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

Note

The limitations listed in this section only apply to versions R2025b and earlier.

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

For versions R2026a and later, you can modify the input port labels according to the logical expressions used inside the block. To edit the port label, pause the cursor on the default text, u1,u2,...,un, and change the text.

For version R2025b or earlier, the block only allows the default input port label u1,u2,...,un. 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 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 (since R2026a)

Use a range of logical expressions, including expressions that use:

• Tunable parameters.

• General MATLAB^® expressions, such as x1 > x2 + 5.

• Data objects such as Simulink.Parameter and Simulink.Signal with custom storage classes. For more information, see Create and Apply Storage Class Defined in User-Defined Package (Embedded Coder).

• Fixed-point data type.

• Enumerated data type.

• MATLAB structure arrays.

• Expressions with:

  • Arithmetic functions — ceil, floor, abs, and sign.

  • Trigonometric functions — sin, cos, tan, asin, acos, atan, atan2, sinh, cosh, and tanh.

  • Exponential, logarithmic and root functions — log, log10, exp, and sqrt.

Logical expression (R2025b and earlier)

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, in 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 (Since R2026a)

• Use a range of logical expressions that were not previously supported, including expressions that use:

  • Tunable parameters.

  • General MATLAB expressions, such as x1 > x2 + 5.

  • Data objects such as Simulink.Parameter and Simulink.Signal with custom storage classes. For more information, see Create and Apply Storage Class Defined in User-Defined Package (Embedded Coder).

  • Fixed-point data type.

  • Enumerated data type.

  • MATLAB structure arrays.

  • Expressions with:

    • Arithmetic functions — ceil, floor, abs, and sign.

    • Trigonometric functions — sin, cos, tan, asin, acos, atan, atan2, sinh, cosh, and tanh.

    • Exponential, logarithmic and root functions — log, log10, exp, and sqrt.

Logical expressions (R2025b and earlier)

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

More About

Model Coverage

If blocks that cause an If Action Subsystem block to execute receive condition, decision, and MCDC coverage, and:

• The if condition, and each elseif condition, if they exist, receive decision coverage.

• Any if or elseif condition that contains a logical expression with multiple conditions, such as u1 && u2, also receives MCDC and condition coverage for each condition in the expression.

• The else condition does not receive coverage directly. The false case of the if condition or last elseif condition, if it exists, is the true case of the else condition, and vice-versa. Therefore, full coverage on the rest of the If block implies full coverage of the else condition.

Simulink Coverage™ reports the total number of time steps that each if and elseif condition evaluates to true and to false. If the if or elseif condition evaluates to true at least once, and evaluates to false at least once, decision coverage is 100%. If no if or elseif conditions are true, or if no if or elseif conditions are false, decision coverage is 50%. If the previous if or elseif condition is not false for any time step, an elseif condition can have 0% decision coverage.

Because the If block compares its input signals, if you select the Relational Boundary coverage metric, the If block receives relational boundary coverage. For more information, see Relational Boundary Coverage (Simulink Coverage).

The coverage report displays the expression that Simulink Coverage evaluates and reports the number of time steps that the expression is true and false. For example, consider a case where an If block evaluates whether the input not equal to zero, the coverage report displays if(u1 ~= 0) as the decision.

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).
SynthesisAttributes	Specifies the synthesis attributes for the blocks and block output signals in the model. The generated HDL code contains these attributes. For more information, see SynthesisAttributes (HDL Coder).

Version History

Introduced before R2006a

R2026a: Specify synthesis attributes for blocks and block output signals

Use the SynthesisAttributes HDL block property to specify the synthesis attributes for the block and its output signals. HDL Coder includes these attributes in the generated HDL code.

R2026a: Enhancements to input port and enhanced support for logical expressions

Starting in R2026a, the block allows you to:

• Tailor the If block input port label per your modeling requirements. Previously, you could only use the default names such as u1, u2, and so on for input port label.

• Use a range of logical expressions that were not previously supported for the if and elseif conditions. For more information about the list of expressions, see the if parameter.

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.