R Syntax Examples

PDF file at the link. Short, prototypical programs exploring the basics of programming with Processing. If you need to read in a csv with R the best way to do it is with the command read. For example, it's crucial to remember that film is primarily a visual medium. R Presentations are a feature of RStudio that enable easy authoring of HTML5 presentations using a combination of Markdown and R. Each person is an instance, and the properties about these people, such as for example their name, their age and whether they have children are. R-Forge offers a central platform for the development of R packages, R-related software and further projects. They have heard about reverse mortgage loans, but didn’t know the details. Although these examples don’t show it, it is possible to define sequences within any parent structure. By default, the owner of a filesystem object is the user that created it. EXAMPLE: Mr. It was not my usual stint to explore optimization problems or looking into game theory. SYNTAX EXAMPLES FOR R STATISTICAL PROGRAM by Matthew Keller, matthew. The lavaan tutorial Yves Rosseel Department of Data Analysis Ghent University (Belgium) July 5, 2019 Abstract If you are new to lavaan, this is the place to start. The lower and upper limit of each subscript in an array can be specified; the default lower limit is 1. Keep the controller as simple as possible. To compensate for this, R functions are written to "understand"Perl regular expression syntax if you specify argument "perl"=TRUE. N-Mixture models. Now you have learned the very basic concepts of R Markdown. You might also want to add our useful summary function for ncdf objects in R: summary. fruit colour, fruit size, plant height, and compare one group of plants. This is built-in to many functions and standard operators. The reason for this is that the model is not identified (non-positive degrees of freedom) if there are two random slopes (plus the intercept) the case when there are only three time points. Bus dashboard. And we usually see what a function does with the input: f(x) = x 2 shows us that function "f" takes "x" and squares it. Discover alternatives using R's vectorization feature. Below we redo the example using R. Get to know many of the input and output widgets that are available in Shiny with these examples. Bibliography--> CRAN--> R-Forge. The lower and upper limit of each subscript in an array can be specified; the default lower limit is 1. Then select the Advanced tab and set the "Show all filename extensions" Finally, the last option is to just change your code to look for example. packages("ggplot2"); install. More examples on data mining with R can be found in my book "R and Data Mining: Examples and Case Studies", which is downloadable as a. Examples and tutorials. The app is designed to showcase d3 graphics capabilities and R programming for organizational data visualizations to assist in executive decision making. Here, sequence is a vector and val takes on each of its value during the loop. Hey, I would like to analyze some Data from a Titanic Example. This example shows basic techniques and functions for working with matrices in the MATLAB® language. The "e" stands for extract, and it means to unzip or expand an archive. Note in this circuit there are three resistors, a current source, and an op-amp From Ohm’s Law we know: 1 1 1 v i R = 2 2 2 v i R = 3 3 3 v R And from the current. Case studies and code from papers. R \dontrun{ # this is a long running example for(i in seq(1, 1e5)) { lm(mpg ~ wt, data = mtcars) } } # some other shorter example 2 + 2. SCP Command Examples - Linux / Unix Tutorials SCP stands for secure copy is used to copy data (files or directories) from one unix or linux system to another unix or linux server. clear all Example 1: Bootstrapping instead of a t-test (with unequal sample sizes). There is more information available about the Selection of Examples. C provides two functions strtok() and strtok_r() for splitting a string by some delimiter. Then select the Advanced tab and set the “Show all filename extensions” Finally, the last option is to just change your code to look for example. If the table or view is in a particular schema, qualify it with the schema name. If no outfile is specified, the name used. N-Mixture models. This post originally appeared on my Wordpress blog on May 23, 2010. Syntax Examples By YourDictionary Along with diction, syntax is a fundamental part of written language. Posted in Data Science, Machine Learning, Math & Statistics, Programming, R | Tags: lightgbm, machine-learning, r Tags 1-line anon bash big-data big-data-viz C data-science econ econometrics editorial hacking HBase hive hql infosec java javascript linux lists machine-learning macro micro mssql MySQL nosql padb passwords postgres programming. Query by Example (QBE) is a method of query creation that allows the user to search for documents based on an example in the form of a selected text string or in the form of a document name or a list of documents. An R tutorial on the concept of data frames in R. This is because they are loaded into the global environment of the R session; all R code in a Shiny app is run in the global environment or a child of it. An example of a completed DA Form 4604-R (Security Construction Statement). Replace the first occurrence of a pattern with sub or replace all occurrences with gsub. An alternative approach to these last two examples would be to use the FIPS datasets that we’ve built-in to the acs. Each of those smaller goals can be applied to the SMART criteria in the same way as shown here. You can run an Oracle R Enterprise example script with the demo function. Home » Tutorials - SAS / R / Python / By Hand Examples » K Means Clustering in R Example K Means Clustering in R Example Summary: The kmeans() function in R requires, at a minimum, numeric data and a number of centers (or clusters). 