If

Select subsystem execution using logic similar to if-else statement

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Libraries:
Simulink / Ports & Subsystems

Description

The If block, along with If Action Subsystem blocks that contain an Action Port block, implements if-else logic to control subsystem execution. For an example that uses the If block, see Select Subsystem Execution.

Examples

Simulink Subsystem Semantics

This set of examples shows different types of Simulink® Subsystems and what semantics are used when simulating these subsystems. Each example provides a description of the model and the subtleties governing how the model is executed.

• Open Live Script

If-Then-Else Blocks

The effect of feeding a sine wave into If Action Subsystem blocks. It is designed to illustrate the similarity between the If Action Subsystem block and the Enabled Subsystem block.

• Open Model

Modeling Clutch Lock-Up Using If Blocks

Use If/Else subsystems to build a clutch model. An 'If' subsystem models the clutch dynamics in the locked position while an 'Else' subsystem models the unlocked position. One or the other is enabled using the 'If' block. The dot-dashed lines from the 'If' block denote control signals, which are used to enable If/Else (or other conditional) subsystems. Checking any of the boxes on the GUI produces a plot of any of the selected variables (versus time).

• Open Model

Limitations

The If block has the following limitations:

• It does not support tunable parameters. Values for an if or elseif expression cannot be tuned during a simulation in normal or accelerator mode, or when running generated code.

  To implement tunable if-else expressions, tune the expression outside the If block. For example, use the Relational Operator block to evaluate the expression outside of the If block or add the tunable parameter as an input to the If block.

• It does not support custom storage classes. See Organize Parameter Data into a Structure by Using Struct Storage Class (Embedded Coder).

• The if expression and elseif expressions cannot accept certain operators, such as +, -, *, and /.

Ports

Input

Logical operands — Values for evaluating logical expressions

scalar | vector

Inputs u1,u2,...,un must have the same data type. The inputs cannot be of any user-defined type, such as an enumerated type.

The If block does not directly support fixed-point data types. However, you can use the Compare To Constant block to work around this limitation. See Support for Fixed-Point Data Type in Select Subsystem Execution.

Data Types: single | double | int8 | int16 | int32 | uint8 | uint16 | uint32 | Boolean

Output

Action — Action signal for an If Action Subsystem block

scalar

Outputs from the if, else, and elseif ports are action signals to If Action Subsystem blocks.

Parameters

Number of inputs — Specify number of input ports

1 (default) | integer

1

Specify one input port.

integer

Specify the number of input ports. Block ports are labeled with a 'u' character followed by a number, 1,2,...,n, where n equals the number of inputs that you specify.

Programmatic Use

Block Parameter: NumInputs

Type: character vector

Values: '1' | integer in quotes

Default: '1'

Show else condition — Control display of else port

on (default) | off

When the if port and all elseif port expressions are false, the else port sends an action signal to execute the attached If Action Subsystem block.

on

Display else port.

off

Hide else port.

Programmatic Use

Block Parameter: ShowElse

Type: character vector

Values: 'on' | 'off'

Default: 'on'

Enable zero-crossing detection — Control zero-crossing detection

on (default) | off

Control zero-crossing detection.

on

Detect zero crossings.

off

Do not detect zero crossings.

Programmatic Use

Block Parameter: ZeroCross

Type: character vector

Values: 'on' | 'off'

Default: 'on'

Provide if-else statements in generated code — Option to include only if-else statements in code generated from If block

off (default) | on

Since R2025a

Select this parameter to ensure only if-else statements are included in the code generated from an If block.

on

Only if-else statements are included in the generated code. This option prevents inclusion of switch-case statements in the generated code.

off

When this parameter is off, the code generated from an If block might contain switch-case statements, which perform the same operations as the If block.

Programmatic Use

To set the block parameter value programmatically, use the set_param function.

