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# Engineering 1020: Introduction to Programming

.title[
  .lecture[Lecture 10:]
  .title[Slices]
  
  
]

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

---

# Recall: 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?

---

# Problem

> Find the position of a name within a list of names

### Think about:

1. Approach: how would _you_ do this?
--

2. Data representation: what do we need? How is it stored?
--

3. Algorithm: how can you _explain_ the approach?
--

4. Code: how can you express this in Python?

---

# Slicing

### Lists, strings (and more) can be _sliced_

$$
\textrm{Given } X = \left\\{
    0, \frac{\pi}{2}, \pi, \frac{3 \pi}{2}
\right\\},
s = \sum_{i=1}^{3} X_i
$$

$$
s = X_1 + X_2 + X_3
  = \frac{\pi}{2} + \pi + \frac{3 \pi}{2}
  = \frac{6 \pi}{2}
  = 3 \pi
$$

```python
X = [0, pi/2, pi, 3*pi/2]
s = sum(X[1:3])
```

---

# Slices

### A _slice_ of something (list, range, string, etc.)

### From a beginning (_inclusive_) to and end (_exclusive_)

### Much like a `range`!

```python
start = 1
end = 3

# The following are almost the same thing:
print(X[start:end])

for i in range(start, end):
    print(X[i])
```

---

# Advanced slicing

### Also like a `range`, can specify an _increment:_

```python
X[0:4:2]
X[::2]
```

### Can even specify a negative increment:

```python
X[4:0:-2]
X[::-1]
```

---

# Sequence functions

### `sorted()`

### `reversed()`

### `sum()`

### ... all _built in_, no `import` necessary

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# Summary

### Slicing and indexing

### Reversing and sorting

### Sorting and summing

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(here endeth the lesson)