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====== Expression syntax ======

Here follows a description of often-used operators, functions and the most important syntax rules.

===== Operators =====

|==== Mathematical Operators ====|  |
|**Plus**                            |**+ **|
|**Minus**                           |**- **|
|**Times**                           |*** **|
|**Power**                           |**^ **|
|**Divide**                          |**/** |
|==== Relational Operators ====  |  |
|**Equal**                           |**==**|
|**Not equal**                       |**~=**|
|**Less than**                       |**< **|
|**Greater than**                    |**> **|
|**Less than or equal**              |**<=**|
|**Greater than or equal**           |**>=**|
|==== Logical Operators ====     |  |
|**And**                             |**&&**|
|**Or**                              |**||**|

===== Conventions =====

  * The “.” (dot-symbol) is used as decimal symbol.
  * The default angle unit is radians.

===== Conditions =====

|Note|If you want a statement like Time==100, testing if Time is exactly equal to 100, you will have to make sure the solver is forced to include the time point 100 by specifying this in the [[Simulation_settings|simulation settings]]. This is because the variable step-size solver is not guaranteed to solve the states of the system at exactly time = 100. This is not a problem when using a fixed step-size solver where the step-size of the solver corresponds to the required time being included exactly.|

Ecolego allows you to enter conditions directly in equations. There are two ways to enter conditions: using multiplication (classic) or by using the //if()// function (since Ecolego 5).

=== Conditions using the if() function ===

The //if()// function has the following syntax:

''%%if( condition, value if true[, value if false] )%%''
The last argument is optional and will default to 0 if not specified.

Example 1: Simple condition

|''%%IF a>b THEN     0.5  ELSE     0.2 %%''|if( a>b, 0.5, 0.2 )|

Example 2: Nested condition

|''%%IF a>b THEN     IF c>d THEN         0.5     ELSE        0.2  ELSE     0.0%%''|if( a>b, if( c>d, 0.5, 0.2 ), 0.0 )|

Example 3: Using logical operators

|''%%IF a>b AND c>d THEN     0.5  ELSE     0.2 %%''|if( a>b && c>d, 0.5, 0.2 )|

=== Conditions using multiplication ===

When a logical operator is used in an expression, the expression will evaluate to either 0 (if false) or 1 (if true). If the logical operator is used in a statement within parenthesis, the parenthesis will evaluate to either 0 or 1. This is the approach used in ancient versions of Ecolego and is also the approach used in MATLAB.

Example 1: Simple condition

|''%%IF a>b THEN     0.5  ELSE     0.2 %%''|(a>b)*0.5+(a<=b)*0.2|

==== Avoiding division by zero ====

It is common that conditions are used to avoid division by zero (which will abort a simulation).

Example:

The expression ''%%k*Reservoir/time%%'' will result in a division by zero at //time//=0. To avoid the division, change the expression to:

''%%if(time>0, k*Reservoir/time)%%''
Note: When using multiplication (see above) instead of the //if()// function, it is important that the **whole** expression that might cause a division of zero is placed **within parenthesis**:

(time>0)*(Comp1/time)
==== Help the solver! ====

For conditions that control the transfer to [[Compartment|compartments]] it is important to notify the solver when the condition changes. This is done by using a [[Discrete_event|discrete event]] block.

===== Index specific data =====

If you want to use the value for specific index of a vectorised parameter or inventory, the notation is the parameter name followed by the material name in brackets (e.g. Kd_clay[Cs-137], Dose[Cs][Man])

===== See also =====

  * [[Function|Functions]]
  * [[Expression_editing_tool|Expression editing tool]]
  * [[constants_editing_tool|Constant editing tool]]
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