Presentation on how to divide a complex problem into smaller tasks

Ashu Guru Author

03/13/2019 Added

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Presentation on how to divide a complex problem into smaller tasks by Ashu Guru, Univ of Nebraska Raikes School (2 mins)

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[00:00:00.560]We will now talk about three of the most common variable types or data types that we will
[00:00:07.760]be using in R and they are vectors, lists, and tables.
[00:00:14.720]So let's just define what a programming variable is.
[00:00:20.040]So you can think of programming variable as a container that will maintain a certain value
[00:00:25.560]which we can refer to later in that same program.
[00:00:30.540]So for example, I could define a variable called myFirstVariable and assign it a value
[00:00:37.440]of 24.
[00:00:38.800]Similarly, I could define another variable which I name it mySecondVariable and I assign
[00:00:44.700]it a value 2.
[00:00:47.480]What if I want to store a value of the sum of the myFirstVariable and mySecondVariable.
[00:00:54.740]If I want to do that, I can do that.
[00:00:55.540]I will do that as myThirdVariable.
[00:00:58.580]In this case, I'm naming it as myThirdVariable.
[00:01:01.620]It contains the value of myFirstVariable plus mySecondVariable.
[00:01:05.620]So by the time this instruction is run, the ThirdVariable contains the value of 26.
[00:01:11.980]That is 24 plus 2.
[00:01:13.580]So one thing that if you have previously programmed in other programming environments, you may
[00:01:19.500]notice immediately is that the way the assignment operator works in R is
[00:01:25.520]different.
[00:01:26.860]In majority of the programming languages, the assignment operator is equal to sign,
[00:01:31.380]but in R it is a combination of less than and the hyphen.
[00:01:36.360]I am going to launch my R studio and observe these details interactively with the R console.
[00:01:44.760]Here is my R studio and I have created a script by creating a new R script and then typing
[00:01:51.280]the commands that I will be using.
[00:01:53.520]So my first instruction is similar
[00:01:55.500]to what you saw in the PowerPoint that I am assigning a value of 24.
[00:02:00.040]So if I have my cursor here and I click on run, R studio copies this instruction to the
[00:02:07.900]console and runs it.
[00:02:10.320]So now I have a variable called my first variable and it has a value of 24 assigned to it.
[00:02:18.020]Now my cursor on the script is on line number 2.
[00:02:23.360]So I am going to run the second instruction.
[00:02:25.480]Now I have a second variable which has a value of 2 assigned to it.
[00:02:32.720]Similarly I have now a third instruction which actually stores the value of some of my first
[00:02:41.100]variable and second variable in the variable called my third variable.
[00:02:45.640]So I could just print the value of my third variable and I do that.
[00:02:50.800]So it shows me that the value of my third variable is currently 26.
[00:02:55.460]So one thing you may want to notice is that I use camel case for defining the names of
[00:02:59.820]the variables.
[00:03:00.820]So I have my with a capital F and a capital V in my first variable.
[00:03:06.580]I would recommend that your variables are descriptive of what value are they holding.
[00:03:12.380]So if I have a variable that needs to maintain value of number of fruits in a box then I
[00:03:19.140]would name it as my num fruits.
[00:03:23.200]That way the code is much more readable.
[00:03:25.440]And easy to maintain or modify.
[00:03:32.300]Vector is the most fundamental object in R environment.
[00:03:37.040]And you can think of a vector as a linear container with columns.
[00:03:40.620]Here is a diagram that you can think of that a vector is basically a container with multiple
[00:03:46.460]cells and these cells are arranged in a line and each of the cell has its index so the
[00:03:55.420]first cell has an index 1, the second cell has index 2 and so on.
[00:03:59.300]So in this case we have 1, 2, 3, 4, 5, 6 cells and of course that means we have 6 column
[00:04:07.060]indexes and so if I define a vector which is called as my vector a then my vector a
[00:04:15.460]currently contains 23, 12, 8, 6, 8 and 1 as the integers.
[00:04:25.400]So the graphical representation and how do you actually create it in R is by defining my vector a and then we
