Introduction to Biological Data Analysis

TP 1: Introduction to coding in R

Prof. Patrick Meyer

BioSys Lab - Université de Liège

Assignations

1. If the variables a, b, d have as value, respectively 2, 6 et 1, what are their values after the following lines of codes (this execise should be performed mentally)?

   a)	> a <- b		d)	> d <- a
   	> d <- b			> a <- b
   				> b <- d
   b)	> a <- a + 1		e)	> b <- -a
   	> b <- d + 1			> b <- 2 * b
   	> d <- 2 * d			> a <- b
   c)	> a <- b		f)	> a <- a * a
   	> d <- a			> a <- a * a

2. Write a few lines of R code allowing to swap the values of $a$ and $b$.

Functions

1. What is the purpose of the following function (mental exercise)?

   > fction1 <- function(eu)
   {
       fb <- 40.3399*eu
       fb
   }
   

   What will be displayed on screen if you type the following instructions:

       > fction1(100)
       > fction1(200000)
       > fction1(fction1(1))
   

2. What is the purpose of the following function (mental exercise)?

   > fction2 <- function(a,b)
   {
       a <- a + b
       a + b
   }
   

   What will be displayed on screen if you type the following instructions:

       > b <- 8
       > a <- 13
       > fction2(b,a)
       > a
   

3. Write a function $bmi()$ which given two parameters $weight$ and $height$, computes the corresponding body mass index $bmi=\frac{weight}{height^2}$.

4. Write a function that converts Farhenheit into Celsius, knowing that you just have to substract $32$ degrees before multiplying by $\frac{5}{9}$.

5. The Euclidean distance between two points $(x_1, y_1)$ and $(x_2, y_2)$ is given by the formula:
   
   $$s = \sqrt{(x_1-x_2)^2 + (y_1-y_2)^2}$$
   
   
   Write a function that receives four values corresponding to the xy coordinates of two points and computes the Euclidean distance between them (you can use the function $sqrt$).

Vectors

1. What would be displayed on screen if you type the following lines of code (this execise should be performed mentally):

     > weight <- c(60, 72, 57, 90, 95, 72)
     > height <- c(1.75, 1.80, 1.65, 1.90, 1.74, 1.91)
     > bmi(weight,height)
   

2. What would be displayed on screen if you type the following lines of code (this execise should be performed mentally):

     > a <- c(TRUE,FALSE,FALSE,TRUE)
     > b <- c(FALSE,FALSE,TRUE,TRUE)
     > b|a
     > a&!b
   

3. Given the following assignations:

    > e <- c(1,2,3,5,8,13,21,34,55)
    > f <- c(4,6,7,9)
    > g <- rep("hello",10)
    > h <- 50:60
   

   What would be displayed on screen if you type the following lines of code (this execise should be performed mentally):

    > e[5]
    > e[6:9]
    > h[f[1:3]]
    > e[f-2]
    > g[e[f-3]]
    > c(e,f)
    > f[c(TRUE,FALSE,TRUE,FALSE)]
    > f > 7
    > f[f > 6]
    > f[-2]
    > h[-c(4,6)]
   

4. Given two vectors $POS$ and $DAT$, write the code allowing to display the elements of $DAT$ in the order given by $POS$.

   Example

   DAT: 9 6 8 2
   POS: 2 4 3 1
   

   returns $6 \;\; 2 \;\; 8 \;\; 9$.

5. Write a code that performs the scalar product of two vectors ($u,v$) given that the formula for vectors of length $n$ is :
   
   $$\sum_{i=1}^n u_i \cdot v_i$$
   
   
   You may use the function $sum()$.