For Loop in R Programming Language: Syntax and Examples

A for loop is used to iterate over a vector in R programming.

Syntax of for loop

for (val in sequence)
{
statement
}

Here, sequence is a vector and val takes on each of its value during the loop. In each iteration, statement is evaluated.

Flowchart of for loop

Example: for loop

Below is an example to count the number of even numbers in a vector.

x <- c(2,5,3,9,8,11,6)
count <- 0
for (val in x) {
if(val %% 2 == 0)  count = count+1
}
print(count)

Output

[1] 3

In the above example, the loop iterates 7 times as the vector x has 7 elements.

In each iteration, val takes on the value of corresponding element of x.

We have used a counter to count the number of even numbers in x. We can see that x contains 3 even numbers.

Useful example of for loop

Code

<span class="pln">
</span><span class="com"># Nice example</span><span class="pln">

</span><span class="kwd">for</span><span class="pun">(</span><span class="pln">i </span><span class="kwd">in</span><span class="pln"> </span><span class="lit">1</span><span class="pun">:</span><span class="lit">10</span><span class="pun">)</span><span class="pln">
</span><span class="pun">{</span><span class="pln">
  
  </span><span class="kwd">print</span><span class="pun">(</span><span class="pln">i</span><span class="pun">)</span><span class="pln">
</span><span class="pun">}</span><span class="pln">

</span><span class="kwd">print</span><span class="pun">(</span><span class="lit">1</span><span class="pun">:</span><span class="lit">10</span><span class="pun">)</span><span class="pln">
fruits </span><span class="pun">&lt;-</span><span class="pln"> c</span><span class="pun">(</span><span class="str">"apple"</span><span class="pun">,</span><span class="pln"> </span><span class="str">"Banana"</span><span class="pun">,</span><span class="pln"> </span><span class="str">"Pomegranate"</span><span class="pun">)</span><span class="pln">
fruitLength </span><span class="pun">&lt;-</span><span class="pln"> rep</span><span class="pun">(</span><span class="pln">NA</span><span class="pun">,</span><span class="pln"> length</span><span class="pun">(</span><span class="pln">fruits</span><span class="pun">))</span><span class="pln">
fruitLength
names</span><span class="pun">(</span><span class="pln">fruitLength</span><span class="pun">)</span><span class="pln"> </span><span class="pun">&lt;-</span><span class="pln"> fruits
fruitLength

</span><span class="kwd">for</span><span class="pun">(</span><span class="pln">a </span><span class="kwd">in</span><span class="pln"> fruits</span><span class="pun">){</span><span class="pln">
  fruitLength</span><span class="pun">[</span><span class="pln">a</span><span class="pun">]</span><span class="pln"> </span><span class="pun">&lt;-</span><span class="pln"> nchar</span><span class="pun">(</span><span class="pln">a</span><span class="pun">)</span><span class="pln">
</span><span class="pun">}</span><span class="pln">

fruitLength

</span><span class="com"># another way to do the above thing</span><span class="pln">

fruitLength2 </span><span class="pun">&lt;-</span><span class="pln"> nchar</span><span class="pun">(</span><span class="pln">fruits</span><span class="pun">)</span><span class="pln">
fruitLength2
names</span><span class="pun">(</span><span class="pln">fruitLength2</span><span class="pun">)</span><span class="pln"> </span><span class="pun">&lt;-</span><span class="pln"> fruits
fruitLength2

identical</span><span class="pun">(</span><span class="pln">fruitLength</span><span class="pun">,</span><span class="pln"> fruitLength2</span><span class="pun">)</span><span class="pln">


