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<h1 class="title">Loops and Conditionals</h1>
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<div id="learning-objectives" class="section level2">
<h2>Learning Objectives</h2>
<ul>
<li>Write and use <code>for</code> loops to apply the same code to multiple objects or elements</li>
<li>Use <code>if</code>-<code>else</code> statements to conditionally execute code</li>
</ul>
</div>
<div id="required-packages-and-datasets" class="section level2">
<h2>Required Packages and Datasets</h2>
<p>Make sure you can load all these packages and dataset before starting the module</p>
<pre class="r"><code>library(dplyr)
library(uwpols501)
data(iver)</code></pre>
</div>
<div id="for-loops" class="section level2">
<h2>For Loops</h2>
<p>We use <code>for</code> loops to iterate through multiple elements of a list, variable, etc., and do <strong>something</strong> with those elements.</p>
<p>A <code>for</code> loop has the following skeleton:</p>
<pre class="r"><code>for (each_element in here) { # Try to use meaningful names for the
# do something # "each_element" object
print(each_element)
}</code></pre>
<p>A short/silly example:</p>
<pre class="r"><code>list_numbers <- sample(x = 1:100, size = 10, replace = FALSE) # sample() function
for (number in list_numbers) {
print(paste0("2 times ", number, " is ", number * 2))
}</code></pre>
<pre><code>## [1] "2 times 56 is 112"
## [1] "2 times 6 is 12"
## [1] "2 times 46 is 92"
## [1] "2 times 47 is 94"
## [1] "2 times 78 is 156"
## [1] "2 times 36 is 72"
## [1] "2 times 52 is 104"
## [1] "2 times 16 is 32"
## [1] "2 times 58 is 116"
## [1] "2 times 81 is 162"</code></pre>
<p>A real example:</p>
<p>Imagine that we want to calculate the <span class="math inline">\(log\)</span> of the variables <code>povred</code> and <code>enp</code> of the <code>iver</code> dataset. We can create a list with these two variable names and then, for each of them, find them in <code>iver</code>, calculate the <span class="math inline">\(log\)</span>, and add the result as a new variable to the dataset.</p>
<pre class="r"><code>numeric_vars <- c("povred", "enp")
for (variable in numeric_vars) {
y <- iver[,variable]
log_y <- log(y)
name_new_var <- paste0(variable, "_log")
iver[,name_new_var] <- log_y
}</code></pre>
</div>
<div id="conditional-execution-if-and-else-statements" class="section level2">
<h2>Conditional Execution: if and else statements</h2>
<p>Sometimes we only want to execute certain code if the data fulfills some conditions. To do that we use <code>if</code> and <code>else</code> statements.</p>
<p>How <code>if</code> and <code>else</code> statements look like:</p>
<pre class="r"><code>if (this) {
# do that
} else if (that) {
# do something else
} else {
#
}</code></pre>
<p><strong>This</strong> and <strong>that</strong> in the previous chunk of code are boolean tests: code that returns TRUE/FALSE when the computer executes it. Some examples of boolean tests:</p>
<pre class="r"><code>10 == 2</code></pre>
<pre><code>## [1] FALSE</code></pre>
<pre class="r"><code>10 %in% c(2, 5, 13, 20, 10)</code></pre>
<pre><code>## [1] TRUE</code></pre>
<pre class="r"><code>10 == 2 & 10 %in% c(2, 5, 13, 20, 10)</code></pre>
<pre><code>## [1] FALSE</code></pre>
<pre class="r"><code>10 == 2 | 10 %in% c(2, 5, 13, 20, 10)</code></pre>
<pre><code>## [1] TRUE</code></pre>
<p>A short/silly example:</p>
<pre class="r"><code>some_numbers <- c(1, 4, 6, 10,12, 16, 45, 88, 102)
for (number in some_numbers) {
if (number < 10) {
print(paste0(number, " is smaller than 10"))
} else if (number < 50) {
print(paste0(number, " is smaller than 50 but greater or equal to 10"))
} else {
print(paste0(number, " is greater than 50"))
}
}</code></pre>
<pre><code>## [1] "1 is smaller than 10"
## [1] "4 is smaller than 10"
## [1] "6 is smaller than 10"
## [1] "10 is smaller than 50 but greater or equal to 10"
## [1] "12 is smaller than 50 but greater or equal to 10"
## [1] "16 is smaller than 50 but greater or equal to 10"
## [1] "45 is smaller than 50 but greater or equal to 10"
## [1] "88 is greater than 50"
## [1] "102 is greater than 50"</code></pre>
<p>Real example:</p>
<p>We previously calculated the <span class="math inline">\(log\)</span> and added it to the dataset for the numeric variables in <code>iver</code>. To do that we first created a list with the numeric variables. This time we won’t create that list. Instead, we’ll use conditional execution to indicate that we only want to take the <span class="math inline">\(log\)</span> of numeric variables.</p>
<pre class="r"><code>data(iver) # load the data set again
iver <- as.data.frame(iver)
for (variable in names(iver)) {
y <- iver[,variable]
if (is.numeric(y)) {
log_y <- log(y)
name_new_var <- paste0(variable, "_log")
iver[,name_new_var] <- log_y
} else{
print(paste0(variable, " is not a numeric variable."))
}
}</code></pre>
<pre><code>## [1] "cty is not a numeric variable."
## [1] "elec_sys is not a numeric variable."</code></pre>
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