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    <p>This is documented on <code>?logical</code>. The pertinent section of which is:</p> <pre><code>Details: ‘TRUE’ and ‘FALSE’ are reserved words denoting logical constants in the R language, whereas ‘T’ and ‘F’ are global variables whose initial values set to these. All four are ‘logical(1)’ vectors. Logical vectors are coerced to integer vectors in contexts where a numerical value is required, with ‘TRUE’ being mapped to ‘1L’, ‘FALSE’ to ‘0L’ and ‘NA’ to ‘NA_integer_’. </code></pre> <p>The second paragraph there explains the behaviour you are seeing, namely <code>5 == 1L</code> and <code>5 == 0L</code> respectively, which should both return <code>FALSE</code>, where as <code>1 == 1L</code> and <code>0 == 0L</code> should be TRUE for <code>1 == TRUE</code> and <code>0 == FALSE</code> respectively. I believe these are not testing what you want them to test; the comparison is on the basis of the numerical representation of <code>TRUE</code> and <code>FALSE</code> in R, i.e. what numeric values they take when coerced to numeric.</p> <p>However, only <code>TRUE</code> is guaranteed to the be <code>TRUE</code>:</p> <pre><code>&gt; isTRUE(TRUE) [1] TRUE &gt; isTRUE(1) [1] FALSE &gt; isTRUE(T) [1] TRUE &gt; T &lt;- 2 &gt; isTRUE(T) [1] FALSE </code></pre> <p><code>isTRUE</code> is a wrapper for <code>identical(x, TRUE)</code>, and from <code>?isTRUE</code> we note:</p> <pre><code>Details: .... ‘isTRUE(x)’ is an abbreviation of ‘identical(TRUE, x)’, and so is true if and only if ‘x’ is a length-one logical vector whose only element is ‘TRUE’ and which has no attributes (not even names). </code></pre> <p>So by the same virtue, only <code>FALSE</code> is guaranteed to be exactly equal to <code>FALSE</code>.</p> <pre><code>&gt; identical(F, FALSE) [1] TRUE &gt; identical(0, FALSE) [1] FALSE &gt; F &lt;- "hello" &gt; identical(F, FALSE) [1] FALSE </code></pre> <p>If this concerns you, always use <code>isTRUE()</code> or <code>identical(x, FALSE)</code> to check for equivalence with <code>TRUE</code> and <code>FALSE</code> respectively. <code>==</code> is not doing what you think it is.</p>
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    1. POWhat evaluates to True/False in R?
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    1. USGavin Simpson
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    1. POWhat evaluates to True/False in R?
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    1. COOne more thing. In an `if` condition, non-zero numbers are treated like `TRUE` and 0 is treated like `FALSE`.
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      1. USRichie Cotton
    2. CO@Richie: it's not just numbers; every condition is coerced to logical. See my comment on Rafe's answer.
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      1. USJoshua Ulrich
 

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