Operator Precedence Description
Inversion ~ Preoperator Performs a bitwise inversion of the operand
Shift Left << 22 Shifts the bits of the first operand to the left
Shift Right (Arithmetic) >> 22 Shifts the bits of the first operand right, maintaining its signum
Shift Right >>> 22 Shifts the bits of the first operand right, filling with zeroes
And & 28 Bitwise and
Or | 28 Bitwise or
Exclusive Or ^ 28 Bitwise exclusive or

For the shift operators, only the 5 least significant bits of the second operand are used.
What does that mean? Well, it's a neat feature that allows you to specify, how many bits you want to stay instead of how many you want to get rid of:

// IntToHex() is a custom function that converts an integer into its hexadecimal string representation
 
// -12 = 0xfffffff4 -> five least significant bits: 0xfffffff4 & 0x0000001f = 0x00000014 = 20
// 12 bits = 3 hex digits; 20 bits = 5 hex digits
 
// shift positive ints                 log output
log(IntToHex(0x02468ace <<   12));  //  68ace000
log(IntToHex(0x02468ace <<  -12));  //  ace00000 = same as 0x02468ace <<  20
log(IntToHex(0x02468ace >>   12));  //  00002468
log(IntToHex(0x02468ace >>  -12));  //  00000024 = same as 0x02468ace >>  20
log(IntToHex(0x02468ace >>>  12));  //  00002468
log(IntToHex(0x02468ace >>> -12));  //  00000024 = same as 0x02468ace >>> 20
 
// shift negative ints (to prove there's no difference from the usual behavior)
log(IntToHex(0xfdb97531 <<   12));  //  97531000
log(IntToHex(0xfdb97531 <<  -12));  //  53100000 = same as 0xfdb97531 <<  20
log(IntToHex(0xfdb97531 >>   12));  //  ffffdb97
log(IntToHex(0xfdb97531 >>  -12));  //  ffffffdb = same as 0xfdb97531 >>  20
log(IntToHex(0xfdb97531 >>>  12));  //  000fdb97
log(IntToHex(0xfdb97531 >>> -12));  //  00000fdb = same as 0xfdb97531 >>> 20

Vector Operators

See also vector.

Operation Operator Precedence Description
Reverse - Preoperator Returns the negative value of the operand
Multiply components * 16 Multiplies the corresponding components of both vectors
Multiply by scalar * 16 (vector * float or float * vector)
Divide by scalar / 16 (vector / float)
Dot product Dot 16 Calculates the dot product (inner product) of the vectors
Cross product Cross 16 Calculates the cross product (outer product) of the vectors
Addition + 20 Adds both vectors
Subtraction - 20 Reverses the second vector and adds it to the first one

Here the assignment operator = can also be combined with +, -, * and /. All combined assignment operators have a precedence of 34, so be careful when combining them with the string operators $ and @ since those have a precedence of 40.

Rotator Operators

See also rotator.

Operation Operator Precedence Description
Multiplication * 16 Multiplies all components of the rotator
Division / 16 Divides all components
Addition + 20 Adds the rotations
Subtraction - 20 Reverses the second rotator and adds it to the first one
Rotate vector (reversed) << 22 Rotates the vector in the way described by the rotator
Rotate vector >> 22 Rotates the vector in the way described by the rotator, but in reversed direction
Check for clockwise rotation (UnrealEngine 2 and above) ClockwiseFrom 24 Pass the same components from different rotators to this operator and it returns, whether the rotation was clockwise or not.

Again, the assignment operator = can be combined with +, -, * and /.

Note that rotators can't be inverted with the - preoperator. You will have to use -1 * theRotator- for this operation.

The effect of the rotator/vector operators << and >> can be explained using the result of the global functions GetAxes() and GetUnAxes() function:

static final operator(22) vector << (vector A, rotator B)
{
  local vector X, Y, Z;
  GetAxes(B, X, Y, Z);
  return X * A.X + Y * A.Y + Z * A.Z;
}
 
static final operator(22) vector >> (vector A, rotator B)
{
  local vector X, Y, Z;
  GetUnAxes(B, X, Y, Z);
  return X * A.X + Y * A.Y + Z * A.Z;
}

The << operator performs exactly the calculations required to position a player's weapon in first person view based on a constant view offset vector and the player's view rotation.

String Operators

Operation Operator Precedence Description
String Concatenation @ 40 The two strings are put together with a space in between.
String Concatenation $ 40 The two strings are put together without any space in between.
String Concatenation and Assign @= 44 The two strings are put together with a space in between and the result is assigned to the first variable and returned.
String Concatenation and Assign $= 44 The two strings are put together without any space in between and the result is assigned to the first variable and returned.
Remove ocurances and Assign -=- 45 Removes all occurances of the second string from the first string.

Aside from the string operators, there are other things that you can do with strings that have been implemented as Global Functions.

Color Operators

Color operators are only available in subclasses of Actor in UnrealEngine 1 and 2. Starting with UnrealEngine 3, they are defined as "global" operators in the Object class.

Operation Operator Precedence Description
Multiplication * 16 Multiplies all components of the color by a float
Addition + 20 Adds the colors
Subtraction - 20 Subtracts the colors (what happens if the RH component is bigger than the LH?)

There are no combined assignment operators for colors.

UnrealEngine 3 also features the "LinearColor" type, which is a color expressed in floating point values. Two operators exist for this type:

Operation Operator Precedence Description
Multiplication * 16 Multiplies all components of the linear color by a float
Addition + 20 Adds the linear colors

Linear colors don't have combined assignment operators either.

Complete Table Of Precedences

The higher an operator is in this table the more tightly it binds.

Operator class Examples
Parentheses ( )
Unary preoperators - ! ~ ++ --
Unary postoperators ++ --
Exponentiation **
Multiplicative operators * / Cross Dot
Additive operators + -
Bit shifting and vector rotating operators << >> >>>
Comparison operators (except inequality) < > <= >= ~= == ClockwiseFrom
Inequality !=
Bitwise Integer operators ^
Logical AND, logical XOR && ^^
Logical OR |
Combined assignment operators += -= *= /=
String concatenation @ $
String concatenation and assign @= $=
String removal and assign -=-
Conditional operator ?: ??:
Assignment (actually a statement!) =

Related Topics