95/12/18 - با استفاده از افزونه دانلود فایرفاکس و کروم٬ چکیده مقالات به صورت خودکار تشخیص داده شده و دکمه دانلود فری‌پیپر در صفحه چکیده نمایش داده می شود. Apriorit Software/R&D Outsourcing Non-Disclosure Agreement (NDA) Sample. Edit, edit, edit. Below is an example to count the number of even numbers in a vector. %%parameter: A replaceable parameter: in a batch file use %%G (on the command line %G) This command walks down the folder tree starting at [drive:]path, and executes the DO statement against each matching file. Robocopy (Robust File Copy) is a command-line file copy utility that comes with Windows Vista / Windows 2008 or newer. The R Function of the Day series will focus on describing in plain language how certain R functions work, focusing on simple examples that you can apply to gain insight into your own data. They have heard about reverse mortgage loans, but didn’t know the details. What's even better is that the tool's man page contains a lot of details that might be of help to users (especially newbies). The complexity of a writer's or speaker's sentences creates a formal or informal level of diction that is presented to its audience. What is here at present are links to three example pages. Read on to learn more! Syntax is the order or arrangement of words and phrases to form proper sentences. Morale improved significantly. Machine Learning A-Z™: Hands-On Python & R In Data Science If you want to do decision tree analysis, to understand the decision tree algorithm / model or if you just need a decision tree maker - you’ll need to visualize the decision tree. Part 3: Introduction to ARIMA models for forecasting. In the first example we simply hand the plot function two vectors. It is based on FusionForge offering easy access to the best in SVN, daily built and checked packages, mailing lists, bug tracking, message boards/forums, site hosting, permanent file archival, full backups, and total web-based. —The_Psuedonymouse. One-Sample t Test and Confidence Interval in R with Example: Learn how to conduct the one-sample t-test and calculate the confidence interval in R (using RSt. 5 practical examples of shutdown command in Linux Now that you know the syntax of the shutdown command, let's see how to use it. This R tutorial on loops will look into the constructs available in R for looping, when the constructs should be used, and how to make use of alternatives, such as R's vectorization feature, to perform your. A tutorial on loops in R that looks at the constructs available in R for looping. If, for example, one of the arguments in the Julia function is used to index variables, it needs to be integer and you may have to convert in the Julia function or use L in R (0L, 1L). What does %>% function mean in R? For example, this function will return a string consisting of its left argument followed by a comma and space and then it's. February 9, 2017 This section shows you how to use the DO cplex cloud R Client to create a job, submit it, and get results. To load a table from a set of unload files, simply reverse the process by using a COPY command. for example, the + operator can add two arrays of numbers without the need for an explicit loop. Example: for loop. Flowchart of for loop. Example 5: The usual ordering of the real numbers in R, in which =is the weak ordering and > is the strict ordering, is analogous to the relations de ned in Example 3, but generally not quite the same. percent is set to 0. Download Open Datasets on 1000s of Projects + Share Projects on One Platform. For example, c(1, 2) + c(3, 4) = c(4, 6) — you can read more about vectorization here. Binary operators in R are really just functions in disguise, and take two arguments (the one before and the one after the operator become the first two arguments of the function). 5", , it can be used like a gide to build a more complex web app. sftp> mput file1 file2 file3. Florida adopted a red flag law last year after the shooting that killed 17 people at Marjory Stoneman Douglas High School. In some cases, you will need to make multiple choices in R. They think if their product is good, their Gage R&R should be too. r; R/blsAPI. This stuff won't work unless you have loaded astsa and the data files at the start of the session. In Finder, click on Finder -> Preferences. This site may not work in your browser. For example, using the -R, recursive option, ls provides a tree-like listing of a directory structure. Today, I will discuss the table function. Function - VHDL Example. Whereas a logistic regression model tries to predict the outcome with best possible accuracy after considering all the variables at hand. You create a dataset from external data, then apply parallel operations to it. type -a Command Examples. R outside shinyServer(), with one important difference: they are also visible