Loop in R markdown

Could that be what you want?

---
title: "Untitled"
author: "Author"
output: html_document
---


```{r, results='asis'}
for (i in 1:2){
   cat('\n')  
   cat("#This is a heading for ", i, "\n") 
   hist(cars[,i])
   cat('\n') 
}
```

This answer was more or less stolen from here.

As already mentioned, any loop needs to be in a code chunk. It might be easier to to give the histogram a title rather than add a line of text as a header for each one.

```{r}
    for i in length(somelist) {
        title <- paste("The following graph shows a histogram of", somelist[[ i ]])
        hist(somelist[[i]], main=title)
    }
```

However, if you would like to create multiple reports then check out this thread.

Which also has a link to this example.
It seems when the render call is made from within a script, the environmental variables can be passed to the Rmd file.

So an alternative might be to have your R script:

for i in length(somelist) {
    rmarkdown::render('./hist_.Rmd',  # file 2
               output_file =  paste("hist", i, ".html", sep=''), 
               output_dir = './outputs/')
}

And then your Rmd chunk would look like:

```{r}
    hist(i)
```

Disclaimer: I haven't tested this.