5 Common MATLAB Error Messages and How to Fix Them

MATLAB error messages like “Inner matrix dimensions must agree” or “Index exceeds matrix dimensions” almost always trace back to five common mistakes: confusing matrix multiplication with elementwise multiplication, indexing past the bounds of an array, forgetting that MATLAB indexing starts at 1, mixing up the assignment operator with the equality operator, and assigning a value of the wrong size into a matrix slot. Each has a quick fix.

Key Takeaways

• Using * instead of .* between two vectors of the same size, such as A = [1 2 3] and B = [4 5 6], triggers “Error using * Inner matrix dimensions must agree.”
• MATLAB indexing begins at 1, not 0, so A(0) on any matrix returns “Subscript indices must either be real positive integers or logicals.”
• Writing “if n = 4” instead of “if n == 4” causes MATLAB to reject the line with “The expression to the left of the equals sign is not a valid target for an assignment.”
• Functions such as numel(), size(), and length() can be used to check array dimensions before indexing into them.
• Cleve Moler, co-founder of MathWorks, addressed MATLAB’s 1-based indexing in a comment on an April Fools blog post from MathWorks.

Why does “Error using * Inner matrix dimensions must agree” appear?

This is the most frequently posted MATLAB error message among new users. It happens when someone creates two matrices or vectors and multiplies them with the * operator, expecting an elementwise result. For example, running A = [1 2 3]; B = [4 5 6]; A*B produces this exact error.

The * operator performs true matrix multiplication, which requires an N-by-M matrix to be multiplied by an M-by-P matrix, producing an N-by-P result. The shared dimension “M” is the “inner matrix dimension” the error message refers to. When two same-sized row vectors like A and B don’t share a compatible inner dimension for matrix multiplication, MATLAB throws this error.

The fix is almost always to replace * with .* to perform elementwise multiplication, where corresponding elements are multiplied and the result matches the size of the inputs. Running A.*B on the vectors above returns ans = 4 10 18. This same distinction applies to other operators: use .^ instead of ^, ./ instead of /, and .\ instead of \ when an elementwise operation is intended rather than a matrix operation. MathWorks documents the distinction in its Array vs. Matrix Operations reference page.

What causes “Index exceeds matrix dimensions”?

This error occurs when code tries to reference a matrix element that doesn’t exist. For a 5-by-5 matrix created with magic(5), which has 25 total elements, attempting to access A(26) produces “Index exceeds matrix dimensions” because there is no 26th element.

To resolve this, confirm that the matrix is actually the size you expect and that the index you’re using falls within that range. If the index comes from a calculation or a loop counter, check the loop’s iteration count and any arithmetic that produces the index value. The functions numel(), size(), and length() are useful for checking the actual dimensions and element counts of a matrix before indexing into it.

Why does MATLAB reject “Subscript indices must either be real positive integers or logicals”?

This message most often appears when someone coming from a language with 0-based indexing tries to access the first element of a MATLAB array using A(0). In MATLAB, A(1) is the first element of a vector or matrix, equivalent to A(1,1) for a matrix, not A(0).

Cleve Moler, co-founder of MathWorks, addressed why MATLAB uses 1-based indexing in a comment on an April Fools blog post published on the MathWorks blog: “We have always had BOTH 0-based indexing and 1-based indexing. In order to distinguish between the two, 0-based indices are followed by ‘+1’. The 1-based indices are preferred because they are the language of mathematics.” The full exchange appears on the MathWorks Cleve’s Corner blog post from April 1, 2015.

This error can also appear when a noninteger or negative value is used as an index, such as A(1.5) or A(-3). MATLAB cannot resolve either of these as valid subscripts. When this happens inside a loop, check the loop bounds to confirm they never produce decimal or negative index values.

Why does “The expression to the left of the equals sign is not a valid target for an assignment” appear?

This error results from confusing the = operator with the == operator. The = operator performs assignment, while == performs a logical test for equality. An if statement expects a logical condition, so writing if n = 4 instead of if n == 4 produces this exact error message.

The fix is to replace = with == inside the conditional. For example, n = 5; if n == 4; n = n.^2; end runs without error, while if n = 4 does not. A quick way to see the difference between the operators: for A = 1:5 and B = 5, the expression A == B returns the logical vector 0 0 0 0 1, showing which elements of A equal B, while A = B would instead overwrite A entirely with the value 5. Use == to compare values and = to assign them.

What causes “Subscripted assignment dimension mismatch”?

This error appears when code tries to assign a vector or matrix into a slot too small or too large to hold it. For instance, attempting A(1) = [4 5 6] on a 3-by-3 matrix created with magic(3) fails because a single matrix element can only hold one value, not a three-element vector.

In large matrices or loops, this error can be harder to trace because a vector’s size may grow unexpectedly across iterations. The most reliable fix is to check the size of both sides of the assignment independently. For example, size(A(1:3)) and size([4 5 6]) both return 1 3, confirming the sizes match, so A(1:3) = [4 5 6] executes successfully and updates the first row of A to 4 1 6 (with the surrounding rows unchanged).

Frequently Asked Questions

Why does MATLAB use 1-based indexing instead of 0-based indexing?

MathWorks co-founder Cleve Moler has stated that MATLAB supports both 0-based and 1-based indexing internally, but 1-based indices are preferred because they align with standard mathematical notation, where the first element of a sequence is typically labeled 1 rather than 0.

What is the difference between * and .* in MATLAB?

The * operator performs matrix multiplication, requiring the number of columns in the first matrix to match the number of rows in the second. The .* operator performs elementwise multiplication, multiplying corresponding elements of two arrays that are the same size and returning a result of that same size.

How do I check the size of a matrix in MATLAB to avoid indexing errors?

Use size() to return the dimensions of a matrix as rows and columns, numel() to return the total number of elements, and length() to return the size of the largest dimension. Checking these before indexing helps confirm that an index falls within valid bounds.

What is the difference between = and == in MATLAB?

The = operator assigns a value to a variable, for example n = 5 sets n equal to 5. The == operator tests whether two values are equal and returns a logical result of 0 or 1. Conditional statements such as if require ==, not =.

Why does “Subscripted assignment dimension mismatch” happen inside loops?

Inside a loop, the size of a vector or matrix being built up can change unexpectedly between iterations, so a later assignment may no longer match the size of the target it’s being assigned to. Checking the size of both sides of an assignment statement independently is the most reliable way to locate the mismatch.

Where can I find more MATLAB error message explanations?

The MATLAB Programming Error Messages page on Wikibooks documents additional common error messages beyond the ones covered here.