## Iteration

`while`

Statements

Execution rule:

- Evaluate the header’s expression
- If it’s a true value, execute the (whole) suite, then return to step 1

## Designing Functions

Three characteristics of functions that are really important:

- A function’s
*domain*is the set of all inputs it might possibly take as arguments. - A function’s
*range*is the set of output values it might possible return. - A pure function’s
*behavior*is the relationship it creates between input and output.

### A Guide to Designing Functions

- Give each function eactly one job, but make it apply to many related situations
- Don’t repeat yourself. Implement a process just once.

```
>>> round(1, 23)
1
>>> round(1.23, 1) # round to 1 decimal place
1.2
>>> round(1.23, 0) # roudn to integer
1
```

## Higher Order Functions

### Generalizing over Computational Processes

A higher order function is a function that takes in another function as an argument.

Example, `summation`

and `make_adder`

## Locally Defined Functions

Functions defined within other function bodies are bound to names in a local frame.

An *operator* is any expression that evaluates to a function.
So, in

```
make_adder(1)(2)
```

`make_adder(1)`

is the operator.

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