Part15 Conditional statement

“if” statement

Structure of the if statement:

if(condition){
    action_command
}

If the condition is TRUE, then proceed to the action_command; if it is FALSE, nothing happens.

k <- 10
# print if value is > 3
if(k > 3){
  print(k)
}
# print if value is < 3
if(k < 3){
  print(k)
}

With else

if(condition){
    action_command1
}else{
    action_command2
}

If the condition is TRUE, then proceed to the action_command1; if the condition is FALSE, proceed to action_command2.

k <- 3
if(k > 3){
  print("greater than 3")
}else{
  print("less than 3")
}

With else if

if(condition1){
    action_command1
}else if(condition2){
    action_command2
}else{
    action_command3
}

If the condition1 is TRUE, then proceed to the action_command1; if the condition1 is FALSE, test for condition2: if the condition2 is TRUE, proceed to the action_command2; if neither condition1 nor condition2 are TRUE, then proceed to the action_command3.

Note that you can add up as many else if statements as you want.

• Example without else

k <- -2
# Test whether k is positive or negative or equal to 0
if(k < 0){
    print("negative")
}else if(k > 0){
    print("positive")
}else if(k == 0){
    print("is 0")
}

• Example with else

k <- 10

# print if value is <= 3
if(k <= 3){
  print("less than or equal to 3")
}else if(k >= 8){
  print("greater than or equal to 8")
}else{
  print("greater than 3 and less than 8")
}

• If statement in For loop:

# Matrix
mymat <- matrix(rnorm(800), 
        nrow=50)
    
# Loop over rows of mymat and print row if its median value is > 0
for(i in 1:nrow(mymat)){
    # extract the current row
    rowi <- mymat[i,]
    # if median of row is > 0, print row
    if(median(rowi) > 0){
        print(rowi)
    }
}