R - Control Structure

• If... else...
• Example: print msg if your age is under 19

 > if(age <19){  
 +   print("you are not adult")  
 + } else {  
 +   print("you are adult")  
 + }  
 [1] "you are not adult"  

• For loop
• Takes an iterator variable by successive values from a vector. 
• Example: loop a vector

 > x <- c("a","B","c") 
 > for(i in x){  
 +   print(i)  
 + }  
 [1] "a"  
 [1] "B"  
 [1] "c"  

• Use seq_along() function for looping

 > for(i in seq_along(x)){  # seq_along returns integer vector 1:3
 +   print(x[i])  
 + }  
 [1] "a"  
 [1] "B"  
 [1] "c"  

• Loop for matrix using seq_len()

 > x <- matrix(1:4,2,2)  
 > for(i in seq_len(nrow(x))){  
 +   for(j in seq_len(ncol(x))){  
 +     print(x[i,j])  
 +   }  
 + }  
 [1] 1  
 [1] 3  
 [1] 2  
 [1] 4  

# difference between seq_len() and seq_along()
 > nrow(x)  
 [1] 2  
 > seq_len(nrow(x))  
 [1] 1 2  
 > seq_along(nrow(x))  
 [1] 1  

• While loop
• Loops while the testing condition is true

 > while(age <19){  
 +   print("you can't enter the club")  
 +   age <- age+1  
 + }  
 [1] "you can't enter the club"  
 [1] "you can't enter the club"  

• next keyword
• Skip an iteration of a loop 

 > for(i in 1:4){  
 +   if(i<2){  
 +     next  # skip to next iteration
 +   }  
 +   print("Hello")  
 + }  
 [1] "Hello"  
 [1] "Hello"  
 [1] "Hello"  

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