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 a variable's size in this way can spend most of their run time in this inefficient activity.

For the same reasons, there is significant overhead in shrinking an array or in changing the size of a variable on each iteration.

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 by using concatenation, preallocation alone does not improve the performance. The code must also use explicit indices.

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

If any of the following conditions are true, it might be appropriate to suppress this message, as described in Adjust Code Analyzer Message Indicators and Messages:

• The loop code contains a conditional block where the array grows. (In this case, it can be reasonable to grow the array only as the loop finds such conditions.)

• For each iteration in the loop, the length of the concatenation varies (such as character vectors with variable length). The total size of the array is not calculable before entering the loop.

• The array is small and will remain small; therefore, the impact of recopying is also small.

• The code preallocates the array, but in a way that Code Analyzer does not recognize.