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How to convert cell to array in Matlab?

Published in MATLAB Cell Array Conversion 4 mins read

The standard way to convert a cell array into an ordinary array in MATLAB is by using the cell2mat function.

The Primary Tool: cell2mat

The cell2mat function is specifically designed for converting cell arrays into standard arrays (like numeric arrays, logical arrays, etc.).

The syntax is straightforward:

A = cell2mat(C)

Here, C is your input cell array, and A is the resulting ordinary array.

As stated in the reference: `A = cell2mat( C ) converts a cell array into an ordinary array. The elements of the cell array must all contain the same data type, and the resulting array is of that data type. The contents of C must support concatenation into an N-dimensional rectangle. Otherwise, the results are undefined.

How cell2mat Works (Detailing the Requirements)

For cell2mat to successfully convert a cell array, two crucial conditions must be met, based on the reference:

1. Data Type Consistency

  • Requirement: All elements within the cell array C must hold data of the same fundamental data type.
  • Explanation: MATLAB needs to know what type of array A will be (e.g., double, single, int32, logical). If cells contain mixed types (like numbers and text), it cannot form a single, homogeneous array.
  • Example of Failure:
    C_mixed = {1, 'hello'; 3, 4};
% A_mixed = cell2mat(C_mixed); % This will cause an error because of mixed data types (double and char)

2. Rectangular Concatenation

  • Requirement: The contents of the cells must be shaped such that they can be horizontally and vertically concatenated to form a single, rectangular array.
  • Explanation: MATLAB effectively tries to "stitch" the contents of the cells together based on their arrangement in the cell array C. If the dimensions of the contents don't align properly for concatenation (like trying to put a 1x2 matrix next to a 2x1 matrix within the same row of the cell array, or putting a 1x3 matrix above a 1x2 matrix in the same column), the operation fails or yields undefined results.
  • Example of Success:
    C_numeric = {[1 2], [3 4]; [5 6], [7 8]};
A_numeric = cell2mat(C_numeric);
% A_numeric will be:
% 1 2 3 4
% 5 6 7 8
% This works because [1 2] and [3 4] can concatenate horizontally, and
% the resulting rows ([1 2 3 4] and [5 6 7 8]) have the same width.
  • Example of Failure:
    C_non_rect = {[1 2], [3; 4]};
% A_non_rect = cell2mat(C_non_rect); % This will cause an error because [1 2] (1x2) and [3; 4] (2x1) cannot form a rectangular block when concatenated.

Practical Usage and Considerations

  • Ideal Use Case: cell2mat is most commonly and successfully used when your cell array contains numeric (like double, single, int) or logical arrays of potentially varying sizes, but structured in a way that they tile perfectly into a larger matrix.
  • Limitations: It is generally not suitable for cell arrays containing strings (unless they are char arrays that can be dimensionally concatenated, which is rare) or heterogeneous data types.
  • What Happens When Requirements Aren't Met: As the reference states, the results are either an error or, in some edge cases, undefined. Relying on undefined behavior is not recommended; you should ensure your cell array meets the criteria.

In summary, use cell2mat for direct conversion when your cell contains uniform data types that can form a single, rectangular array through concatenation.

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