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While Iterator

Repeatedly execute contents of subsystem at current time step while condition is satisfied


Ports & Subsystems


The While Iterator block, when placed in a subsystem, repeatedly executes the contents of the subsystem at the current time step while a specified condition is true.

    Note:   Placing a While Iterator block in a subsystem makes it an atomic subsystem if it is not already an atomic subsystem.

The output of a While Iterator subsystem cannot be a function-call signal. Otherwise, Simulink® displays an error when you simulate the model or update the diagram.

You can use this block to implement the block-diagram equivalent of a C program while or do-while loop. In particular, use While loop type to select one of the following while loop modes:

  • do-while

    In this mode, the While Iterator block has one input, the while condition input, whose source must reside in the subsystem. At each time step, the block runs all the blocks in the subsystem once and then checks whether the while condition input is true. If the input is true, the iterator block runs the blocks in the subsystem again. This process continues as long as the while condition input is true and the number of iterations is less than or equal to the Maximum number of iterations.

  • while

    In this mode, the iterator block has two inputs: a while condition input and an initial condition (IC) input. The source of the initial condition signal must be external to the while subsystem. At the beginning of the time step, if the IC input is true, the iterator block executes the contents of the subsystem and then checks the while condition input. If the while condition input is true, the iterator executes the subsystem again. This process continues as long as the while condition input is true and the number of iterations is less than or equal to the Maximum number of iterations. If the IC input is false at the beginning of a time step, the iterator does not execute the contents of the subsystem during the time step.

      Tip:   Specify a maximum number of iterations to avoid infinite loops, which you can break only by terminating MATLAB®.

Data Type Support

Acceptable data inputs for the condition ports are any numeric data type that Simulink supports, as well as any fixed-point type that includes a 0 value. For more information, see Data Types Supported by Simulink in the Simulink documentation.

The optional output port can output any of the following data types: double, int32, int16, or int8.


Maximum number of iterations

Specify the maximum number of iterations allowed. A value of -1 allows any number of iterations as long as the while condition input is true. Note that if you specify -1 and the while condition never becomes false, the simulation will run forever. In this case, the only way to stop the simulation is to terminate the MATLAB process. Therefore, do not specify –1 as the value of this parameter unless you are certain that the while condition becomes false at some point in the simulation.

While loop type

Specify the type of while loop that this block implements.

States when starting

Set this field to reset if you want the iterator block to reset the states of the blocks in the while subsystem to their initial values at the beginning of each time step (i.e., before executing the first loop iteration in the current time step). To cause the states of blocks in the subsystem to persist across time steps, set this field to held (the default).

Show iteration number port

If you select this check box, the While Iterator block outputs its iteration value. This value starts at 1 and is incremented by 1 for each succeeding iteration. By default, this check box is not selected.

Output data type

If you select the Show iteration number port check box (the default), this field is enabled. Use it to set the data type of the iteration number output to int32, int16, int8, or double.


The While Iterator block can optionally output the current iteration number, starting at 1. The following model uses this capability to compute N, where N is the first N integers whose sum is less than 100.

The contents of the While Iterator subsystem are:

The While Iterator block uses the following parameter settings:

  • Maximum number of iterations set to 20

  • States when starting set to reset

The model is the diagrammatic equivalent of the following pseudocode:

max_sum = 100;
sum = 0;
iteration_number = 0;
cond = (max_sum > 0);
while (cond != 0) {
	iteration_number = iteration_number + 1;
	sum = sum + iteration_number;
	if (sum > max_sum OR iteration_number > max_iterations) 
		cond = 0;


Direct Feedthrough


Sample Time

Inherited from the driving block

Scalar Expansion




Zero-Crossing Detection


Introduced before R2006a

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