The size of the indicated variable or array appears to be changing with each loop iteration. Commonly, this message appears because an array is growing by assignment or concatenation. Growing an array by assignment or concatenation can be expensive. For large arrays, MATLAB must allocate a new block of memory and copy the older array contents to the new array as it makes each assignment.

Programs that change the size of a variable in this way can spend most of their run time in this inefficient activity. There is also significant overhead in shrinking an array on each iteration or in changing the size of a variable on each iteration. In such cases, MATLAB must reallocate and copy the contents repeatedly.

For scripts, Code Analyzer cannot determine whether the array was preallocated before the script was called. Depending on your use of scripts, you might want to enable or disable this message independently of the related message (with message ID AGROW) that applies to functions and class methods.

Consider preallocating a variable or array before entering the loop by using zeros, ones, cell, or a similar function. Preallocating avoids the need for MATLAB to copy the data from one array to another inside the loop. For examples of code that do and do not preallocate an array, see “Preallocating Arrays”

When an array grows by assigning past the end of the array or using concatenation, preallocation alone does not improve the performance. The code must use explicit indices also.

Assigning to an array in a script that the caller of the script preallocated provides an opportunity to create bugs that are difficult to detect. If the script is called multiple times, these bugs are particularly difficult to detect. However, there are other uses of scripts where preallocating in this manner is necessary or even desirable. In this case, suppress the message for specific assignments or for the whole script, as described in Adjust Code Analyzer Message Indicators and Messages. It might also be appropriate to suppress this message if any of the following is true:

• The loop code is searching for rare or exceptional events (especially if you do not expect it to find any).

  In this case, it can be reasonable to grow the array only as the loop code finds such events.

• The array is small and will always be small.

  In this case, the impact of recopying is also small.

• The array was preallocated, but in a way that Code Analyzer does not recognize.

If you do not know the size of an array before the loop begins, preallocate it, and then shrink it, if necessary, after the loop completes.