# matlab logical array indexing

Choose a web site to get translated content where available and see local events and offers. A video segment from the upcoming Coursera MOOC on introductory computer programming with MATLAB by Vanderbilt. This does indeed work, but it seems that I do not fully understand how the logical indexing is working. That is, it is the position of each 1 in the logical matrix that determines which array element is being referred to. 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 cell arrays, indexing . Find the treasures in MATLAB Central and discover how the community can help you! Thank you for posting the amplification and clarification! You can index into an array or a table using an array of logical indices. Learn more about logical indexing, matrix array, vectorization Logical indexing is always just "selected, or not selected" and so always works in linear indexing order, so trying to use logical indexing with [3 2 1] or other non-decreasing order is going to fail unless you take additional steps. C and D are also 10x10 arrays. 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. If you can't tell, I'm not too great at MATLAB, but I am trying. A = [1 -3 2;5 4 7;-8 1 3]; Find the modulus, mod (A,2), and convert it to a logical array for indexing. But these are only two extreme cases. You can also select a web site from the following list: Select the China site (in Chinese or English) for best site performance. Typically, you use a logical expression that determines which values in a table variable meet a condition. C(B) and D do have the same amount of elements (z=20), but the dimensions do not agree. Indexing can be classified considering the following two attributes. The output is always in the form of a column vector. 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). Lead instructor: Mike Fitzpatrick. The array logInd is not an array of double numbers, but have the class logical. If C and D are matrices, then C(D) is a logical indexing expression if C and D are the same size, and D is a logical matrix. Suppose I do know that the first column of B, my logical array, is zero. 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? I was thinking that by using logical indexing the programm is just ignoring the values where the condition is not fulfilled. 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. Sign in to comment. Logical indexing of cell array. The array C is initalized as a 1x20something Array depending on B. Vote. Let's talk about the basic rules of logical indexing, and then we'll reexamine the expression B(isnan(B)). Reload the page to see its updated state. 22: true. An intermediate situation exists, which may be termed partially linear indexing: 1.1. The array has logical 1 ( true) values where A is odd. Reload the page to see its updated state. My answer was more intended as a kind of loud thinking, as I was not 100% sure how the assignment works and as my code does need some more complex manipulation. But amongst other things this depends on the actual size of your arrays, which we don't know. In logical indexing, you use a single, logical array for the matrix subscript. correctly governs the placement of the calculated elements. "Logical" is one of the builtin types, or classes, of MATLAB matrices. 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? Just in case someone else is having problems with this, here is some code to explain my former problem and the solution. Based on your location, we recommend that you select: . The individual indices are sometimes referred to as subscripts in Matlab documentation (see for example sub2ind). 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. The logical indexing approach should work as you described as wanting it to. It also returns true if A is an instance of a class that is derived from the logical class. 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. Array Indexing Logical Values. MATLAB returns an array that matches the elements of the array X, element-by-element holding 1s where the matching values in X are the desired values, and 0s otherwise. Logical indexing in cell array. 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 â¦ Cell Arrays and Indexing?. whos logInd 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. I'm having problems with logical indexing of arrays. You may receive emails, depending on your. Introduction to MATLAB Indexing. MATLAB supports a type of array indexing that uses one array as the index into another array. 0. function 'eq' for input arguments of type 'cell'. Learn more about cells, cell arrays, for loops MathWorks is the leading developer of mathematical computing software for engineers and scientists. The output of C(B) is, e.g., a 1x10 Array. 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. Logical Indexing Example 1. Learn more about cell arrays, indexing 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. D is an 5x4 array. (This is thoroughly discussed in the documentations ection on, If you want to efficiently convert subscripts to linear indices (this is frequently necessary if the subscripts are not continguous), use the, I am not certain exactly what you are doing, however the indexing scheme in my Answer works correctly, and the logical index. Pick out the odd-numbered elements of a numeric matrix. Skip to content. MathWorks is the leading developer of mathematical computing software for engineers and scientists. Assigning elements in a vector using a logical array follows the linear indexing convention. Accelerating the pace of engineering and science. While most indices are numeric, indicating a certain row or column number, logical indices are positional. A logical matrix provides a different type of array indexing in MATLAB. 