Block Parameter: EnsureIfElseCode

Type: character vector

Values: 'on' | 'off'

Default: 'off'

if — Logical expression

u1 > 0 (default) | logical expression

The If Action Subsystem block attached to the if port executes when the associated if expression evaluates to true.

u1 > 0

Specify sending an action signal on the output port when the input u1 is greater than 0.

logical expression

Note

Starting in R2025a, to edit the default if expression (u1 > 0), use the Expressions table. For a complex expression with multiple inputs and operators, select the if expression and edit it in the Expression editor box.

For versions prior to R2025a, this parameter appears as If expressions in the block dialog box. You can specify logical expressions using this parameter.

However, across all Simulink^® versions, this expression appears on the If block adjacent to the if output port.

The expression can include only the operators <, <=, ==, ~=, >, >=, &, |, ~, () , and unary-minus. Operators such as +, -, *, /, and ^ are not allowed. The expression must not contain data type expressions, for example, int8(6), and must not reference workspace variables whose data type is anything other than double or single.

Programmatic Use

Block Parameter: IfExpression

Type: character vector

Values: 'u1 > 0' | logical expression in quotes

Default: 'u1 > 0'

elseif — Logical expression

empty (default) | list of logical expressions

The If Action Subsystem block attached to an elseif port executes when the associated elseif expression evaluates to true and all other if and elseif expressions are false.

empty

Logical expressions not specified.

list of logical expressions

Note

Starting in R2025a, to add and edit elseif expressions, use the Expressions table. Use the action buttons on the left of the table to add, delete, and move the expressions. The table allows you to write each elseif expression separately instead of in a comma-separated list and organize the expressions per your modeling requirements. For a complex expression with multiple inputs and operators, select the row containing that elseif expression and edit it in the Expression editor box.

For versions prior to R2025a, this parameter appears as Else expressions in the block dialog box. In those versions, specify a list of logical expressions separated by commas.

However, across all Simulink versions, the expressions appear on the If block below the if port and above the else port when you select the Show else condition check box.

Expressions can include only the operators <, <=, ==, ~=, >, >=, &, |, ~, (), and unary-minus. Operators such as +, -, *, /, and ^ are not allowed. The expressions must not contain data type expressions, for example, int8(6), and must not reference workspace variables whose data type is anything other than double or single.

Programmatic Use

Block Parameter: ElseIfExpressions

Type: character vector

Values: '' | list of logical expressions separated by commas placed in quotes

Default: ''

Block Characteristics

Data Types	Boolean | double | integer | single
Direct Feedthrough	yes
Multidimensional Signals	no
Variable-Size Signals	no
Zero-Crossing Detection	yes

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using Simulink® Coder™.

HDL Code Generation

Generate VHDL, Verilog and SystemVerilog code for FPGA and ASIC designs using HDL Coder™.

HDL Coder™ provides additional configuration options that affect HDL implementation and synthesized logic.

HDL Architecture

This block has one default HDL architecture.

HDL Block Properties

ConstrainedOutputPipeline	Number of registers to place at the outputs by moving existing delays within your design. Distributed pipelining does not redistribute these registers. The default is 0. For more details, see ConstrainedOutputPipeline (HDL Coder).
InputPipeline	Number of input pipeline stages to insert in the generated code. Distributed pipelining and constrained output pipelining can move these registers. The default is 0. For more details, see InputPipeline (HDL Coder).
OutputPipeline	Number of output pipeline stages to insert in the generated code. Distributed pipelining and constrained output pipelining can move these registers. The default is 0. For more details, see OutputPipeline (HDL Coder).

Version History

Introduced before R2006a

R2025a: Enhancements to block dialog box and code generation

• The updated block dialog box allows you to specify the logical expressions in a table. You no longer need to specify the elseif expressions in a comma-separated list. This change makes the overall logic flow easier to visualize and parse. The table also contains new action buttons that allow you to add, delete, and move if-else expressions up and down. These buttons help you to organize the expressions per your modeling requirements. Additionally, the new expression editor box allows you to edit complex expressions.

• Generated code contains if-else expressions. This change prevents inclusion of switch-case statements and enhances readability of the generated code.