to the code in ui. When you’re searching for an example of a resume consider that the style of your resume will depend on the industry you work in. Case studies and code from papers. “Trump’s new public charge rule is yet one more example of his administration turning its back on people fighting to make a better life for them and their families,” James said in a statement. Shaw Stuart Wagenius November 3, 2003 As part of a research program to assess the evolutionary consequences of extreme population fragmentation, Stuart Wagenius has conducted a field ex-periment to study seedling recruitment in Echinacea angustifolia (purple cone-flower). If you want to recode from car you have to first install the car package and then load it for use. Based on the version 3. Replace the first occurrence of a pattern with sub or replace all occurrences with gsub. When rm is executed with the -r or -R options, it recursively deletes any matching directories, their subdirectories, and all files they contain. Robocopy (Robust File Copy) is a command-line file copy utility that comes with Windows Vista / Windows 2008 or newer. Mixed Effects Logistic Regression | R Data Analysis Examples Mixed effects logistic regression is used to model binary outcome variables, in which the log odds of the outcomes are modeled as a linear combination of the predictor variables when data are clustered or there are both fixed and random effects. As an example, you can certainly learn using the online documents on the boot() package, by saying: As an example, you can certainly learn using the online documents on the boot() package, by saying:. $ scp -c blowfish some_file [email protected] Native speakers of a language learn correct syntax without realizing it. If no outfile is specified, the name used. you can find it in the knitr main manual or its graphics manual. sftp> mkdir directory sftp> put -r directory/ 5. R functions are presented in the context of interesting applications with real data. Syntax is a tool used in writing proper grammatical sentences. Our negative example for “Los Angeles” resulted in the best-ranked program changing from looking for a line starting with Camel Case to looking for a line followed by a line that starts with a number. 1) MS1 (one link goes into one node, FF of the link = 0) FF of A, B, C, and E= 0. Obtain the (binary) netCDF file: example. Normally you would use double spacing in the paper. js can live anywhere you want, but you will likely need to adjust the paths accordingly in these examples. Free download PostScript(R) by Example Free Ebook PDF Download and read Computers and Internet Books Online. The command can run across multiple lines, all but the last ending with a back-slash. SCP uses secured shell (ssh) to transfer the data between the remote hosts. An integrated development environment for R, with a console, syntax-highlighting editor that supports direct code execution, and tools for plotting, history, debugging and workspace management. R functions are presented in the context of interesting applications with real data. Based on the version 3. Get to know many of the input and output widgets that are available in Shiny with these examples. You can also use tee command to store the output of a command to a file and redirect the same output as an input to another command. (3) All the files are. Download Open Datasets on 1000s of Projects + Share Projects on One Platform. In R: data (iris) From the iris manual page: This famous (Fisher’s or Anderson’s) iris data set gives the measurements in centimeters of the variables sepal length and width and petal length and width, respectively, for 50 flowers from each of 3 species of iris. The following example creates a new table, LOADVENUE, and loads the table from the data files created in the previous example. We have divided the section into Five parts from basic to advance usage of the find command. User-written Functions. KNN example in R Ranjit Mishra Tuesday, November 03, 2015. for example, the + operator can add two arrays of numbers without the need for an explicit loop. See the options page. $\endgroup$ - Matt Reichenbach Oct 8 '13 at 19:58 $\begingroup$ Plug here for a package by Patrick Breheny called ncvreg which fits linear and logistic regression models penalized by MCP, SCAD, or LASSO. However, we recommend you to write code on your own before you check them. lakes and inland seas) and many large flowing streams of water ( i. It supports both simple calls to R functions and a full running REPL. This document provides examples of different things that can be done in emacs org-mode files. Example of Deep Learning With R and Keras Recreate the solution that one dev created for the Carvana Image Masking Challenge, which involved using AI and image recognition to separate photographs. To prepare for the Portfolio Probe blog post called “Implied alpha and minimum variance”, I tried to update a matrix of daily stock prices using a function I had written for the purpose. 