How to set types of variables in a raw frame?

Question

How to set types of variables in a raw frame?

I wonder if anyone can explain to me how I can set the type for each variable in my data framework.

I created a function to call quotes from the yahoo API as JSON, and each of them bound each of them to a data framework called a portfolio. The unfortunate data is very raw, and all this happens as characters.

I ask you if there is a simple way by which r can automatically save values as numeric or characters without setting each variable through a data frame.

Here I insert you my script:

library(jsonlite)
# function that sets all NULL values to NA

nullToNA <- function(x) {
    x[sapply(x, is.null)] <- NA
    return(x)
}

# function that gets Quotes from Yahoo API

getQuote <- function(ticker) {
    path <- paste("https://query.yahooapis.com/v1/public/yqlq=select%20*%20from%20yahoo.finance.quotes%20where%20symbol%20in%20(%22",ticker,"%22)&format=json&env=store%3A%2F%2Fdatatables.org%2Falltableswithkeys&callback=", sep="")
    stock <- fromJSON(path, flatten = TRUE)
    q <- stock$query$results$quote
    q <- nullToNA(q)
}

# define the tickers to include in portfolio

tickers <- c("AAPL","GOOG", "AMZN") 

# portfolio construction

portfolio <- c()

for (ticker in tickers){
    portfolio <- rbind(portfolio, getQuote(ticker))
}

portfolio < unlist(portfolio)
portfolio <- data.frame(portfolio, stringsAsFactors = FALSE)