# Load packages

# Core
library(tidyverse)
library(tidyquant)

Goal

Collect individual returns into a portfolio by assigning a weight to each stock

five stocks: “SPY”, “EFA”, “IJS”, “EEM”, “AGG”

from 2012-12-31 to 2017-12-31

1 Import stock prices

symbols <- c("SPY", "EFA", "IJS", "EEM", "AGG")

prices  <- tq_get(x    = symbols, 
                  get  = "stock.prices", 
                  from = "2012-12-31",
                  to   = "2017-12-31")

2 Convert prices to returns

asset_returns_tbl <- prices %>%
    
    group_by(symbol) %>%
    
    tq_transmute(select     = adjusted, 
                 mutate_fun = periodReturn, 
                 period     = "monthly", 
                 type       ="log") %>%
    
    slice(-1) %>%
   
    ungroup() %>%

    set_names(c("asset", "date", "returns"))

3 Get market returns

market_returns_tbl <- tq_get(x = "SPY", 
                  get  = "stock.prices", 
                  from = "2012-12-31",
                  to   = "2017-12-31") %>%
    
    tq_transmute(select     = adjusted, 
                 mutate_fun = periodReturn, 
                 period     = "monthly", 
                 type       ="log") %>%
    
    slice(-1)

4 Calculate CAPM Beta by Asset

asset_beta_tbl <- asset_returns_tbl %>% 
    
    nest(data = -asset) %>% 
    
    # Calculate CAPM Beta 
    mutate(model = map(.x = data, 
                       .f = ~lm(returns ~ market_returns_tbl$monthly.returns, 
                               data = .x))) %>%
    
    # Extract beta 
    mutate(model = map(.x = model, .f = broom::tidy)) %>%
    unnest(model) %>%
    filter(term != "(Intercept)") 

asset_beta_tbl %>%
    
    ggplot(aes(x = estimate, 
               y = fct_reorder(asset, estimate),
               fill = asset)) +
    geom_col() + 
    
    scale_fill_tq() +
    theme_tq() +
    theme(legend.position = "none") +
    
    labs(y = NULL, x = "Beta Coefficient",
         title = "The Beta Coefficient by Asset")