So the next contract is a for loop. So this is probably the most common type of loop oper, operator that you're going to use in r. The basic idea is that you have a loop index which is typically called i, but if you have many loops you might say j, k, l, et cetera. And a loop index is going to go from, it's going to cover a sequence of numbers typically in. integers. So for example here I've got a for loop. And it's looping over the numbers 1 through 10. So I've created the sequence of colon operator 1 through 10. And this for loop is not doing anything particularly interesting, it's just printing out the numbers so printing out i at each iteration. After the loop is finished it continues on to the next block of code. So there's different ways to use a for loop and the r is pretty flexible in how you can kind of index the different type of r objects. So here I've created a vector a character with the letters a b c d in it. And all these four loops are equivalent. So for example, in the first one, I kind of done what you might commonly see in other languages like C that have four looping. Here I have an indexed i that I sequenced through 1 through 4, I create an imager sequence. Then I print the i'th element of x, each time. So here this loop just prints out a, b, c, d. Another thing I could do is use the seek along function, so seek_along what it does is it takes a vector as an input and it creates an integer sequence that's. That's equal to the length of that vector. So this is a vector of length 4, so it's going to create an integer sequence 1 through 4. It's exactly the same as, as the previous sequence where I've got the sequence 1:4. However, here I generate the sequence based on the length of the variable x. So now I'm, my index serial i is going through this sequence. It's printing out the i filament of x and exactly the same behavior occurs. I print out a, b, c and d. The third example here, I'm, I've got a different index variable, I'm calling it letter, and the index variable is actually going to be index, is going to be taking values from the vector itself. So there's no reason why the index variable has to be an integer. It can take elements from any arbitrary vector. So now the for loop is going through this vector of letters, a,b,c,d. An it's just going to print out the letter itself, so now I'm printing out the index variable, which happens to be equal to the letters in the vector. And so these three for loops are exactly the same so far. The last version here is, is the, is the same as the first one except I haven't included the curly braces, and so for a single expression if you. If the for loop only has a single expression in this body you can omit the curly braces and put everything on one line, which maybe useful sometimes because it's a little bit more compact. But it's not my particular style. For for loops, I generally like to put the curly braces in, regardless of how many elements are in the body. For loops can be nested so you can have a for loop inside of a for loop, so the common thing for example, and and is, is with a matrix that has two dimensions and then you might want to loop over the rows and then loop over the columns. So here the outer loop the ought with the i index is seek, is looping over the rose. And I'm using a function that's called seq_len, so, the idea is that, it takes, in, seq_len takes an integer, which in this case happens to be the number of rows in x, and then it creates an integer sequence out of that. So this particular matrix has two rows, so it's going to create the sequence of 1 to 2. For example, similarly the the nested loop with the j index is using the number of columns to create a sequence so this matrix has three columns, and so the seq_len function is going to create an integer sequence 1 through 3. So the, so this doubling nested for loop, is basically printing out all the elements of the matrix. So one thing to be careful with, with nested for loops is that it's going beyond 2-to-3 levels, while it's theoretically okay in r, it makes reading the code a little bit difficult to do. And so if you're reading code that has three or four nested for loops it gets difficult to understand what's, what's going on. Sometimes that's the logical thing to do, but many a times there's ways to get around that, for example, by using functions.
