[R] loops over regression models

[Petr PIKAL]

Hi
> 
> Dear R help listers,
> 
> I am trying to replicate results in Gelman and Hill's book (Chapter 3
> in regressions and multilevel models). Below I estimated two models
> (chp3.1 and chp3.3 in R codes) with the same data and dependent
> variable but different independent variables. I have been using Stata
> for quite a while, and I know I can use foreach to build a loop to
> condense the codes (especially if I have a large number of models to
> run).
> 
> In Stata, it would be something like:
> 
> ****************************************************
> // read in data
> use kidiq, clear
> 
> // run two regression
> reg kid_score mom_hs
> reg kid_score mom_iq
> 
> // the next three lines are equivalent of the previous two lines
> foreach var in mom_hs mom_iq {
>  reg kid_score `var'
> }
> ***************************************************
> 
> 
> So I want to figure out how to use R to do this. Below are my codes:
> 
> ####################################################
> library(foreign)
> # read in stata data file
> kidiq <-data.frame(read.dta('kidiq.dta', convert.factor=FALSE))
> 
> # bivariate regressions
> chp3.1 <- lm(kid_score ~ mom_hs, data=kidiq)
> summary(chp3.1)
> 
> chp3.3 <- lm(kid_score ~ mom_iq, data=kidiq)
> summary(chp3.3)
> 
> clist <- c("mom_iq", "mom_hs")
> 
> for (x in clist) {
>   lm(kid_score ~ x, data = kidiq)

use 
as.formula(paste("kid_score ~ ", eval(x)))

as I understand x is unevaluated and you need to evaluate it inside your 
data.

And you also need to assign values of lm inside cycle or explicitly print 
them.

Regards
Petr 

> 
> }
> Error in model.frame.default(formula = kid_score ~ x, data = kidiq,
> drop.unused.levels = TRUE) :
>   variable lengths differ (found for 'x')
> ##################################################
> 
> But I got an error message that says variable length differ. I tried
> various ways to work around this, for example, I tried:
> 
> clist <- c("mom_iq", "mom_hs")
> 
> for (x in 1:length(clist)) {
>   lm(kid_score ~ clist[x], data = kidiq)
> 
> }
> 
> 
> 
>  But none of these work. So I am wondering if anyone could give me
> some hint. Thanks a lot
> 
> Jun Xu, PhD
> Assistant Professor
> Department of Sociology
> Ball State University
> Muncie, IN
> 
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