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# Introduction

The recipes in this chapter lie somewhere between problem-solving ideas and tutorials. Yes, they solve common problems, but the Solutions showcase common techniques and idioms used in most R code, including the code in this Cookbook. If you are new to R, I suggest skimming this chapter to acquaint yourself with these idioms.

# 2.1. Printing Something

## Problem

You want to display the value of a variable or expression.

## Solution

If you simply enter the variable name or expression at the command prompt, R will print its value. Use the `print` function for generic printing of any object. Use the `cat` function for producing custom formatted output.

## Discussion

It’s very easy to ask R to print something: just enter it at the command prompt:

```> `pi`
[1] 3.141593
> `sqrt(2)`
[1] 1.414214```

When you enter expressions like that, R evaluates the expression and then implicitly calls the `print` function. So the previous example is identical to this:

```> `print(pi)`
[1] 3.141593
> `print(sqrt(2))`
[1] 1.414214```

The beauty of `print` is that it knows how to format any R value for printing, including structured values such as matrices and lists:

```> `print(matrix(c(1,2,3,4), 2, 2))`
[,1] [,2]
[1,]    1    3
[2,]    2    4
> `print(list("a","b","c"))`
[[1]]
[1] "a"

[[2]]
[1] "b"

[[3]]
[1] "c"```

This is useful because you can always view your data: just `print` it. You needn’t write special printing logic, even for complicated data structures.

The `print` function has a significant limitation, however: it prints only one object ...

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