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<li class="toctree-l1"><a class="reference internal" href="Section_intro.html">1. Introduction</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_errors.html">12. Errors</a></li>
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<li>if command</li>
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<div class="section" id="if-command">
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<span id="index-0"></span><h1>if command</h1>
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<div class="section" id="syntax">
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<h2>Syntax</h2>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="k">if</span> <span class="n">boolean</span> <span class="n">then</span> <span class="n">t1</span> <span class="n">t2</span> <span class="o">...</span> <span class="k">elif</span> <span class="n">boolean</span> <span class="n">f1</span> <span class="n">f2</span> <span class="o">...</span> <span class="k">elif</span> <span class="n">boolean</span> <span class="n">f1</span> <span class="n">f2</span> <span class="o">...</span> <span class="k">else</span> <span class="n">e1</span> <span class="n">e2</span> <span class="o">...</span>
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</pre></div>
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</div>
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<ul class="simple">
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<li>boolean = a Boolean expression evaluated as TRUE or FALSE (see below)</li>
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<li>then = required word</li>
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<li>t1,t2,...,tN = one or more LAMMPS commands to execute if condition is met, each enclosed in quotes</li>
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<li>elif = optional word, can appear multiple times</li>
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<li>f1,f2,...,fN = one or more LAMMPS commands to execute if elif condition is met, each enclosed in quotes (optional arguments)</li>
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<li>else = optional argument</li>
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<li>e1,e2,...,eN = one or more LAMMPS commands to execute if no condition is met, each enclosed in quotes (optional arguments)</li>
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</ul>
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</div>
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<div class="section" id="examples">
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<h2>Examples</h2>
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<div class="highlight-default"><div class="highlight"><pre><span></span>if "${steps} > 1000" then quit
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if "${myString} == a10" then quit
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if "$x <= $y" then "print X is smaller = $x" else "print Y is smaller = $y"
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if "(${eng} > 0.0) || ($n < 1000)" then &
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"timestep 0.005" &
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elif $n<10000 &
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"timestep 0.01" &
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else &
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"timestep 0.02" &
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"print 'Max step reached'"
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if "${eng} > ${eng_previous}" then "jump file1" else "jump file2"
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</pre></div>
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</div>
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</div>
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<div class="section" id="description">
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<h2>Description</h2>
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<p>This command provides an if-then-else capability within an input
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script. A Boolean expression is evaluted and the result is TRUE or
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FALSE. Note that as in the examples above, the expression can contain
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variables, as defined by the <a class="reference internal" href="variable.html"><span class="doc">variable</span></a> command, which
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will be evaluated as part of the expression. Thus a user-defined
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formula that reflects the current state of the simulation can be used
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to issue one or more new commands.</p>
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<p>If the result of the Boolean expression is TRUE, then one or more
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commands (t1, t2, ..., tN) are executed. If it is FALSE, then Boolean
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expressions associated with successive elif keywords are evaluated
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until one is found to be true, in which case its commands (f1, f2,
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..., fN) are executed. If no Boolean expression is TRUE, then the
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commands associated with the else keyword, namely (e1, e2, ..., eN),
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are executed. The elif and else keywords and their associated
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commands are optional. If they aren’t specified and the initial
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Boolean expression is FALSE, then no commands are executed.</p>
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<p>The syntax for Boolean expressions is described below.</p>
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<p>Each command (t1, f1, e1, etc) can be any valid LAMMPS input script
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command, except an <a class="reference internal" href="include.html"><span class="doc">include</span></a> command, which is not
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allowed. If the command is more than one word, it must enclosed in
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quotes, so it will be treated as a single argument, as in the examples
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above.</p>
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<div class="admonition note">
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<p class="first admonition-title">Note</p>
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<p class="last">If a command itself requires a quoted argument (e.g. a
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<a class="reference internal" href="print.html"><span class="doc">print</span></a> command), then double and single quotes can be used
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and nested in the usual manner, as in the examples above and below.
