## 10.3 While Loops and Repeat

In addition to for loop where you have to specify the range of values to iterate, it is sometimes more convenient to use another type of loop: the while loop.

### 10.3.1while loop

Let’s first review the syntax of for loops.

for (val in val_seq){
statement1
statement2
}

Here, we have to specify the val_seq along which we will assign each element to val and execute the statements, unless break is called. Sometimes, it is not clear what the val_seq would be. For example, if we want to find the Fibonacci sequence up to 100, it is not clear how many elements we will have in the val_seq. In this case, the while loop comes to rescue. Let’s take a look of the syntax of while loop.

while(cond_expr){
statement1
statement2
}

In the while loop, we put a logical statement in the cond_expr. The loop will continue as long as cond_expr take the value of TRUE. Let’s see the actual code in action.

fib_seq <- c(0, 1)
fib_last_value <- fib_seq
fib_cur_value <- fib_seq
fib_next_value <- fib_last_value + fib_cur_value
while(fib_next_value < 100){
fib_seq <- c(fib_seq, fib_next_value)
fib_cur_value <- fib_next_value
fib_last_value <- fib_cur_value
fib_next_value <- fib_last_value + fib_cur_value
}
fib_seq                #print the sequence
#>   0  1  1  2  4  8 16 32 64

### 10.3.2repeat Loop

The last type of loops is the repeat loop, which doesn’t have neither the range of values, nor the logical expression. The syntax is as below.

repeat{
expr1
expr2
if(cond_expr){
break
}
expr3
}

As you can see in the syntax of the repeat loop, you always need to put a conditional break statement to avoid infinite loops.

Let’s rewrite the previous example using repeat loop.

fib_seq <- c(0, 1)
fib_last_value <- fib_seq
fib_cur_value <- fib_seq
fib_next_value <- fib_last_value + fib_cur_value
repeat{
fib_seq <- c(fib_seq, fib_next_value)
fib_cur_value <- fib_next_value
fib_last_value <- fib_cur_value
fib_next_value <- fib_last_value + fib_cur_value
if(fib_next_value >= 100){
break
}
}
fib_seq                #print the sequence
#>   0  1  1  2  4  8 16 32 64

You can see that the same sequence was generated using the repeat loop.