title: “Class5_Loops” author: “Nick Vohra” date: “9/3/2019” output: html_document
Class 5 - Loops
In this Class, we looked at the very important coding principle of Loops
Loops take three main forms:
###### for ###### while ###### if…else ##### In this class we will focus on if…else and while loops
# Lets first focus on the if...else loop.
# What an if...else loop is really saying is,
# if somethign is true, do this
# if something is false, do this instead.
# heres a simple example, lets say I ain't to run a code if x is positive and run a different code if x is negative.
# TO do this, we need somehting called operators.
# Step 1: Operators:
# Logical Operators
# A > B A is greater than B
# A >= B A is greater than or equal to B
# A < B A is less than B
# A <= B A is less than or equal to B
# A == B A is equal to B
# A != B A is NOT equal to B
# Mathematical Operators
# + Addition
# - Subtraction
# * Multiplication
# / Division
# ^ Exponent
# %% Modulus (Remainder from Division)
# From our desire to tell if a number is positive or negative,
# we need the > or < sign to distinguish if a number is below 0 or not
# Step 2:
# The syntax and the form of the if...else loop
# if(something is true){
# Do this chunk of code
# }else{
# do this chunk of code }
# Step 3:
# Test it out
x = 5
if(x>0){
print("Positive")
}else{
print("Negative")}## [1] "Positive"# Thus, we have created a simple loop that tells us if x is positive or negativeThe for Loop
The for loop operated through each element in a vector and applies code to it. You can also couple the for loop and the if/else loop to properly apply code to each element is it meets a certain criteria.
# Lets create a for loop that iterates through an entire vector and count up the even numbers in the vector.
# We need to first create an arbitrary number vector:
MyVector = c(5,15,78,95,12,74,32,10,5,42,65,78,94,51,12,20)
# How to count up the even numbers:
# 1) go through each element in the vector
# 2) think of a way to determine if a number is even or not
# 3) Count those up and deliveer the total number
# # Form of a for loop
# for (item in vector){
# Do this code to the item
# }
#
count = 0
for (item in MyVector){
if(item %% 2 == 0){
count = count + 1
}
}
count # equals 7 meaning that the total count of even numbers## [1] 10# We have now gone through each end of a vector
# We checked if the number is even
# then added to the count variable. Position vectors in R
# using a for loop to sift though a data set and separate data based on treatment
Treatment = c(1,2,1,3,2,1,3,2,3,2)
Sex = c(1,1,0,1,0, 1, 0, 1, 0, 0)
Cell_Count = c(132, 123,548,421,848,951,951,357,854,212)
df = data.frame(Treatment, Sex, Cell_Count)
df## Treatment Sex Cell_Count
## 1 1 1 132
## 2 2 1 123
## 3 1 0 548
## 4 3 1 421
## 5 2 0 848
## 6 1 1 951
## 7 3 0 951
## 8 2 1 357
## 9 3 0 854
## 10 2 0 212# How to contruct a loop that we need to sift through the data and store it based on out citeria
CellCountTreat_1 = c()
CellCountTreat_2 = c()
CellCountTreat_3 = c()
# Whats the magic word? - INDEXING
for(item in 1:length(Treatment)){
if(Treatment[item] == 1){
CellCountTreat_1 = c(CellCountTreat_1,Cell_Count[item])
}
if(Treatment[item] == 2){
CellCountTreat_2 = c(CellCountTreat_2, Cell_Count[item])
}
if(Treatment[item] == 3){
CellCountTreat_3 = c(CellCountTreat_3, Cell_Count[item])
}
}
CellCountTreat_1## [1] 132 548 951CellCountTreat_2## [1] 123 848 357 212CellCountTreat_3## [1] 421 951 854boxplot(CellCountTreat_1, CellCountTreat_2, CellCountTreat_3)
# Quick note on the use of the tilde in Boxplot
boxplot(Cell_Count ~ Treatment) # No need for loops