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See <a class="reference internal" href="Section_commands.html#cmd-2"><span class="std std-ref">Section_commands 2</span></a> of the manual for
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more details on using quotes in arguments. Only one of level of
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nesting is allowed, but that should be sufficient for most use cases.</p>
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</div>
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<p>Note that by using the line continuation character “&”, the if command
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can be spread across many lines, though it is still a single command:</p>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="k">if</span> <span class="s2">"$a < $b"</span> <span class="n">then</span> <span class="o">&</span>
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<span class="s2">"print 'Minimum value = $a'"</span> <span class="o">&</span>
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<span class="s2">"run 1000"</span> <span class="o">&</span>
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<span class="k">else</span> <span class="o">&</span>
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<span class="s1">'print "Minimum value = $b"'</span> <span class="o">&</span>
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<span class="s2">"minimize 0.001 0.001 1000 10000"</span>
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</pre></div>
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</div>
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<p>Note that if one of the commands to execute is <a class="reference internal" href="quit.html"><span class="doc">quit</span></a>, as in
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the first example above, then executing the command will cause LAMMPS
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to halt.</p>
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<p>Note that by jumping to a label in the same input script, the if
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command can be used to break out of a loop. See the <a class="reference internal" href="variable.html"><span class="doc">variable delete</span></a> command for info on how to delete the associated
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loop variable, so that it can be re-used later in the input script.</p>
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<p>Here is an example of a loop which checks every 1000 steps if the
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system temperature has reached a certain value, and if so, breaks out
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of the loop to finish the run. Note that any variable could be
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checked, so long as it is current on the timestep when the run
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completes. As explained on the <a class="reference internal" href="variable.html"><span class="doc">variable</span></a> doc page,
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this can be insured by includig the variable in thermodynamic output.</p>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">variable</span> <span class="n">myTemp</span> <span class="n">equal</span> <span class="n">temp</span>
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<span class="n">label</span> <span class="n">loop</span>
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<span class="n">variable</span> <span class="n">a</span> <span class="n">loop</span> <span class="mi">1000</span>
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<span class="n">run</span> <span class="mi">1000</span>
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<span class="k">if</span> <span class="s2">"$</span><span class="si">{myTemp}</span><span class="s2"> < 300.0"</span> <span class="n">then</span> <span class="s2">"jump SELF break"</span>
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<span class="nb">next</span> <span class="n">a</span>
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<span class="n">jump</span> <span class="n">SELF</span> <span class="n">loop</span>
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<span class="n">label</span> <span class="k">break</span>
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<span class="nb">print</span> <span class="s2">"ALL DONE"</span>
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</pre></div>
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</div>
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<p>Here is an example of a double loop which uses the if and
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<a class="reference internal" href="jump.html"><span class="doc">jump</span></a> commands to break out of the inner loop when a
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condition is met, then continues iterating thru the outer loop.</p>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">label</span> <span class="n">loopa</span>
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<span class="n">variable</span> <span class="n">a</span> <span class="n">loop</span> <span class="mi">5</span>
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<span class="n">label</span> <span class="n">loopb</span>
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<span class="n">variable</span> <span class="n">b</span> <span class="n">loop</span> <span class="mi">5</span>
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<span class="nb">print</span> <span class="s2">"A,B = $a,$b"</span>
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<span class="n">run</span> <span class="mi">10000</span>
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<span class="k">if</span> <span class="s2">"$b > 2"</span> <span class="n">then</span> <span class="s2">"jump SELF break"</span>
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<span class="nb">next</span> <span class="n">b</span>
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<span class="n">jump</span> <span class="ow">in</span><span class="o">.</span><span class="n">script</span> <span class="n">loopb</span>
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<span class="n">label</span> <span class="k">break</span>
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<span class="n">variable</span> <span class="n">b</span> <span class="n">delete</span>
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<span class="nb">next</span> <span class="n">a</span>
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<span class="n">jump</span> <span class="n">SELF</span> <span class="n">loopa</span>
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</pre></div>
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</div>
