If Statements

If

We can use if to execute commands only when certain conditions are met.

if(<value>){
  # commands
}

Checks the condition, <value> and if it is true, runs the commands. (If <value> is false, R skips directly to the end of the curly brackets.)

x <- 1
if(x > 0){
  print("positive")
}

## [1] "positive"

Often if statements are set up like this with some relational operator as the value.

On Your Own

Write an if statement that prints out a short phrase when x is equal to 1.

If Else

The operator else can be paired with an if statement. If the if statement is false, then the else code will run. If the if statement is true, the if code will run. (One or the other will run, but never both.)

x <- -1

if(x > 0){ # check if x > 0
  print("x > 0") # runs if x > 0 is true
}else{ # note the placement of the else
  print("x <= 0")
}

## [1] "x <= 0"

On Your Own

Copy and paste your code chunk from the last On Your Own prompt. Modify it to print out a different short phrase when x is not equal to 1.

Else If

Sometimes we have reason to do a sequence of checks that are all related, where code will only run if all the previous statements were false and the current criteria is true. The operator else if can be used to implement this.

x = -3

if(x > 0){
    type = "positive"
} else if (x < 0) {
    type = "negative"
} else if(x == 0){
    type = "zero"
} else {
    type = "Error"
}

type 

## [1] "negative"

What’s the logic here?

Another example with else if used in a function:

toyfun <- function(X,Y,Do){
  if(Do == "Add"){
    Z = X+Y

  }else if(Do =="Subtract"){
    Z = X-Y

  }else if(Do =="Multiply"){
    Z = X*Y

    
  }else if(Do =="Penguin"){
    Z = c("<('' )")
    
  } else{
    Z = c(X,Y)
  }
  
  return(Z)
}
toyfun(2,4,"Add")

## [1] 6

toyfun(2,4,"Subtract")

## [1] -2

toyfun(2,4,"Penguin")

## [1] "<('' )"

toyfun(2,4,"typo")

## [1] 2 4

On Your Own

Implement a sequence that checks if a logical statement is TRUE or FALSE. Use the final else statement to allow for the possibility that there is some error and the input is neither true nor false. If TRUE, print “yes”; if FALSE, print “no”; otherwise print “error”.

IfElse

Finally, we have a shortcut function that can sometimes help keep our code clean.

ifelse(test, yes, no)

• test is the logical statement
• yes is what the code does if test results in TRUE
• no is what the code does if test results in FALSE

x <- 0
x.info <- ifelse(x < 0, "negative", "nonnegative"); x.info

## [1] "nonnegative"

We can also pass the ifelse function a vector and it will check each element individually. For example, in the mtcars data set, we can find the proportion of cars that have mpg > 25 and hp > 60

fast_efficient <- ifelse(mtcars$mpg > 25 & mtcars$hp>60, TRUE, FALSE)
sum(fast_efficient)/length(fast_efficient)

## [1] 0.15625

On Your Own

Modify your code from the second On Your Own prompt to use an ifelse function.

Nested If Chains

x <- 105
if(x>0){
  if(x>100){
     type = "large positive number"
  } else {
     type = "positive number"
  }
} else if(x<0) {
  type = "negative number"
} else if(x==0){
  type = "zero"
}else {
  type = "Error"
}

type 

## [1] "large positive number"

We can also nest ifelse statements by making the yes and no results other ifelse statements, but this can get tough to follow pretty quickly!

x <- 0

numInf <- ifelse(x > 0, "positive", 
                 ifelse(x < 0, "negative", 
                        ifelse(x == 0, "zero", "error")))
numInf

## [1] "zero"