variable lengths differ in R using lm

Based on @Dave2e comment.

It is better to use data=NewSetSlide.ex argument inside lm function call to avoid naming conflicts moreover you should avoid using attach function. Please see as below (NewSetSlide.ex data frame was taken from the question above):

resultr <- lm(returnapple ~ returnsandp, data = NewSetSlide.ex)
summary(resultr)

Output:

Call:
lm(formula = returnapple ~ returnsandp, data = NewSetSlide.ex)

Residuals:
         Min           1Q       Median           3Q          Max 
-0.166599181 -0.041838291  0.003778841  0.044034591  0.166774667 

Coefficients:
                Estimate   Std. Error  t value  Pr(>|t|)   
(Intercept)  0.028595156  0.008677931  3.29516 0.0016294 **
returnsandp -0.035466006  0.160976847 -0.22032 0.8263478   
---
Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

Residual standard error: 0.0672294 on 62 degrees of freedom
Multiple R-squared:  0.0007822871,  Adjusted R-squared:  -0.01533413 
F-statistic: 0.04853977 on 1 and 62 DF,  p-value: 0.8263478