Simulink Reference

For Iterator

Implement a C-like for control flow statement in Simulink

Library

Ports & Subsystems/For Subsystem

Description

The For Iterator block, when placed in a subsystem, implements a C-like for control flow statement in Simulink as a For subsystem. In the For subsystem, the For Iterator block has iterative control over any Simulink blocks present. For each iteration value of the For Iterator block, the accompanying blocks execute. The number of iterations is set internally for the For Iterator block or externally with data input.

The following example shows a completed for control flow statement that increments an initial value of zero by 10 over 20 iterations.

In the preceding example, a For subsystem receives an input, which it passes to the For Iterator block inside. The For Iterator block uses this input to determine the number of times it executes the blocks of its subsystem, in this case, 20 times. Each time the blocks execute, a value of 10 is added to a sum, which is initially zero. In addition, for each time the blocks of the subsystem execute, the For Iterator block outputs a value equal to the number of times that the blocks have executed, including the current execution. This is referred to as the iterator value. This value, along with the sum value, is sent to an XY Graph block with the following result.

The preceding for control flow statement example can be represented by the following pseudocode.

• sum = 0;
  iterations = 20;
  sum_increment = 10;
  for (i = 0; i < iterations; i++) {
      sum = sum + sum_increment;
  }
  

You construct a For subsystem like the preceding example as follows:

1. Create a subsystem and place a For Iterator block in it.

1. This changes the subsystem icon to the text for{...}.

   You can use an ordinary subsystem or an atomic subsystem. In either case, the resulting For subsystem is atomic.

2. Double-click the For Iterator block to open its Block Parameters dialog and enter as follows:

• Specify external for the Source of number of iterations field.

• A data input marked N now appears on the For Iterator block. The source for this data input must be outside the For subsystem.

  In the preceding example, the source for the N port is a constant of value 20 that resides outside the For subsystem.

  If you specify internal for the Source of number of iterations field, the Number of iterations field appears for you to specify the number of iterations.

• Make sure that the Show iteration number port check box is selected if you want the For Iterator block to provide an optional data output port for the iteration value. This value begins at 1 for the first iteration and increments by 1 for each succeeding iteration.

• When the Show iteration number port check box is selected (the default), the Output data type field is enabled (grayed out otherwise). This allows you to select the type for the iterations number data output. The default type is int32.

• The setting of the States when starting field applies only to cases in which the For subsystem is called repeatedly. If you select reset (the default value) for this field, every time the For subsystem is called, its states are reset to their initial values. If you select held, the states of the For subsystem are retained between calls. In the preceding example, the For subsystem is called only once.

Data Type Support

The following rules apply to the data type of the number of iterations (N) input port:

• The input port accepts data of mixed types.
• If the input port value is noninteger, it is first truncated to an integer.
• Internally, the input value is cast to an integer of the type specified for the output port.
• If no output port is specified, the input port value is cast to type int32.
• If the input port value exceeds the maximum value of the output port's type, it is truncated to that maximum value.

Data output for the iterator value can be selected as double, int32, int16, or int8 in the Block Properties dialog.

Parameters and Dialog Box

States when starting

Set this field to reset if you want the states of the For subsystem to be reinitialized for each iteration. Otherwise, set this field to held (the default) to make sure that these subsystem states retain their values from the previous iteration.

Source of number of iterations

If you set this field to internal, the value of the Number of iterations field determines the number of iterations. If you set this field to external, the signal at the For Iterator block's N port determines the number of iterations.

Number of iterations

Set the number of iterations for the For Iterator block to this value. This field appears only if you selected internal for the Source of number of iterations field.

Show iteration number port

If this is selected, the For Iterator block outputs its iteration value. This value starts at 1 and is incremented by 1 for each iteration.

Output data type

Set the type for the iteration value output from the iteration number port to double, int32, int16, or int8.

Characteristics

Direct Feedthrough	No
Sample Time	Inherited from driving blocks
Scalar Expansion	No
Dimensionalized	No
Zero Crossing	No

First-Order Hold

For Iterator Subsystem