</span>

Output

<span class="pln">
</span><span class="pun">&gt;</span><span class="pln"> </span><span class="kwd">for</span><span class="pun">(</span><span class="pln">i </span><span class="kwd">in</span><span class="pln"> </span><span class="lit">1</span><span class="pun">:</span><span class="lit">10</span><span class="pun">)</span><span class="pln">
</span><span class="pun">+</span><span class="pln"> </span><span class="pun">{</span><span class="pln">
</span><span class="pun">+</span><span class="pln">   
</span><span class="pun">+</span><span class="pln">   </span><span class="kwd">print</span><span class="pun">(</span><span class="pln">i</span><span class="pun">)</span><span class="pln">
</span><span class="pun">+</span><span class="pln"> </span><span class="pun">}</span><span class="pln">
</span><span class="pun">[</span><span class="lit">1</span><span class="pun">]</span><span class="pln"> </span><span class="lit">1</span><span class="pln">
</span><span class="pun">[</span><span class="lit">1</span><span class="pun">]</span><span class="pln"> </span><span class="lit">2</span><span class="pln">
</span><span class="pun">[</span><span class="lit">1</span><span class="pun">]</span><span class="pln"> </span><span class="lit">3</span><span class="pln">
</span><span class="pun">[</span><span class="lit">1</span><span class="pun">]</span><span class="pln"> </span><span class="lit">4</span><span class="pln">
</span><span class="pun">[</span><span class="lit">1</span><span class="pun">]</span><span class="pln"> </span><span class="lit">5</span><span class="pln">
</span><span class="pun">[</span><span class="lit">1</span><span class="pun">]</span><span class="pln"> </span><span class="lit">6</span><span class="pln">
</span><span class="pun">[</span><span class="lit">1</span><span class="pun">]</span><span class="pln"> </span><span class="lit">7</span><span class="pln">
</span><span class="pun">[</span><span class="lit">1</span><span class="pun">]</span><span class="pln"> </span><span class="lit">8</span><span class="pln">
</span><span class="pun">[</span><span class="lit">1</span><span class="pun">]</span><span class="pln"> </span><span class="lit">9</span><span class="pln">
</span><span class="pun">[</span><span class="lit">1</span><span class="pun">]</span><span class="pln"> </span><span class="lit">10</span><span class="pln">
</span><span class="pun">&gt;</span><span class="pln"> </span><span class="kwd">print</span><span class="pun">(</span><span class="lit">1</span><span class="pun">:</span><span class="lit">10</span><span class="pun">)</span><span class="pln">
 </span><span class="pun">[</span><span class="lit">1</span><span class="pun">]</span><span class="pln">  </span><span class="lit">1</span><span class="pln">  </span><span class="lit">2</span><span class="pln">  </span><span class="lit">3</span><span class="pln">  </span><span class="lit">4</span><span class="pln">  </span><span class="lit">5</span><span class="pln">  </span><span class="lit">6</span><span class="pln">  </span><span class="lit">7</span><span class="pln">  </span><span class="lit">8</span><span class="pln">  </span><span class="lit">9</span><span class="pln"> </span><span class="lit">10</span><span class="pln">
</span><span class="pun">&gt;</span><span class="pln"> fruits </span><span class="pun">&lt;-</span><span class="pln"> c</span><span class="pun">(</span><span class="str">"apple"</span><span class="pun">,</span><span class="pln"> </span><span class="str">"Banana"</span><span class="pun">,</span><span class="pln"> </span><span class="str">"Pomegranate"</span><span class="pun">)</span><span class="pln">
</span><span class="pun">&gt;</span><span class="pln"> fruitLength </span><span class="pun">&lt;-</span><span class="pln"> rep</span><span class="pun">(</span><span class="pln">NA</span><span class="pun">,</span><span class="pln"> length</span><span class="pun">(</span><span class="pln">fruits</span><span class="pun">))</span><span class="pln">
</span><span class="pun">&gt;</span><span class="pln"> fruitLength
</span><span class="pun">[</span><span class="lit">1</span><span class="pun">]</span><span class="pln"> NA NA NA
</span><span class="pun">&gt;</span><span class="pln"> names</span><span class="pun">(</span><span class="pln">fruitLength</span><span class="pun">)</span><span class="pln"> </span><span class="pun">&lt;-</span><span class="pln"> fruits
</span><span class="pun">&gt;</span><span class="pln"> fruitLength
      apple      </span><span class="typ">Banana</span><span class="pln"> </span><span class="typ">Pomegranate</span><span class="pln"> 
         NA          NA          NA 
</span><span class="pun">&gt;</span><span class="pln"> </span><span class="kwd">for</span><span class="pun">(</span><span class="pln">a </span><span class="kwd">in</span><span class="pln"> fruits</span><span class="pun">){</span><span class="pln">
</span><span class="pun">+</span><span class="pln">   fruitLength</span><span class="pun">[</span><span class="pln">a</span><span class="pun">]</span><span class="pln"> </span><span class="pun">&lt;-</span><span class="pln"> nchar</span><span class="pun">(</span><span class="pln">a</span><span class="pun">)</span><span class="pln">
</span><span class="pun">+</span><span class="pln"> </span><span class="pun">}</span><span class="pln">
</span><span class="pun">&gt;</span><span class="pln"> fruitLength
      apple      </span><span class="typ">Banana</span><span class="pln"> </span><span class="typ">Pomegranate</span><span class="pln"> 
          </span><span class="lit">5</span><span class="pln">           </span><span class="lit">6</span><span class="pln">          </span><span class="lit">11</span><span class="pln"> 
</span><span class="pun">&gt;</span><span class="pln"> fruitLength2 </span><span class="pun">&lt;-</span><span class="pln"> nchar</span><span class="pun">(</span><span class="pln">fruits</span><span class="pun">)</span><span class="pln">
</span><span class="pun">&gt;</span><span class="pln"> fruitLength2
</span><span class="pun">[</span><span class="lit">1</span><span class="pun">]</span><span class="pln">  </span><span class="lit">5</span><span class="pln">  </span><span class="lit">6</span><span class="pln"> </span><span class="lit">11</span><span class="pln">
</span><span class="pun">&gt;</span><span class="pln"> names</span><span class="pun">(</span><span class="pln">fruitLength2</span><span class="pun">)</span><span class="pln"> </span><span class="pun">&lt;-</span><span class="pln"> fruits
</span><span class="pun">&gt;</span><span class="pln"> fruitLength2
      apple      </span><span class="typ">Banana</span><span class="pln"> </span><span class="typ">Pomegranate</span><span class="pln"> 
          </span><span class="lit">5</span><span class="pln">           </span><span class="lit">6</span><span class="pln">          </span><span class="lit">11</span><span class="pln"> 
</span><span class="pun">&gt;</span><span class="pln"> identical</span><span class="pun">(</span><span class="pln">fruitLength</span><span class="pun">,</span><span class="pln"> fruitLength2</span><span class="pun">)</span><span class="pln">
</span><span class="pun">[</span><span class="lit">1</span><span class="pun">]</span><span class="pln"> TRUE
</span>