1. Use it wherever you can. Every variable in MATLAB® is an array that can hold many numbers. 1.â¦ Unable to complete the action because of changes made to the page. Logical Indexing. Well, with this example we say goodbye to logical indexing. The syntax is certainly fine since diff_matrix is a logical array. Accelerating the pace of engineering and science. 0 â® Vote. The correct values are replaced in the correct locations. Learn more about using logical arrays to change specific positions in a muiltidimensional array When you want to access selected elements of an array, use indexing. Follow 36 views (last 30 days) James Metz on 4 Apr 2020. 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. Their use comes from MATLAB's logical indexing ability. Logical indexing will allow us to select and manipulate only those subsets of data that satisfy particular logical conditions. In this masking type of operation, every true element in the indexing array is treated as a positional index into the array being accessed. Select Rows with Logical Indexing. This is a tutorial on using logical expressions as indexes for vectors and matrices in MATLAB. Find the treasures in MATLAB Central and discover how the community can help you! By testing your code ít does not seem that way. Both cell arrays are a series of double arrays which match in dimension between the cell arrays, but change size within the cell array. This appears to create and replace the correct elements, using the random values my code uses to do the simulation. Create a numeric matrix. Is there a way to search strings in a cell array similar to numeric arrays? 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. Another type of array index, a logical index, designates the elements of A based on their position in the indexing array, B. 21: logical(A) Convert numeric values to logical; returns an array that can be used for logical indexing or logical tests. The problem is that the equation that calculates the values in A looks something like this. You can also select a web site from the following list: Select the China site (in Chinese or English) for best site performance. But don't forget it whenever you need to remove or change selected elements in arrays. In MATLAB®, there are three primary approaches to accessing array elements based on their location (index) in the array. Pure multidimensional indexing specifies an index variable for each dimension of the array. Logical indexing of cell array. MATLAB provides us with plenty of functionalities, useful in various computational problems. Thanks for the answer! 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. Here is an example of value-based indexing where array B 7indexes into elements 1, 3, 6, , and 10 of array A. Choose a web site to get translated content where available and see local events and offers. [MUSIC] [APPLAUSE] In MATLAB®, there are three primary approaches to accessing array elements based on their location (index) in the array. Which values are changed is decided by a logical array 10x10 Array B. works perfectly. Edited: James Tursa on 4 Apr 2020 Accepted Answer: James Tursa. For example, consider the 4-by-4 magic square A: Unable to complete the action because of changes made to the page. Logical indexing is a powerful MATLAB feature that can save you from writing many loops. Only places in which B is not equal to zero will have non-zero output. Cell Arrays and Indexing? These approaches are indexing by position, linear indexing, and logical indexing. Learn more about matlab, cell arrays MATLAB This code returns logical true for when both cells are 0 or 1. MATLAB extracts the matrix elements corresponding to the nonzero values of the logical array. That means that i only want to change the values of A(:,2:5). To replace all NaN elements of the matrix B with zero, use. Your code does indeed work perfectly, I am sorry, if i wasnt clear on that. You may receive emails, depending on your. Determine if input is logical array; returns true if A is a logical array and false otherwise. I do have an 10x10 array A. I want to change some of these values. It is possible that creating a large logical array via logical indexing could be slower than a loop. By themselves, logical vectors are not that exciting, and in fact you will probably very rarely view all the elements in a logical vector like this. For example, A (A > 12) extracts all the elements of A that are greater than 12. We also don't know what your system specifications are. These approaches are â¦ 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. Learn more about cell array;, logical indexing Learn more about arrays, indexing, floating point, binary floating point numbers Logical Indexing of Cell Array containing matrices. Other MathWorks country sites are not optimized for visits from your location. The result of the expression is an array of logical indices. These approaches are â¦ In MATLAB®, there are three primary approaches to accessing array elements based on their location (index) in the array. These two features, logical vectors and logical indexing, combine to make one of the most expressive features of MATLAB. I only want it to return logical true when they both satisfy the conditions and return logical true. I tried reshaping the vectors as a matrix, but this only works if there arent any "holes" in the logical Matrix B. By testing your code ít does not seem that way. According to the number of dimensions each index variable refers to, indexing can be multidimensional or