R: Apply Functions

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R: Apply Functions

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R: Apply Functions

  1. 1. Apply functions<br />The Dataminingtools.net Team<br />
  2. 2. Apply functions<br />Apply functions are used to execute a function repetitively. &quot;Apply&quot; functions keeps us from having to write loops to perform some operation on every row or every column of a matrix or data frame, or on every element in a list.<br />
  3. 3. Apply family<br />sapply()<br />lapply()<br />apply()<br /><ul><li>mapply()
  4. 4. tapply()
  5. 5. rapply()</li></li></ul><li>Usage<br />Using Loops!<br />&gt; avg &lt;- numeric (8)<br />&gt; avg<br />[1] 0 0 0 0 0 0 0 0<br />&gt; for(i in 1:8)<br />+ avg[i]&lt;-mean(state.x77[,i])<br />&gt; avg[i]<br />[1] 70735.88<br />&gt; avg<br />[1] 4246.4200 4435.8000 1.1700 70.8786 7.3780<br />[6] 53.1080 104.4600 70735.8800<br />
  6. 6. Usage<br />Using ‘apply’<br />&gt; apply (state.x77, 2, median)<br />Population Income Illiteracy Life Exp Murder <br /> 2838.500 4519.000 0.950 70.675 6.850 <br /> HS Grad Frost Area <br /> 53.250 114.500 54277.000<br />The 2 means &quot;go by column&quot; -- a 1 would have meant &quot;go by row.&quot;<br />
  7. 7. Usage<br />We construct a function and pass it to apply. It computes the median and maximum of each column of state.x77.<br />
  8. 8. Usage<br /> apply() works on each row, one at a time, to find the smallest number in each row. which() function, returns the indices within a vector for which the vector holds the value TRUE<br />
  9. 9. lapplyand sapply<br />The lapply() function works on any list. The &quot;l&quot; in &quot;lapply&quot; stands for &quot;list.&quot;<br />The &quot;s&quot; in &quot;sapply&quot; stands for &quot;simplify.&quot;<br />
  10. 10. tapply<br />tapply() is a very powerful function that lets us break a vector into pieces and apply some function to each of the pieces. It is like sapply(), except that with sapply() the pieces are always elements of a list. With tapply() we get to specify how the breakdown is done. <br />&gt;tapply(barley$yield, barley$site, mean) <br />Grand Rapids Duluth University Farm Morris Crookston Waseca <br />24.93167 27.99667 32.66667 35.4 37.42 48.10833<br />

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