[00:04:34.220]assign the value with a small c and a parentheses and then the numbers inside
[00:04:39.520]that parentheses. This means is that my vector a1 contains the value 23 and
[00:04:45.860]similarly my vector a3 contains the value 8 because this is index 1 so
[00:04:51.260]my vector a1 has 23, this is index 2, this is
[00:04:55.380]index 3 and my vector a3 will contain the value 8. Let's try this
[00:05:01.020]interactively. So here is the RStudio. So I have defined a vector called my vector
[00:05:07.420]a. Once I run that if I want to print the value of the cell 1 then I run the
[00:05:15.120]instruction my vector a1 and it shows that the value is 23 and similarly I can run what is the
[00:05:25.360]value in my vector a cell number 3 and it shows that the value is number 8 like
[00:05:32.740]we expected. One thing interesting that we should also notice is that even
[00:05:38.200]though when we define a variable which has a single value so let's say my
[00:05:42.480]variable a has a value of 25 that I assigned to it, it is equivalent to
[00:05:47.680]defining it as a my variable with a vector notation so my variable a has a
[00:05:55.340]single cell with the value of 25. Let's look at it interactively as well so I
[00:06:02.460]have I'm defining a variable called my variable a and here and I assign it a
[00:06:11.340]value of 25 so I run that so now my my name variable a has a value of 25. I run
[00:06:20.340]the next instruction where I print the value. It shows that it has
[00:06:25.320]25 as the value. I'm defining the same variable with a vector notation so my
[00:06:32.040]variable a is assigned the value of 25. And next I'm going to print the my
[00:06:40.260]variable again and you can see that the output is similar so as we expected a
[00:06:48.800]variable which holds a single cell value even though if you define it
[00:06:55.300]without an vector notation is actually stored in as a vector. Now let's look at
[00:07:01.000]lists so many time we want a vector of vectors and in that case we will turn to
[00:07:09.060]what is called as a list so a list is basically a vector of vectors and here
[00:07:15.760]is an example. So if I want to define a new list, let's say I want to name a
[00:07:22.020]variable my list and it should hold
[00:07:25.280]the value of a list then the way I will define it is by saying my list is
[00:07:30.000]assigned a value of list. And then this is the index of the list is equal to a
[00:07:37.160]vector 2, 4, 6 the second vector is I want that to be known as my odds and it is a
[00:07:46.360]of values 3, 7, 9 and 11 and then similarly I want a third vector in my list which is my primes
[00:07:53.810]which is a vector of 2, 3, 5, 7 and 11. So if I want to print the second element or the second
[00:08:05.590]vector of my list then I will type in my list double square bracket 2 because then I'm referring
[00:08:11.850]it by index 2, 1, 2 and it will give me the odds vector. If I want I can also print it by the index
[00:08:21.770]string that I have provided. So in this case I'm printing what is the vector for the index string
[00:08:30.550]my odds. So here is the interactive mode. So here is my definition of the list. I'm calling it as
[00:08:39.190]my list and I'm assigning
[00:08:41.230]I'm creating it by the list command and the first element or the first vector of
[00:08:49.050]this list is that may also be referred by the string my evens and has the
[00:08:54.470]vector 2, 4, and 6. The second one may be referred by my odds. It has the values
[00:09:02.530]3, 7, 9 and 11 as a vector. Then I also have a third vector for this list that
[00:09:10.610]has the values 2, 3, 5, 7 and 11. So now if I run this, I have defined a new
[00:09:23.010]variable my list. So now if I want to print the second vector by the index
[00:09:28.550]number, I will run the command my list double square parentheses 2 and it
[00:09:36.410]prints 3, 7, 9 and 11 as we expected.
[00:09:39.990]I could also refer to the same vector by the index string and I can do that by my
[00:09:51.090]list double square parentheses and in double quotes my odds. So when I do that
[00:09:56.370]I get the same result. Now just let's just print the whole list and you can
[00:10:07.470]see that it has
[00:10:09.370]the first vector is my evens, the second one is my odds, the third one is my
