Q:

Missing horizontal scroll bar in R Markdown HTML code chunks and output

How can I make my knitted .Rmd document not wrap code when producing an HTML document? Whenever I knit my file I get output like the following:

You can see that the first line containing a cbind call is wrapped. This was produced by the following Rmd code. Basically, I'd like to see the resultant HTML file look like it does here on StackOverflow (i.e. with a horizontal scroll bar).
---
title: "Title"
author: "Author"
date: "March 25, 2016"
output: html_document
---

```{r}
myveryveryveryveryverylongvariablenameanditsdataaaaaaaaaaaaaaaaaaaaaaaaaaa <- cbind(iris, iris, iris, iris, iris, iris, iris)
head(myveryveryveryveryverylongvariablenameanditsdataaaaaaaaaaaaaaaaaaaaaaaaaaa )
```

Then separately, how can I do this for the text output on the second line? I've tried options(width=...) but this only seems to jarble up the output more. I'd like to it also look just as it does here on StackOverflow (no wrapping, with horizontal scroll bar):
  Sepal.Length Sepal.Width Petal.Length Petal.Width Species Sepal.Length Sepal.Width Petal.Length Petal.Width Species Sepal.Length Sepal.Width Petal.Length Petal.Width Species Sepal.Length Sepal.Width Petal.Length Petal.Width
1          5.1         3.5          1.4         0.2  setosa          5.1         3.5          1.4         0.2  setosa          5.1         3.5          1.4         0.2  setosa          5.1         3.5          1.4         0.2
2          4.9         3.0          1.4         0.2  setosa          4.9         3.0          1.4         0.2  setosa          4.9         3.0          1.4         0.2  setosa          4.9         3.0          1.4         0.2
3          4.7         3.2          1.3         0.2  setosa          4.7         3.2          1.3         0.2  setosa          4.7         3.2          1.3         0.2  setosa          4.7         3.2          1.3         0.2
4          4.6         3.1          1.5         0.2  setosa          4.6         3.1          1.5         0.2  setosa          4.6         3.1          1.5         0.2  setosa          4.6         3.1          1.5         0.2
5          5.0         3.6          1.4         0.2  setosa          5.0         3.6          1.4         0.2  setosa          5.0         3.6          1.4         0.2  setosa          5.0         3.6          1.4         0.2
6          5.4         3.9          1.7         0.4  setosa          5.4         3.9          1.7         0.4  setosa          5.4         3.9          1.7         0.4  setosa          5.4         3.9          1.7         0.4

A:

With some help from folks in the comments, I was able to piece together a solution. There are two steps:

Include a custom style sheet in the .Rmd YAML header:
css: report_styles.css
Which contains these styles:
pre, code {white-space:pre !important; overflow-x:scroll !important}
This makes it so that echoed code chunks will not wrap and that they have a horizontal scroll bar. It will also make is so that chunk outputs will not wrap further when the browser window is resized.
Now, to get the chunk output to not wrap initially, we need to set options(width=a-big-number) as per this question.

A:

This is basically @JeffKeller's answer but instead of having a separate CSS file and fiddling around in the YAML header you can just write your CSS inside your .Rmd file as a code snippet! For example,
```{css, echo=FALSE}
pre, code {white-space:pre !important; overflow-x:auto}
```

I tend to do this at about the same early point where I do other bits of setup e.g.
```{r setup, include=FALSE}
knitr::opts_chunk$set(echo = TRUE)
```

(Perhaps this is more of a comment but I can't really format it as such. Judging from the comments on the other answer though, I think some people might find this useful.)