4, this book will help you develop strong fundamentals when working with R by taking you through a series of full representative examples, giving you a holistic view of R. As a screenwriter, you must show what's happening in a story, rather than tell. (f) Utilizing the open circuit, closed loop gain (5 gain obtained when a resistance R L=10k (g) As an alternative approach to (f) above, redo the analysis of the the values of R 1 and R 2 found in (d) to determine Example #6 Ω. Generalized Likelihood Ratio Test Example a. If you want to store binary data and make it available to the user, put it in data/. Last for final actions at the close of an R session. The general formula is A = P(1+ r. Introduction. The recode() command from the car package is another great way to recode data in R. txt Add read permission on all, remove execute on all, and set user/group to read write for file myfile. "for example" or "for the sake of an example": used to introduce an illustrative example or short list of examples. In addition to the above scripts, you can add your own, thanks to the extensive configuration options. Meet The animation package. But this example also introduces a special kind constructor: the default constructor. of EECS There are seven device equations Finally, we add in the device equations. From Learning SAS® by Example. If applicable DA Form 5500-R/5501 - R (Body Fat Content Worksheet), DA Form Weapons Qualification Card (DA Form 3595-R or DA Form 5790)(must be. Software Architecture Document. Draw deflected shape of loaded truss. To upload multiple files use the following command. In both cases, the results are undefined if the command to be executed contains a NUL character. edu:~ It is often suggested that the -C option for compression should also be used to increase speed. The Sales Analytics Dashboard uses Shiny to monitor simulated bicycle sales for a bicycle manufacturer, Cannondale. It can make a significant difference to the business,. R instead of example. packages("lattice") ` * Paste in the contents of this gist (which contains the R Markdown file used to produce this post) and save the file with an `. Please use a supported browser. R has various functions for regular expression based match and replaces. tikzDevice combines the computational power of R with the graphical beauty of and font consistancy of TikZ. EXAMPLE OF A RESEARCH PAPER. Help pages for functions usually include a section with executable examples illustrating how the functions work. R&D Tax Credits: Example Calculations. 6 Choosing the Final Model; 5. Example Domain. An integrated development environment for R, with a console, syntax-highlighting editor that supports direct code execution, and tools for plotting, history, debugging and workspace management. Morale improved significantly. Kaplan ESHRI/R&D PMO June 2011 Readers should note that the enclosed report is only an example. gzip -cd file. For Example. Example 5: The usual ordering of the real numbers in R, in which =is the weak ordering and > is the strict ordering, is analogous to the relations de ned in Example 3, but generally not quite the same. In this example, the chkdsk command is used to perform an extensive check of the C: drive to correct any errors and to locate any recovery information from bad sectors. Dhillon is a trial lawyer and a partner in the Dhillon Law Group in San. path can be used to set the cache directory. Below is an example to count the number of even numbers in a vector. In Finder, click on Finder -> Preferences. Rule: if entity eis represented in a subclass, then eis represented in the superclass(and recursively up the tree). By default, it does not remove directories. We will consider the synthetic example considered in the Zou et. Even an assignment is an expression whose result is the value assigned, and it may be used wherever any expression may be used; in particular multiple assignments are possible. [4] For example, in non-rhotic varieties of English, the sound /r/ does not occur in a word such as tuner when it is spoken in isolation, before an intonation break (in pausa ), or before a word beginning with a consonant. Here, just the basic structure of modeling in R is given, using anova and linear regression as examples. $ R CMD BATCH [options] my_script. R code examples for data mining are posted here. Bus dashboard. what goes into the function is put inside parentheses after the name of the function: So f(x) shows us the function is called "f", and "x" goes in. Precisely speaking, it is Pandoc's Markdown. 1, Windows 10 and their server variants. x and r Chartin Class Example - Free download as Powerpoint Presentation (. Simply put, the test compares the expected and observed number of events in bins defined by the predicted probability of the outcome. Example in R. You should get a grip on the Linux grep command. Parallel Computing for Data Science: With Examples in R, C++ and CUDA is one of the first parallel computing books to concentrate exclusively on parallel data structures, algorithms, software tools, and applications in data science. It is assumed that you know how to enter data or read data files which is covered in the first chapter, and it is assumed that you are familiar with the different data types. The R Function of the Day series will focus on describing in plain language how certain R functions work, focusing on simple examples that you can apply to gain insight into your own data. This tutorial is about using of R visualization in Power BI. Most netCDF files are substantially more complicated than this simple example. Wget Command Examples. As an HR Manager, this job seeker successful filled positions in management, technical and professional fields while developing