Control Structures

Controlling your program execution

Two primary methods:

• if/else statements
• loops

If Statements

R

set.seed(1432309)
x <- sample(1:10, 1)
y <- NA # NA is a good placeholder value

if(x > 3) {
  if (x > 5) {
    y <- "above 5"
  } else {
    y <- "between 3 and 5"
  }
} else {
  y <- "below 3"
}

print(paste("according to y, x is ", y))

[1] "according to y, x is  between 3 and 5"

print(paste("x =", x))

[1] "x = 5"

Python

import random
import numpy as np

random.seed(1432309)
x = random.randint(1, 11)
y = np.nan # NA is a good placeholder value

if x > 3:
  if x > 5:
    y = "above 5"
  else :
    y = "between 3 and 5"
else :
  y = "below 3"

"according to y, x is " + y

'according to y, x is above 5'

"x = " + str(x)

'x = 6'

Flow Charts

Program Flow Diagrams or control diagrams are used to describe the logical steps followed by a program

Loops

Loops do the same set of steps for multiple values (usually either in a sequence or in a vector/column).

R

set.seed(1432309)
x <- sample(1:10, 5) # Sample 5 values, this time
y <- rep(NA, length(x)) # NA is a good placeholder value

for(i in 1:length(x)) { # First val in x, second, and so on...
  if(x[i] > 3) {
  if (x[i] > 5) {
    y[i] <- "above 5"
  } else {
    y[i] <- "between 3 and 5"
  }
} else {
  y[i] <- "below 3"
}
}


print(paste0("according to y, x is ", y, " (x = ", x, ")"))

[1] "according to y, x is between 3 and 5 (x = 5)"
[2] "according to y, x is below 3 (x = 2)"        
[3] "according to y, x is above 5 (x = 9)"        
[4] "according to y, x is above 5 (x = 6)"        
[5] "according to y, x is above 5 (x = 7)"        

Python

import random
import numpy as np

random.seed(1432309)
x = random.choices(range(1, 11), k = 5)
y = np.full(shape=5, fill_value="*"*15) # NA is a good placeholder value

for i, xval in enumerate(x):
  if xval > 3:
    if xval > 5:
      y[i] = "above 5"
    else :
      y[i] = "between 3 and 5"
  else :
    y[i] = "below 3"


# This is a "list comprehension" - an abbreviated for loop used to work with vectors
["according to y, x is " + y[i] + " (x=" + str(x[i]) + ")" for i, xval in enumerate(x)]

['according to y, x is above 5 (x=10)', 'according to y, x is above 5 (x=9)', 'according to y, x is above 5 (x=6)', 'according to y, x is below 3 (x=3)', 'according to y, x is below 3 (x=1)']