Lecture 4 - Introduction to R III
# Topics for today! We will demo the final two elements of programming 1. If else statements (Ch. 3.2.1 in Book 1) 2. for loops (Ch.3.2.2, 3.2.4.1 in Book 1) # 1. Topic: If else statements ```{r} ## Syntax: #if(boolean){ # run code here #}else{ # run code here #} ``` ## Example 1: Write an if statement to check if variable is a certain size. ```{r} x = rnorm(1,1,5) # x is one sample from a normal distribution with mean 1 and std 10 if(x >= 1){ print("x is huge!") }else{ print("x is hopelessly tiny") } ``` If theres more than two cases we can use the else if to chain together commands! ```{r} ## Syntax: #if(boolean){ # run code here #}else if{ # run code here #}else{ # run code here #} ``` ## Example 2: Write an if/else if/else statement to check the size of a variable. ```{r} # Using x from the previous block! if(x > 2){ print("x is huge!") }else if(x < 0){ print("x is hopelessly tiny") }else{ print("x is boring") } # If statements are particular helpful in conjuction with loops and functions. ``` # 2. Topic: for loops General info: A for loop consists of a *loop varible* (usually we call it i,j, or k) and a *loop vector* (usually 1:N where N is some integer, often). ```{r} ## Syntax: # for(loop_var in loop_vec){ # run code # } ``` ## Example 1: Print the elements of a vector. ```{r} data = c(1,1,2,3,5,7) for(i in data){ print(i) } # In the above loop "i" is the name of the loop variable, and it is pushed through the loop vector named "data" element by element. ## Loop Trace # At iteration 1: i = data[1] which is 1, then print(i) is run # At iteration 2: i = data[2] which is 1, then print(i) is run # At iteration 3: i = data[3] which is 2, then print(i) is run # ... # At iteration 6: i = data[6] which is 7, then print(i) is run and the loop terminates. ``` ## Example 2: Remove thhe negative elements from an array. This example will combine if statements with for loops! ```{r} data = rnorm(10, 0, 1) # 10 samples from a normal random variable clean_data = rep(0,10) for(i in 1:length(data)){ if(data[i] < 0){ clean_data[i] = NA }else{ clean_data[i] = 1 } } print(clean_data) ``` ## Some finer details of for loops Recall the motivation for for-loops is to repeat pieces of code in a unified way. In this example we will write a piece of code as a for loop, and then equivalently as a number of code indepent code blocks. First lets recall some things about strings. Namly, recall from lecture 2 we learned the substring function. Its syntax was substring(char,start_ind, end_ind) ```{r} strg = "hello" substring(strg,1,2) # Will return "he" # Lets learn another R function, nchar(). It will tell us how long a character is (i.e. how many letters are in the string) # Example: Using nchar() nchar(strg) # will return 5 ``` Now suppose we want to count the number of "l"'s in a given string. For this task we can use for loop to examine all the letters, and everytime we find an "l" we will increment a counter. ## Example 3: Counting "l"'s in a string. ```{r} string_we_care_about = "lala" # here I'm making up a string, but it will be good because it has l's in it. N = nchar(string_we_care_about) # N is now the number of letters in the char string_we_care_about count = 0 # This is a initialization of a counter. Note it is define outside the for loop. If we defined it inside the for loop, it would be reset to 0 every time the loop ran this line. vec = 1:N # this a vector 1,2,3,..,N. We loop "over" this vector. for(i in vec){ if(substring(string_we_care_about,i,i) == "l"){ count = count + 1 # this line adds 1 to the variable count, then redefines count as the sum } } print(count) # Two notes: ## Note 1: the variable i defined in line 48 is the loop variable. It exists to keep track of how many times lines 49-51 are run. To ill ``` Finally to cement these concepts lets trace the execution of the previous example iteration by iteration. ## Example 4: Tracing a for loop ```{r} string_we_care_about = "lala" count = 0 # Intialization ## Iteration 1 ## i = vec[1] # in the first iteration i is equal 1 # Then the following if statement is checked. if(substring(string_we_care_about,i,i) == "l"){ # Note substring(string_we_care_about,1,1) = "l", so it will return true count = count + 1 # this line adds 1 to the variable count, then redefines count as the sum } # At the the end of iteration 1, count = 1, i = 1 ## Iteration 2 i = vec[2] # in the second iteration i is equal 2 # Then the following if statement is checked. if(substring(string_we_care_about,i,i) == "l"){ # Note substring(string_we_care_about,2,2) = "a", so it will return false count = count + 1 # This line thus does not get run } # At the the end of iteration 2, count = 1, i = 2 ## Iteration 3 i = vec[3] # in the second iteration i is equal 3 # Then the following if statement is checked. if(substring(string_we_care_about,i,i) == "l"){ # Note substring(string_we_care_about,3,3) = "l", so it will return true count = count + 1 # This line gets run and count is set to 2 } # At the the end of iteration 3, count = 2, i = 3 ## Iteration 4 i = vec[4] # in the second iteration i is equal 4 # Then the following if statement is checked. if(substring(string_we_care_about,i,i) == "l"){ # Note substring(string_we_care_about,4,4) = "a", so it will return flase count = count + 1 # This line does not get run } # At the the end of iteration 4, count = 2, i = 4 # This is the last element of the vector vec, thus the loop ends. print(count) # at the end of the last iteration, count was equal to 2. Thus 2 is printed. ```
