Flow control and loops

class: center, middle, inverse, title-slide

.title[
# Flow control and loops
]
.subtitle[
## Controlling program flow and looping
]
.author[
### Lincoln Colling
]

---

# Flow Control

It's common to want to write a program (a script or a function) that will do
something different depending on whether some condition is met. That is, 
an **IF A THEN B** rule.

- In `Matlab`, we can do this with an `if` statement

```matlab
birthyear = 2005
if birthyear < 2002
 disp('You are old enough to buy beer')
end % notice that we put end when we've finished the rule
```

If the condition `birthyear < 1999` produces a logical `1`, then the path is
followed:

```matlab
>> birthyear = 2015;
>> birthyear < 2005

ans =

logical

1
```

---

## `if` then `else`

You can also use `else` to specify what to do if the condition _isn't_ met:

```matlab
if birthyear < 2005
 disp('You are old enough to buy beer')
else
 disp('You are not old enough to buy beer!')
end
```

Or `elseif` for more complex branching:

```matlab
if birthyear < 2005
 disp('You are old enough to buy beer')
elseif birthyear == 2005 
 disp('You might be old enough to buy beer')
else 
 disp('You are not old enough to buy beer!')
end
```

---

## `switch` and `case`

If you need to check through **many** conditions, then there's a more efficient
way than stringing together a bunch of `if`, and `elseif` statements.

Instead, you can use `switch` and `case`:

```matlab
n = 10 % set n to a number

switch n 
  case 1
      disp('one')
  case 2
      disp('two')
  otherwise 
      disp('I can only count to two')
end
```

---

## `switch` and `case`

`switch` and `case` also works with strings (`chars`)

```matlab
n = 'one'

switch n
  case 'one'
      disp('1')
  case 'two'
      disp('2')
  case {'three','four'}
      disp('more than 2 less than 5')
end
```

---

# Loops

Loops allow us to run a bit of code over and over.

- Loops are extremely useful in, e.g., PsychToolbox where we present multiple trials,
  or in data analysis where we might want to process multiple data files.

`Matlab` has **two** basic forms of loops:

- The `for` loop, which loops a certain number of times.

- The `while` loop, which loops continuously as long as some condition is met.

Whether you use a `while` loop or `for` loop will depend on the **problem you
want to solve** or how you **structure your solution**.

---

## The `for` loop

- The simplest **loop** is the `for` loop.

- If we pass a range of values (e.g., a vector or cell array) to a `for`
  statement, then one value of that range will be available on each iteration of
  the loop

An example with a vector of numbers:

```matlab
for i = 1 : 10
    disp(num2str(i))
end
```

An example with a cell array:

```matlab
for n = {'one','two','three'}
    disp(n)
end
```

---

## The `while` loop

- `for` loops loop over some series of values.

- `while` loops, on the other hand, loop continuously as long as some condition
  is met.

```matlab
x = 1
y = 0
while y < 100
 y = x^2
 disp(['x squared is ' num2str(y)])
 x = x + 1
end 
```

This means `while` loops are useful when we don't know how long we have to loop
for.

```matlab
% read lines from a file
% carry on looping while you're not at the end of the file
while ~feof(fid)
    fgetl(fid)
end
```

<!--
/* --- */

## Alternatives to loops1

If you've used `map`-style functional syntax in `R` (either through `purrr::map()` or `apply()`) or `python` (either with `map()` or a list-comprehension with a lambda)

- In `Matlab` you can use `arrayfun` and `cellfun` to **apply** a function to every element of a 1-D matrix or 1-D cell array

```matlab
>> list_of_numbers = [1 6 8 12 12];
>> logs = arrayfun(@(x) log(x), list_of_numbers)

logs =

0    1.7918    2.0794    2.4849    2.4849
```

.footnote[1Knowing this isn't vitally important, but if you like using functional style programming in `R` or `python` then you might want to do the same in `Matlab`]

-->