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Details

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Zelig uses the syntax of R, which has several essential elements:

1. R is case sensitive. Zelig, the package or library, is not the same as zelig, the command.

2. R functions accept user-defined arguments: while some arguments are required, other optional arguments modify the function's default behavior. Enclose arguments in parentheses and separate multiple arguments with commas. For example, print(x) or print(x, digits = 2) prints the contents of the object x using the default number of digits or rounds to two digits to the right of the decimal point, respectively. You may nest commands as long as each has its own set of parentheses: log(sqrt(5)) takes the square root of 5 and then takes the natural log.

3. The <- operator takes the output of the function on the right and saves them in the named object on the left. For example, z.out <- zelig(...) stores the output from zelig() as the object z.out in your working memory. You may use z.out as an argument in other functions, view the output by typing z.out at the R prompt, or save z.out to a file using the procedures described in Section .

4. You may name your objects anything, within a few constraints:
   • You may only use letters (in upper or lower case) and periods to punctuate your variable names.
   • You may not use any special characters (aside from the period) or spaces to punctuate your variable names.
   • Names cannot begin with numbers. For example, R will not let you save an object as 1997.election but will let you save election.1997.

5. Use the names() command to see the contents of R objects, and the $ operator to extract elements from R objects. For example:

   # Run least squares regression and save the output in working memory:  
   > z.out <- zelig(y ~ x1 + x2, model = "ls", data = mydata)
   # See what's in the R object: 
   > names(z.out)
    [1] "coefficients"  "residuals"  "effects"  "rank"
   # Extract and display the coefficients in z.out: 
   > z.out$coefficients
   

6. All objects have a class designation which tells R how to treat it in subsequent commands. An object's class is generated by the function or mathematical operation that created it.

7. To see a list of all objects in your current workspace, type: ls(). You can remove an object permanently from memory by typing remove(goo) (which deletes the object goo), or remove all the objects with remove(list = ls()).

8. To run commands in a batch, use a text editor (such as the Windows R script editor or emacs) to compose your R commands, and save the file with a .R file extension in your working directory. To run the file, type source("Code.R") at the R prompt.

If you encounter a syntax error, check your spelling, case, parentheses, and commas. These are the most common syntax errors, and are easy to detect and correct with a little practice. If you encounter a syntax error in batch mode, R will tell you the line on which the syntax error occurred.