length_one_tuples.Rmd

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# Length 1 tuples

Remember {ref}`tuples`.  For example, here are a couple of length two tuples:

```{python}
first_tuple = (1, 2)
first_tuple
```

```{python}
second_tuple = (3, 4)
```

As for lists, you can add tuples, to concatenate the contents:

```{python}
tuples_together = first_tuple + second_tuple
tuples_together
```

## Length 1 tuples

Let us say you want to write code for a tuple with only one element.

You might think this would work:

```{python}
# Python interprets this parentheses in arithmetic.
just_a_number = (1)
just_a_number
```

Just a single number or string, with parentheses around it, does not make a
tuple, because Python interprets this as your usual brackets for arithmetic.  That means that:

```{python}
(1)
```

is exactly the same as:

```{python}
1
```

Why?  Because, Python has to decide what expressions like this mean:

```python
# Wait - what do the parentheses mean?
(1 + 2) + (3 + 4)
```

Is this adding two one-element tuples, to give a two-element tuple `(3, 7)`? Or
is it our usual mathematical expression giving 3 + 7 = 10.  The designer of the Python language decided it should be an arithmetical expression.

```{python}
# They mean parentheses as in arithmetic.
(1 + 2) + (3 + 4)
```

To form a length-1 tuple, you need to disambiguate what you mean by the parentheses, with a trailing comma:

```{python}
short_tuple = (1,)  # Notice the trailing comma
short_tuple
```

So, for example, to add two length one tuples together, as above:

```{python}
# Notice the extra comma to say - we mean these to be length-1 tuples.
(1 + 2,) + (3 + 4,)
```