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copied!<p>Since matrices are stored in mode of numeric, type of integer/double, and storage mode integer/double (assuming that matrix holds numerical values) you can use simple square bracket indexing to address specific values in them, e.g.</p> <pre><code># create dummy data > m <- matrix(rnorm(25), 5) > mode(m) # check mode [1] "numeric" > typeof(m) # check type [1] "double" > storage.mode(m) [1] "double" </code></pre> <p>Now, in order to create diagonal matrix, you can use <code>lower.tri</code> and <code>upper.tri</code> functions, which return logical values for matrix elements beneath and above the main diagonal, respectively.</p> <p>You can use <code>lower.tri</code> and <code>upper.tri</code> functions to set off-diagonal elements to 0.</p> <pre><code>> m[lower.tri(m)] <- 0 > m[upper.tri(m)] <- 0 > m [,1] [,2] [,3] [,4] [,5] [1,] 0.3356640 0.0000000 0.0000000 0.000000 0.0000000 [2,] 0.0000000 -0.2940369 0.0000000 0.000000 0.0000000 [3,] 0.0000000 0.0000000 -0.4490546 0.000000 0.0000000 [4,] 0.0000000 0.0000000 0.0000000 -1.093924 0.0000000 [5,] 0.0000000 0.0000000 0.0000000 0.000000 0.3199157 </code></pre> <p>You can, of course, set any constant to lower/upper diagonal elements, be it numerical, character, logical, NA, etc...</p> <p>Once you have set zeros as an off-diagonal elements, you can simply use square-bracket indexing. A little diversion: bare in mind that mathematically, vectors are special cases of matrices (and <em>vice versa</em>), but the same doesn't stand for <em>R</em>:</p> <pre><code>> is.vector(m) [1] FALSE > is.matrix(1:10) [1] FALSE </code></pre> <p>But, finally, you <strong>can</strong> do:</p> <pre><code>> m[m == 0] <- 1 </code></pre> <p>This will insert 1 in matrix wherever zeros occur. You can use <code>diag</code> function to access/change the diagonal elements, for example:</p> <pre><code>> m[m == 1] <- 0 > diag(m) <- rep(pi, 5) </code></pre> <p>will change your diagonal matrix to scalar one.</p> <p>After this boring post of mine, let us all hope that it helped someone, somehow...</p> <p>Cheers!</p>

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