lass="op">$</span>b <span class="op">==</span><span class="st"> </span><span class="dv">-99</span>] &lt;-<span class="st"> </span><span class="ot">NA</span></span>
<span id="cb24-4"><a href="Intro.html#cb24-4"></a>df<span class="op">$</span>c[df<span class="op">$</span>c <span class="op">==</span><span class="st"> </span><span class="dv">-99</span>] &lt;-<span class="st"> </span><span class="ot">NA</span></span>
<span id="cb24-5"><a href="Intro.html#cb24-5"></a>df<span class="op">$</span>d[df<span class="op