succession plans and avoidance programs. This value is much larger than Total Gage R&R, which is 7. SparkR also supports distributed machine learning using MLlib. xlsx example data set (shown below) holds corporate data on 22 U. Search for text/string in a file: findstr pattern filename. In a previous post on mail command we saw how the mail command can be used to send emails from the command line on your linux server. The last example listed below shows a vector with zero elements; the output of any and all in this case may at first be confusing. This stuff won't work unless you have loaded astsa and the data files at the start of the session. Using the R sytem. Radermacher and S. jpg as a part of the DA Form 5500-R/5501-R, Body Fat Content Worksheet-if required. You separate each word by & in the \shortex environment, and tell it the number of words you plan to enter (in curly brackets). A research proposal example can serve such a heaven-sent piece of advice for a traveler in the depths of science. Use library e1071, you can install it using install. Additional configuration settings such as http authentication (authenticate()), additional headers (add_headers()), cookies (set_cookies()) etc. Apache Spark Examples. , example(lm). If the type of the returned objects is known in advance, accessor methods can be called to obtain the Java object corresponding to the R value, in our case a regular String. This example illustrates how to use XLMiner to perform a cluster analysis using hierarchical clustering. Wget is a free utility that can be used for retrieving files using HTTP, HTTPS, and FTP. An R tutorial on the concept of data frames in R. Revised January 2010 Linear interpolation example. Built-in Function. Splitting a string is a very common task. The output file lists the commands from the script file and their outputs. For example, the following would tell it to use the times of file4 for file5: touch -r file4 file5. It offers a wide range of functionality, including to easily search, share, and collaborate on KNIME workflows, nodes, and components with the entire KNIME community. “Joining the President is his husband…” 3/13/2014. For example, if directory /backup does not exists, create a new one so that your shell script. The following are code examples for showing how to use re. To integrate a one-dimensional integral over a nite or in nite interval, use R function integrate. On-line help is available for all R methods using the ?method syntax at the command prompt; for example ?lm opens a window with help for the lm ( t linear models) method. If you need to read in a csv with R the best way to do it is with the command read. Data Exploration. Example- Current Density. A positive-definite matrix is defined as a symmetric matrix where for all possible vectors \(x\), \(x'Ax > 0\). For dlply, the syntax means d for input data is data frame and l for output data is list. In this forecasting example, we will look at how to interpret the results from a forecast model and make modifications as needed. Once the basic R programming control structures are understood, users can use the R language as a powerful environment to perform complex custom analyses of almost any type of data. In this tutorial, we shall learn R switch statement based on index and matching value, its syntax and examples to demonstrate the realization of decision making using switch statement. Description. This page contains examples on basic concepts of R programming. for example, the + operator can add two arrays of numbers without the need for an explicit loop. It’s implemented by algorithms that have their own built-in feature selection methods. R code for the examples. In R, you can view a function's code by typing the function name without the ( ). In Finder, click on Finder -> Preferences. In this way, the example in Figure 11 creates a frame with three overlays. The department became the most efficient and productive department in the division. Another resource is the StatSoftEquivs wiki. Advertiser Disclosure: Some of the products that appear on this site are from companies from which QuinStreet receives compensation. It supports both simple calls to R functions and a full running REPL. While R can read excel. Markdown is probably the most commonly-used plain text markup used online, and is easy to get started with. ♦ Example: 20 minutes late to group session, slouched in chair, head down, later expressed interest in topic. An Introduction to R for more command-line options: those affecting memory management are covered in the help file for Memory. As an HR Manager, this job seeker successful filled positions in management, technical and professional fields while developing succession plans and avoidance programs. (3) All the files are. if statement runs a set of command if some condition is true. Thus, INTF of P = 0. Functions are small sections of code that perform an operation that is reused throughout your code. A demonstration that a correlation may be decomposed to a within group correlation and a between group correlations and these two correlations are independent. R allows direct access to parsed expressions and functions and allows you to alter and subsequently execute them, or create entirely new functions from scratch. A Simple Example of Parallel Computing on a Windows (and also Mac) Machine More Examples See