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<hr class="docutils" />
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<p>The Boolean expressions for the if and elif keywords have a C-like
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syntax. Note that each expression is a single argument within the if
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command. Thus if you want to include spaces in the expression for
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clarity, you must enclose the entire expression in quotes.</p>
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<p>An expression is built out of numbers (which start with a digit or
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period or minus sign) or strings (which start with a letter and can
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contain alphanumeric characters or underscores):</p>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="mf">0.2</span><span class="p">,</span> <span class="mi">100</span><span class="p">,</span> <span class="mf">1.0e20</span><span class="p">,</span> <span class="o">-</span><span class="mf">15.4</span><span class="p">,</span> <span class="n">etc</span>
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<span class="n">InP</span><span class="p">,</span> <span class="n">myString</span><span class="p">,</span> <span class="n">a123</span><span class="p">,</span> <span class="n">ab_23_cd</span><span class="p">,</span> <span class="n">etc</span>
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</pre></div>
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</div>
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<p>and Boolean operators:</p>
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<div class="highlight-default"><div class="highlight"><pre><span></span>A == B, A != B, A < B, A <= B, A > B, A >= B, A && B, A || B, !A
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</pre></div>
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</div>
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<p>Each A and B is a number or string or a variable reference like $a or
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${abc}, or A or B can be another Boolean expression.</p>
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<p>If a variable is used it can produce a number when evaluated, like an
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<a class="reference internal" href="variable.html"><span class="doc">equal-style variable</span></a>. Or it can produce a string,
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like an <a class="reference internal" href="variable.html"><span class="doc">index-style variable</span></a>. For an individual
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Boolean operator, A and B must both be numbers or must both be
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strings. You cannot compare a number to a string.</p>
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<p>Expressions are evaluated left to right and have the usual C-style
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precedence: the unary logical NOT operator ”!” has the highest
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precedence, the 4 relational operators “<”, “<=”, “>”, and “>=” are
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next; the two remaining relational operators “==” and ”!=” are next;
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then the logical AND operator “&&”; and finally the logical OR
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operator “||” has the lowest precedence. Parenthesis can be used to
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group one or more portions of an expression and/or enforce a different
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order of evaluation than what would occur with the default precedence.</p>
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<p>When the 6 relational operators (first 6 in list above) compare 2
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numbers, they return either a 1.0 or 0.0 depending on whether the
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relationship between A and B is TRUE or FALSE. When the 6 relational
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operators compare 2 strings, they also return a 1.0 or 0.0 for TRUE or
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FALSE, but the comparison is done by the C function strcmp().</p>
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<p>When the 3 logical operators (last 3 in list above) compare 2 numbers,
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they also return either a 1.0 or 0.0 depending on whether the
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relationship between A and B is TRUE or FALSE (or just A). The
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logical AND operator will return 1.0 if both its arguments are
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non-zero, else it returns 0.0. The logical OR operator will return
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1.0 if either of its arguments is non-zero, else it returns 0.0. The
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logical NOT operator returns 1.0 if its argument is 0.0, else it
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returns 0.0. The 3 logical operators can only be used to operate on
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numbers, not on strings.</p>
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<p>The overall Boolean expression produces a TRUE result if the result is
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non-zero. If the result is zero, the expression result is FALSE.</p>
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</div>
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<hr class="docutils" />
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<div class="section" id="restrictions">
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<h2>Restrictions</h2>
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<blockquote>
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<div>none</div></blockquote>
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</div>
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<div class="section" id="related-commands">
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<h2>Related commands</h2>
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<p><a class="reference internal" href="variable.html"><span class="doc">variable</span></a>, <a class="reference internal" href="print.html"><span class="doc">print</span></a></p>
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<p><strong>Default:</strong> none</p>
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