MATLAB supports a type of array indexing that uses one array as the index into another array. In MATLAB®, there are three primary approaches to accessing array elements based on their location (index) in the array. I tried reshaping the vectors as a matrix, but this only works if there arent any "holes" in the logical Matrix B. Follow 36 views (last 30 days) James Metz on 4 Apr 2020. The array has logical 1 ( true) values where A is odd. Skip to content. Accelerating the pace of engineering and science. 21: logical(A) Convert numeric values to logical; returns an array that can be used for logical indexing or logical tests. Indexing can be classified considering the following two attributes. Unable to complete the action because of changes made to the page. According to the number of dimensions each index variable refers to, indexing can be multidimensional or linear. 22: true. If you create an array C with the same size as A, the following code works fine: Even though you know that due to the logical indexing only the second to last columns are changed (5x4), the following code does not work. The sub2ind function should come in handy. The array logInd is not an array of double numbers, but have the class logical. Accelerating the pace of engineering and science. With your help and insight I am confident I can solve the remaining problems by myself. The individual indices are sometimes referred to as subscripts in Matlab documentation (see for example sub2ind). These approaches are … The logical indexing approach should work as you described as wanting it to. [MUSIC] [APPLAUSE] Indexing is the way to select a particular element in an array. Many MATLAB functions that start with is return logical arrays and are very useful for logical indexing. Reload the page to see its updated state. Ideally the Code would look like this: The values in C and D to be used in the equation have to be in the same row and column as in A. Thank you for posting the amplification and clarification! The correct values are replaced in the correct locations. The array C is initalized as a 1x20something Array depending on B. I guess this does mean that this type of Code does work, even though the right hand side is just composed of vectors, but I do have to watch out which dimensions the vectors have? Suppose I do know that the first column of B, my logical array, is zero. Logical indexing of cell array. The output is always in the form of a column vector. Logical Indexing. MATLAB extracts the matrix elements corresponding to the nonzero values of the logical array. I did not know that the assignment of elements using a logical array follows the linear index convention, this does clear up some of my confusion. You can index into an array or a table using an array of logical indices. Find the treasures in MATLAB Central and discover how the community can help you! The output of C(B) is, e.g., a 1x10 Array. For example, one random assignment produces: 0.19812 0.052677 0.94274 0.66634 0.12801, 0.48969 0.73786 0.41774 0.53913 0.99908, 0.33949 0.26912 0.98305 0.69811 0.17112, 0.95163 0.42284 0.30145 0.66653 0.032601, 0.92033 0.54787 0.7011 0.17813 0.5612, 10048 1898 0.94274 0.66634 2173, 4633 0.73786 881 0.53913 0.99908, 3842 3637 0.98305 0.69811 7085, 0.95163 12968 8066 0.66653 11453, 0.92033 0.54787 0.7011 7940 0.5612. We also don't know what your system specifications are. Assigning elements in a vector using a logical array follows the linear indexing convention. Choose a web site to get translated content where available and see local events and offers. MATLAB provides us with plenty of functionalities, useful in various computational problems. For example, A (A > 12) extracts all the elements of A that are greater than 12. The result of the expression is an array of logical indices. Every technique that uses logical matrices as indexing of the entire array is going to have the same problem of being insensitive to order of the panes. But don't forget it whenever you need to remove or change selected elements in arrays. You may receive emails, depending on your. Logical Indexing Example 1. In MATLAB®, there are three primary approaches to accessing array elements based on their location (index) in the array. Pick out the odd-numbered elements of a numeric matrix. For example, you could replace all the NaN elements in an array with another value by using a combination of isnan, logical indexing, and scalar expansion. Logical indexing will allow us to select and manipulate only those subsets of data that satisfy particular logical conditions. https://www.mathworks.com/matlabcentral/answers/678563-manipulating-values-in-an-array-with-logical-indexing#answer_565493, https://www.mathworks.com/matlabcentral/answers/678563-manipulating-values-in-an-array-with-logical-indexing#comment_1183743, https://www.mathworks.com/matlabcentral/answers/678563-manipulating-values-in-an-array-with-logical-indexing#comment_1183803, https://www.mathworks.com/matlabcentral/answers/678563-manipulating-values-in-an-array-with-logical-indexing#comment_1183848, https://www.mathworks.com/matlabcentral/answers/678563-manipulating-values-in-an-array-with-logical-indexing#comment_1183983, https://www.mathworks.com/matlabcentral/answers/678563-manipulating-values-in-an-array-with-logical-indexing#comment_1184983, https://www.mathworks.com/matlabcentral/answers/678563-manipulating-values-in-an-array-with-logical-indexing#comment_1184988, https://www.mathworks.com/matlabcentral/answers/678563-manipulating-values-in-an-array-with-logical-indexing#answer_565483, https://www.mathworks.com/matlabcentral/answers/678563-manipulating-values-in-an-array-with-logical-indexing#comment_1183748. Edited: James Tursa on 4 Apr 2020 Accepted Answer: James Tursa. For example, consider the 4-by-4 magic square A: What i want to achieve is similar to: but i only want to change some values of A, not all of them. Learn more about arrays, indexing, floating point, binary floating point numbers MathWorks is the leading developer of mathematical computing software for engineers and scientists. Let's talk about the basic rules of logical indexing, and then we'll reexamine the expression B(isnan(B)). Another type of array index, a logical index, designates the elements of A based on their position in the indexing array, B. These two features, logical vectors and logical indexing, combine to make one of the most expressive features of MATLAB. Thanks for the answer! When we index an array with a logical variable, Matlab is using find to locate all the true elements and convert them to indices. In MATLAB®, there are three primary approaches to accessing array elements based on their location (index) in the array. I'm having problems with logical indexing of arrays. You can also select a web site from the following list: Select the China site (in Chinese or English) for best site performance. Create a numeric matrix. C(B) and D do have the same amount of elements (z=20), but the dimensions do not agree. The most frequent use of logicals for us, however, will be in indexing. Only places in which B is not equal to zero will have non-zero output. C and D are also 10x10 arrays. You can also select a web site from the following list: Select the China site (in Chinese or English) for best site performance. This code returns logical true for when both cells are 0 or 1. D is an 5x4 array. Well, with this example we say goodbye to logical indexing. correctly governs the placement of the calculated elements. What does confuse me is the fact that the following code does work as well: In this case the output of C(B) and D(B) is, e.g., a 10x1 Array, it is transposed to the Array in your code. Learn more about using logical arrays to change specific positions in a muiltidimensional array Logical Indexing of Cell Array containing matrices. Every variable in MATLAB® is an array that can hold many numbers. Reload the page to see its updated state. If you can't tell, I'm not too great at MATLAB, but I am trying. It is possible that creating a large logical array via logical indexing could be slower than a loop. If you want to make it work the code has to look like this: I guess the colon does indicate that linear indexing should be used, which does agree with the logical array. Based on your location, we recommend that you select: . 1.… B(isnan(B)) = 0 A logical matrix provides a different type of array indexing in MATLAB. With two for-loops it would look like this: I'm not sure if the same can be achieved with logical indexing as C(B) just gives a vector as an output and of course then the dimensions do not agree. If i want to make the following code work, i guess i do have to transpose the 5x4 Array E into a vector with the same length as C(B) and D(B), am I right? function 'eq' for input arguments of type 'cell'. This is a tutorial on using logical expressions as indexes for vectors and matrices in MATLAB. Introduction to MATLAB Indexing. This does indeed work, but it seems that I do not fully understand how the logical indexing is working. Other MathWorks country sites are not optimized for visits from your location. Learn more about cell arrays, indexing . I was thinking that by using logical indexing the programm is just ignoring the values where the condition is not fulfilled. When you want to access selected elements of an array, use indexing. Whole array logical indexing: the mask must have the same shape as the array and the returned value is of rank 1 (a vector in Matlab). You can base this type of indexing on either the values or the positions of elements in the indexing array. Pure multidimensional indexing specifies an index variable for each dimension of the array. This is so because the position of the trues is arbitrary, and thus you cannot guarantee that the result of, basically, poking holes into a matrix will be rectangular. These approaches are indexing by position, linear indexing, and logical indexing. "Logical" is one of the builtin types, or classes, of MATLAB matrices. Just in case someone else is having problems with this, here is some code to explain my former problem and the solution. Both cell arrays are a series of double arrays which match in dimension between the cell arrays, but change size within the cell array. To replace all NaN elements of the matrix B with zero, use. If you are trying to calculate something like the following: As the above equation is only false for the first column, the logical Matrix B looks like this. In this masking type of operation, every true element in the indexing array is treated as a positional index into the array being accessed. An intermediate situation exists, which may be termed partially linear indexing: 1.1. Is there a way to search strings in a cell array similar to numeric arrays? Based on your location, we recommend that you select: . Your code does indeed work perfectly, I am sorry, if i wasnt clear on that. I was thinking that by using logical indexing the programm is just ignoring the values where the condition is not fulfilled. A video segment from the upcoming Coursera MOOC on introductory computer programming with MATLAB by Vanderbilt. Sign in to answer this question. The syntax is certainly fine since diff_matrix is a logical array. By testing your code ít does not seem that way. 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