Other Examples page for more examples on data mining with R, incl. RStudio Server Pro. For |, & and xor a logical or raw vector. Example Essays. { examples :: charting} If there was one area of R that was a bit lacking, it was the ability to visualize financial data with standard financial charting tools. Therefore, the last part of the fitted values vector are one-step forecasts on the test set. The examples should provide a clue of what you need to look up in the org-mode manual. Short, prototypical programs exploring the basics of programming with Processing. This data set was created only to be used as an example, and the numbers were created to match an example from a text book, p. SFTP Command Example #5 – Download files and directories using the. After informal introductions, Johnson announced that his cat recently birthed six kittens. You can vote up the examples you like or vote down the ones you don't like. of Kansas Dept. The best way to learn any programming language is by practicing examples on your own. bak -r 7z: use executable d: delete files archive. txt This is an example of how to capture traffic on your outbound smtp server. Random Walk Theory: The random walk theory suggests that stock price changes have the same distribution and are independent of each other, so the past movement or trend of a stock price or market. " Here are some behavioral questions. In doing so, a worker's motivation can be maximized by applying the particular set of needs to an applicable situation. In addition to the above scripts, you can add your own, thanks to the extensive configuration options. Browse flowchart templates and examples you can make with SmartDraw. Learn about Purdue University's College of Liberal Arts, a college focused on strengthening the Undergraduate Experience, enhancing Graduate Education, and promoting Faculty Excellence. I present it here in its original form. R by Example is an example-based introduction to the statistical computing environment that does not assume any previous familiarity with R or other software packages. Here is a list of worked out examples and tutorials using R-INLA. when class starts in August 2012, with enough inventory to last through the first two weeks of classes. Short, prototypical programs exploring the basics of programming with Processing. Solution If we choose r to be positive, then r = p x2 +y2 = p 12 +(−1)2 = √ 2 tanθ = y x = −1 Since the point (1,−1) lies in the fourth quadrant, we choose θ = −π 4 or θ = 7. r; R/blsAPI. The R Function of the Day series will focus on describing in plain language how certain R functions work, focusing on simple examples that you can apply to gain insight into your own data. If you simply use the shutdown command, it will start the shutdown process after one minute. Load library. The optimizer that is part of r. You can execute these examples in the current R session via the example() command: e. Continents ( e. Keep the controller as simple as possible. R PROGRAMMING ENVIRONMENT. Logit Regression | R Data Analysis Examples Logistic regression, also called a logit model, is used to model dichotomous outcome variables. The best way to learn any programming language is by practicing examples on your own. R Presentations are a feature of RStudio that enable easy authoring of HTML5 presentations using a combination of Markdown and R. Earth's Gravitational Force Is the Centripetal Force Making the Moon Move in a Curved Path (a) Find the acceleration due to Earth's gravity at the distance of the Moon. N-Mixture models. Hello World. The command can run across multiple lines, all but the last ending with a back-slash. Distribute freely. The utilities. Simple examples of in-built functions are seq(), mean(), max(), sum(x) and paste() etc. Another option is to configure Finder to show file extensions. Asia) contain many large bodies of water ( e. method of decomposing a positive-definite matrix. Software, Data and Validation examples for ionospheric and tropospheric radio wave propagation and radio noise Rollup Image You are here Home > ITU-R > Study Groups > SG 3 > Software, Data and Validation examples for ionospheric and tropospheric radio wave propagation and radio noise. This includes infix operators like + , control flow operators like for , if , and while , subsetting operators like [] and $ , and even the curly brace {. The R Function of the Day series will focus on describing in plain language how certain R functions work, focusing on simple examples that you can apply to gain insight into your own data. fruit colour, fruit size, plant height, and compare one group of plants. Edit, edit, edit. For example, as a Human Resources Director, they rewrote policy and compliance documents while also achieve 100% win rate in unemployment claims. 2/21/2011 Example An op amp circuit analysis lecture 10/23 Jim Stiles The Univ. com jump server, and forwards any connection to port 80 on the local machine to port 80 on intra. You can vote up the examples you like or vote down the ones you don't like. In some cases, you can accomplish the same task much easier by using the point-and-click interface. Discover API Examples. Just as con-artists like to lard complexity into bar. It’s often an honor to be asked to write a character letter, but if it’s something you haven’t done before, it may help to look at a reference letter example.