Strange _for_ loop behavior on R programming

问题

Based on the answers of this question, I tried to write the following codes:

for(var in names(Auto)) {print(var)}

---

"mpg" [1] "cylinders" [1] "displacement" [1] "horsepower" [1] "weight" [1] "acceleration" [1] "year" [1] "origin"

Ok, my name variables are in names(Auto), then I moved on:

for(var in names(Auto)) 
{
  summary(lm(paste('mpg ~', var), data = Auto))
}

---

Warning messages:
1: In model.matrix.default(mt, mf, contrasts) :
  the response appeared on the right-hand side and was dropped
2: In model.matrix.default(mt, mf, contrasts) :
  problem with term 1 in model.matrix: no columns are assigned

Strangely for one iteration we have:

var='cylinders'
summary(lm(paste('mpg ~', var), data = Auto)

---

Call:
lm(formula = paste("mpg ~", var), data = Auto)

Residuals:
     Min       1Q   Median       3Q      Max 
-14.2413  -3.1832  -0.6332   2.5491  17.9168 

Coefficients:
            Estimate Std. Error t value Pr(>|t|)    
(Intercept)  42.9155     0.8349   51.40   <2e-16 ***
cylinders    -3.5581     0.1457  -24.43   <2e-16 ***
---
Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

Residual standard error: 4.914 on 390 degrees of freedom
Multiple R-squared:  0.6047,    Adjusted R-squared:  0.6037 
F-statistic: 596.6 on 1 and 390 DF,  p-value: < 2.2e-16

That's what I wanted for each iteration.

来源：https://stackoverflow.com/questions/62364671/strange-for-loop-behavior-on-r-programming