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# Engineering 1020: Introduction to Programming

.title[
  .lecture[Lecture \12\:]
  .title[More fun with lists]
  
  
]

.footer[[/lecture/12/](/lecture/12/)]

---

# Lists so far:

### Creating

### Iterating

### Indexing and slicing

---

# Lists today:

### List-related functions

### List comprehensions

### The `in` keyword

---

# List-related functions

#### These functions get information from iterables (like lists!):

???

Try using the `help()` function to learn a bit about each of these functions...

.floatleft[
* `all()`
* `any()`
* `iter()` (optional)
* `len()`
]

.floatleft[
* `max()`
* `min()`
* `next()` (optional)
* `sorted()`
]

---

# Why lists?

### Can represent arbitrary numbers of things

* Not stuck with pre-defined (_static_) problem sizes
--

* Remember [exercise 3 (EOPA)](../../exercise/3)?
--
 What if you only have seven or eight numeric grades?

### Can be a more elegant representation

* Even if we know the problem size, nice to be general!

---

# Exercise 3

.column[
| Subject | Course |
|---------|---------|
| CHEM | 1050
| ENGI | 1010, 1020, 1030, 1040
| ENGL | 1090
| MATH | 1001, 2050
| PHYS | 1051
]

.column[
> Calculate the Engineering One **average** and **promotability** of a student,
> given their Engineering One grades.
]

???

Let's do this exercise again, this time using **lists**
to make for a simpler, more re-usable implementation.

## Addition operator:

We can **add** lists together using the **`+` operator**.
This is known as **list concatenation**, and it's one way to
make lists **change size**.

---

# A very common pattern:

```python
total = 0
for x in something:
    if x > something_else:
        total += x
```

```python
passing_grades = []
for grade in courses:
    if grade >= 55:
        passing_grades += [grade]
```

```python
aggregate = ...
for x in something:
    if f(x):
        aggregate += g(x)   # or *=, or ...
```

---

# A common solution

#### List comprehension: _an elegant tool for a civilized age_

* syntax for constructing a list from something iterable
--

* Can _filter_ and _transform_ individual elements

#### e.g., The first ten squares of even numbers:

```python
squares = []
for i in range(2, 21):
    if i % 2 == 0:
        squares += [i*i]
```

---

# List comprehensions

### Syntax:

.floatright[
```python
[i*i for i in range(2, 21) if i%2 == 0]
```
]

* `[` — opening bracket
--

* _expression_
--

* `for` _loop parameter_ `in` _iterable_
--

* `if` _condition_
--

* `]` — closing bracket

---

# A familiar problem

### Problem 1 from [Project Euler](https://projecteuler.net):

> Find the sum of all the multiples of 3 or 5 below 1000.

```python
total = 0

for n in range(1000):
    if n % 3 == 0 or n % 5 == 0:
        total += n

```

```python
total = sum([n for n in range(1000) if n % 3 == 0 or n % 5 == 0])
```

---

# More list comprehensions

### Possible to get more complex:

* Can have more than one `for` loop:  
```python
[i+j for i in range(10) for j in range(10,20)]
```
* Not required for this course!
* Further details available at [python.org](python.org)
  ([5.1.3 in the Data Structures tutorial](https://docs.python.org/3/tutorial/datastructures.html#list-comprehensions))

---

# One more thing

### Remember the `in` keyword?

```python
for i in range(10):
```

### Can also use `in` as an operator!

```python
if 100 in grades:
    print('Wow, well done!')
```

```python
is_cool_guy = 'Jon' in username
if letter in 'EAIONRTLSU': # ...
```

???

This new syntax may be helpful for
[assignment 2](../../assignment/2)...

---

# Summary

### How cool are lists?

### List-related functions

### List comprehensions

### `in` keyword

---
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(here endeth the lesson)