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    <p><strong>Edit:</strong> @Dwin's comment to the Q was a bit cryptic, and seeing as my first attempt at part 1 of the Q was not correct due to the unclear Q, I'll try to redeem myself with a go at expanding on DWin's comment:</p> <p>[Assuming <code>dat</code> contains the data you quote in the Q.] First, merge <code>dat</code> with itself:</p> <pre><code>&gt; foo &lt;- merge(dat[, -4], dat, by.x = "Date", by.y = "Date") &gt; head(foo) Date Name.x Price.x Name.y Price.y Interest 1 01.02.10 X 120 X 120 0.2 2 01.02.10 X 120 R 120 0.2 3 01.02.10 X 120 X 130 0.2 4 01.02.10 X 120 X 140 0.2 5 01.02.10 X 120 R 130 0.2 6 01.02.10 R 120 X 120 0.2 </code></pre> <p>Next, get out the rows where <code>Price.x == Price.y</code> <strong>and</strong> where <code>Name.x != Name.y</code></p> <pre><code>&gt; (foo &lt;- foo[with(foo, which(Price.x == Price.y &amp; Name.x != Name.y)),]) Date Name.x Price.x Name.y Price.y Interest 2 01.02.10 X 120 R 120 0.2 6 01.02.10 R 120 X 120 0.2 15 01.02.10 X 130 R 130 0.2 23 01.02.10 R 130 X 130 0.2 </code></pre> <p>Then, get rid of the superfluous columns:</p> <pre><code>&gt; (foo &lt;- foo[, -(4:5)]) Date Name.x Price.x Interest 2 01.02.10 X 120 0.2 6 01.02.10 R 120 0.2 15 01.02.10 X 130 0.2 23 01.02.10 R 130 0.2 </code></pre> <p>And finally, fix-up the column names:</p> <pre><code>&gt; names(foo) &lt;- names(dat) &gt; foo Date Name Price Interest 2 01.02.10 X 120 0.2 6 01.02.10 R 120 0.2 15 01.02.10 X 130 0.2 23 01.02.10 R 130 0.2 </code></pre> <hr> <p>The second thing can be done using <code>ifelse</code></p> <pre><code>with(dat, ifelse(Name == "X", Price + 100*Interest, Price)) </code></pre> <p>Which gives something this</p> <pre><code>&gt; with(dat, ifelse(Name == "X", Price + 100*Interest, Price)) [1] 140 120 150 160 130 </code></pre> <p>The reason that the <code>if()</code> doesn't work, is that <code>if()</code> only take a scalar logical (a single <code>TRUE</code> or <code>FALSE</code>), yet <code>Name == "X"</code> returns a logical vector:</p> <pre><code>&gt; with(dat, Name == "X") [1] TRUE FALSE TRUE TRUE FALSE </code></pre> <p>In these cases, <code>ifelse()</code> is your friend.</p>
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    1. USGavin Simpson
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    1. COHey Gavin! Thanks for the answer. The second part works perfectly. I'm sorry for the bad description of the first part. I accidently took a subset of my dataframe where the interest is actually equal. However, it does change and is not constant. I tried unique but actually nothing happens. I edited my first post to make the first part clearer. Cheers Dani
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    2. COGavin, thank you very much for the explanation.
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