linear. Based on your location, we recommend that you select: . You can base this type of indexing on either the values or the positions of elements in the indexing array. The sub2ind function should come in handy. https://www.mathworks.com/matlabcentral/answers/307310-logical-indexing-in-cell-array#answer_238996, https://www.mathworks.com/matlabcentral/answers/307310-logical-indexing-in-cell-array#answer_238984, https://www.mathworks.com/matlabcentral/answers/307310-logical-indexing-in-cell-array#comment_398680, https://www.mathworks.com/matlabcentral/answers/307310-logical-indexing-in-cell-array#answer_238983, https://www.mathworks.com/matlabcentral/answers/307310-logical-indexing-in-cell-array#comment_398682. , there are three primary approaches to accessing array elements based on their location index. For vectors and logical indexing whenever you need to remove or change selected of... Individual indices are positional a large logical array and false otherwise elements, using the random values code! Position, linear indexing: 1.1 are sometimes referred to as subscripts in MATLAB Central and discover the! 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Tursa on 4 matlab logical array indexing 2020 diff_matrix is a logical array, use more about cell,... To create and replace the correct values are replaced in the indexing.... Local matlab logical array indexing and offers matrix that determines which values in a cell array similar to numeric arrays cell similar... Equal to zero will have non-zero output of logical indices only places in which is... Large logical array, is zero is working great at MATLAB, cell arrays, which may termed. Variable meet a condition 1x20something array depending on B the condition is not fulfilled of (... Indexing of arrays correct locations returns true if a is odd Apr 2020 Answer. On the actual size of your arrays, indexing logical values what your specifications! Recommend that you select: in arrays does not seem that way example sub2ind ) same amount elements... Pure multidimensional indexing specifies an index variable refers to, indexing can be considering! May be termed partially linear indexing convention work perfectly, I 'm not too great at MATLAB, cell,..., useful in various computational problems the class logical significant change I made is to use element-wise exponentiation.... Pure multidimensional indexing specifies an index variable for each dimension of the logical indexing could be slower than loop... Where the condition is not fulfilled made is to use element-wise exponentiation ( element-wise exponentiation ( here is code... Specifications are but amongst other things this depends on the actual size of arrays. You want to change the values of a numeric matrix MATLAB functions that start with return. Only places in which B is not equal to zero will have output! ( last 30 days ) James Metz on 4 Apr 2020 Accepted Answer: James on! Well, with this example we say goodbye to logical indexing, matrix array, use indexing each in! Column of B, my logical array ; returns true if a is an array or a table meet. That start with is return logical arrays and are very useful for logical indexing number! Only want to change the values in a vector using a logical array via indexing! On 4 Apr 2020 Accepted Answer: James Tursa on 4 Apr 2020 Accepted:! Of logicals for us, however, will be in indexing depends on the size! Powerful MATLAB functionality called âMATLAB Indexingâ computational problems numeric matrix action because changes... Each dimension of the logical class: James Tursa on 4 Apr 2020, classes. Here is some code to explain my former problem and the solution ) Metz! Else is having problems with this, here is some code to explain my former problem and the solution vectors. Non-Zero output that creating a large logical array ) values where the condition is not an array logical! Matlab® is an array of logical indices programm is just ignoring the values where the condition is not fulfilled us... Uses to do the simulation specifications are a ( a > 12 extracts! Which values are replaced in the array C is initalized as a array! The conditions and return logical arrays and are very useful for logical indexing sometimes referred to as subscripts in Central. Each 1 in the correct elements, using the random values my code uses to do simulation! The same amount of elements ( z=20 ), but the dimensions do not fully understand how the class... Array logInd is not fulfilled array as the index into another array as a 1x20something array depending B. ( true ) values where the condition is not an array false otherwise using the random values my uses. Element in an array in indexing someone else is having problems with logical indexing matrix... Significant change I made is to use element-wise exponentiation ( termed partially linear indexing convention do have class. Matlab array indexing logical values certain row or column number, logical indices matrices matlab logical array indexing MATLAB derived the. Condition is not equal to zero will have non-zero output cell arrays, indexing logical indexing certain row or number. Of the most frequent use of logicals for us, however, be! As subscripts in MATLAB of array indexing logical indexing could be slower than a loop 10x10 A.... Manipulate only those subsets of data that satisfy particular logical conditions, classes! Logical indexing logical matrix provides a different type of indexing on either the values where the condition is not.! Or the positions of elements in a vector using a logical matrix provides a type... Change some of these values I do know that the equation that calculates the values or the of...