class: big, middle

# Engineering 1020: Introduction to Programming

.title[
  .lecture[Lecture \10\:]
  .title[Ranges and lists]
  
  
]

.footer[[/lecture/10/](/lecture/10/)]

---

# Recall: iteration

### Walking through a bunch of things, _one at a time_

### IRL: design _iterations_

### Computing: loops!

.floatright[
```python
for letter in 'Jonathan Anderson':
	print(letter)
```
]

* get "next" letter, use it
* ... until no more letters

#### Later: lists, arrays and `iter()`

---

# Today

### Iterating over integer ranges

### Lists

### List operations

---

# Motivation

### Example looping problem:

1. What is the sum of the squares of the first 10 whole numbers
(i.e., $1^2 + 2^2 + 3^2 + \dots$)?
--

2. What is the sum of the squares of the first $n$ whole numbers?

---

# Iterating over integers

.column[
```python
total = 0
i = 1

while i <= n:
    total += i*i
    i += 1
```
]

.column[
$$ \sum_{i=1}^{n} {i^2} $$
]

.column[
If only we could write something a bit more like this mathematical expression!
]

* Too bad we can't write "$\textrm{for } i \in [2,n]$"...
--

* or can't we?

---

# Home on the `range()`

### `range()` gives us a _range_ of integers

### A range can be _iterated_ over (just like a string)

.floatleft[
### 1 to 100:
```python
for n in range(100):
       print(n)
```
]

.floatleft[
### 10 to 20:
```python
for n in range(10, 20):
       print(n)
```
]

.floatleft[
### 1000 to 2000, by 500s:
```python
for n in range (1000, 2000, 500):
       print(n)
```
]

---

# Help on the `range()`

#### If we type `help(range)`:

```python
class range(object)
 |  range(stop) -> range object
 |  range(start, stop[, step]) -> range object
 |
 |  Return an object that produces a sequence of integers from start (inclusive)
 |  to stop (exclusive) by step.  range(i, j) produces i, i+1, i+2, ..., j-1.
 |  start defaults to 0, and stop is omitted!  range(4) produces 0, 1, 2, 3.
 |  These are exactly the valid indices for a list of 4 elements.
 |  When step is given, it specifies the increment (or decrement).
```

* some details here that we don't understand (yet)
--

* big picture: two versions of `range()`; more detail later

---

# Mo' Python, Mo' Problems

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

e.g., Problem 1:

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

##### Your solution can be simplified with `range()`

---

# Other collections

### What about non-sequential collections of values?

* student IDs for everyone in the class
--

* grades for everyone in the class
--

* each day's temperature
--

* whether or not class has been cancelled each day

---

# Lists

```python
student_ids = [202012345, 202098765]
grades = [99.7, 94.8, 96.2]
temperatures = [-2, -4, -20, 6]
cancelled = [True, True, False, True]
```

### Syntax:

* `[`
--
 &mdash; opening **bracket**
--

* zero or more expressions
--
 &mdash; **separated by commas**
--

* `]`
--
 &mdash; closing **bracket**

---

# List operations

```python
["Engineering", 1020]
```

### Creation

### Mutation

* for now: `my_list += ["and", 1030]`
--

* later: methods (`my_list.append(...)`)

### Iteration

---

# Iterating over lists

### Just like iterating over ranges!

```python
grades = [99.7, 94.8, 96.2]
for grade in grades:
    if grade >= 55:
        print(grade, ' will count towards promotion average')
```

```python
for temp in [-2, -4, -20, 6]:
    if temp < 10:
        print('brrrr')
```

---

# Iterators

### Common syntax not a coincidence:

.floatleft[
```python
for c in "hello world":
    print(c > 'a')
```
]

.floatleft[
```python
for n in range(10):
    print(n*n)
```
]

.floatleft[
```python
for temp in [-2, -20, 6]:
    print(temp)
```
]

.floatright[
```python
i1 = iter("hello world")
i2 = iter(range(10))
i3 = iter([-2, -20, 6])
```
]

* All use _iterators_
* Can use `for` loop with _any_ iterator
* We'll see more iterators later

---

# Indexing

### Indices in math:

$$
\textrm{Given } X = \left\\{
    0, \frac{\pi}{2}, \pi, \frac{3 \pi}{2}
\right\\}
$$

What is $X_2$?
--
 $X_0 = 0$
--
, $X_1 = \frac{\pi}{2}$
--
, $X_2 = \pi$, ...

### Lists and strings (and more) can be _indexed_

```python
X = [0, pi/2, pi, 3*pi/2]
s = X[2] / pi
```

---

# Python indices

### Start counting from 0

### Will cause an error if _out of range_

### Can be _negative!_

* start counting from the back
* range checks still apply!

---

# Names

### Example:

```python
names = ['Jonathan', 'Anderson']
```

```python
first_name = names[0]
```

```python
initial = first_name[0]
another_way_to_write_initial = names[0][0]
```

### What's the last letter of your last name?

---

# Summary

### `range()`

### Lists

## Go play with [Project Euler](https://projecteuler.net)!

---
class: big, middle

(here endeth the lesson)