[00:10:15.870]primes. The last data type that I would like to talk about is what is called as
[00:10:20.890]a table and a table can be thought of as a fast indexed data structure. So in
[00:10:27.810]many cases we have data which is in in form of a frame or a grid as we see on
[00:10:34.010]the screen. One of the data structures that R provides to handle
[00:10:38.750]this kind of data is a table and you can actually have this data stored in a
[00:10:46.370]regular text file and import the whole CSV file in R and then read that file
[00:10:55.250]into a variable. So these are the steps that we will do and the command I'm
[00:11:00.010]using is read dot table and it takes an argument the file name. It
[00:11:08.130]takes another argument which is header it is true in this case if there is no
[00:11:13.710]header then we can make it false with the capital F and in this case my since
[00:11:19.050]we are using a CSV file the separator is a comma. So here is a working example
[00:11:23.850]code. There is one more concept that I am going to introduce that whenever we work
[00:11:30.330]with a program or any software it has a working directory or a space on the file
[00:11:37.050]system
[00:11:37.510]where it is accessing reading or writing files to if needed. In R if you want to see
[00:11:43.810]what is the current working directory, print that by typing the command get WD,
[00:11:50.170]which is short form for get working directory. R also provides a command
[00:11:56.290]that can allow you to change the working directory so set WD takes an argument
[00:12:01.650]where you provide the path to that directory. Finally we want to read the
[00:12:06.890]CSV into a table and we do that by so in this case I am defining a variable
[00:12:12.590]called my tab data and I use the command read the table from the working
[00:12:18.710]directory there will be a file called one dot average of interval for CSV. It
[00:12:25.250]has headers and the separator is a comma. So if I just want to print the
[00:12:32.210]group on column then I can refer to that by my tab data,
[00:12:36.270]which is the name of the main variable and then a dollar sign and then the
[00:12:39.910]header string. Similarly I can also print get access to that column by using a my
[00:12:48.510]tab data and a square bracket and double quote and the header string. Now let's
[00:12:53.070]work with an interactive mode so I have... let me print the while current working
[00:13:01.270]directory so if I click run I see that R
[00:13:05.650]currently sees users a guru as the working directory and if I want to
[00:13:11.890]change it to my desktop I will run this statement or this instruction "set WD" and
[00:13:20.410]I have changed it to users a guru desktop run that instruction once I do
[00:13:25.630]that it has changed to the desktop I can actually again print get WD and it shows
[00:13:33.730]that now I am
[00:13:35.030]able to read files from users a guru and desktop. On my desktop I have a file
[00:13:42.370]called one dot average of interval dot csv dot txt. I will show that here so
[00:13:50.330]here is the file it has group on attribute 1 attribute 2 attribute 3
[00:13:56.130]separated by a comma and then all the data follows after that so I am going to
[00:14:03.030]import this
[00:14:04.410]in R by saying read the table one dot average of interval dot csv dot text it
[00:14:14.610]has a header and is separated by a comma so I will run this instruction and once
[00:14:26.370]done this variable now has all the value so if I just want to print the value
[00:14:33.790]in the group on column I will use the instruction my table data dollar group
[00:14:40.090]on which is going to be a vector so here it is so it actually has 100 100.2
[00:14:46.970]100.5, 0 which corresponds to 100, 100.2, 100.5, 100.75 and
[00:14:57.350]so on. Similarly I can print it in of another format by using
[00:15:03.410]my tab data group on in the square parentheses by the index column and
[00:15:12.410]here is the printout for that instruction so these are the most common
[00:15:18.830]data types that one may use for writing R scripts of course there are many
[00:15:24.710]other data types but we can explore those as we need to use them.

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Presentation on how to divide a complex problem into smaller tasks

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