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Control Abstraction

Chapter 9

Abstraction

Allows us to hide complexity behind simpler interfaces

Data Abstraction

Create complex data structures with simple interfaces to pass around (e.g. records, structs, dictionaries, etc)

Control Abstraction

Create organized sections of executable code that execute in well-understood ways and can be reused

Subroutine

Performs operations for the caller while the caller waits
Arguments, or actual parameters, are passed to subroutines and mapped to formal parameters from the subroutine definition
Subroutines that can return values can be called functions

Call Stack

Memory space for functions to use to store local variables, return addresses, and other data
Function calls push a new frame to the stack
Returns pop their frame when they are finished with it

Call stack review resources

Parameter Passing

Pass by Value

Actual parameter values are made available to the subroutine
Modification of values will not impact the caller
This may involved making a copy in memory

Pass by Reference

Actual parameters are references to data from the caller
Modification of values will impact caller
Should not involve copying data

C

Pass by value

#include <stdio.h>

void increment(int i) {
  // This will not change the callers value
  i = i + 1;
}

int main(void) {
  int i = 0;
  increment(i);
  printf("%d\n", i);
}

If we want to be able to modify values, we need to pass values by reference
We can’t do this is C, but we can use pointers passed by value to emulate it

#include <stdio.h>

void increment(int* i) {
  // This will change the callers value
  *i = *i + 1;
}

int main(void) {
  int i = 0;
  increment(&i);
  printf("%d\n", i);
}

If we allow C++, we can use a proper pass by reference mode.

#include <stdio.h>

void increment(int &i) {
  // This will change the callers value
  i = i + 1;
}

int main(void) {
  int i = 0;
  increment(i);
  printf("%d\n", i);
}

Python

Pass by object reference
Numbers and strings are passed by value
Composite types such as objects are passed as a reference

def increment(i):
  // Only modifies local value
  i = i + 1

i = 0
increment(i)
print(i)

Callers will be able to see modifications to mutable objects

def append(mylist, value):
  mylist.append(value)

mylist = []
append(mylist, 1)
print(mylist)

Callers will still hold a reference to the exact same object

def empty(mylist):
  # The caller won't be impacted by this reassignment
  mylist = []

mylist = [1,2]
empty(mylist)
print(mylist)

Immutable objects (such as tuples or strings) can’t be modified for the caller

Optional Parameters

Some languages allow us to mark parameters as optional
We may be able to provide a default value
In some languages all parameters are optional

Python allows us to annotate optional parameters by providing default values

def remove_character(mystring, character=" "):
  return mystring.replace(character, '')

print(remove_character("Hello, World!"))

All parameters are optional in Javascript

function print(a) {
  console.log(a)
}

print('Hello, World!')
print() // This is not an error in JS

Named Parameters

We’ve been exploring positional parameters
Some languages allow them to be used by name
This can be very helpful, especially combined with optional parameters

def make_vehicle(type='car', color='red', max_speed=55):
  return (type, color, max_speed)

print(make_car(color='blue'))

Variable Numbers of Arguments

It can sometimes be helpful to accept a different number of arguments
One example of this is printf in C

int printf ( const char * format, ... );

Function Returns

End the function
Return some value
In languages without separate statements, the return is simply the value of the function body
Some languages allow for implicit returns

fn get_num() -> u8 {
  42
}

fn main() {
  println!("{}", get_num());
}

Events

Event happens outside of program at unpredictable times
Running programs wants to respond

Blocking

We simply wait for an event to complete

import urllib.request
import json

junk_foods = [
  'Pizza',
  'Popcorn',
  'Hamburger',
  'Pepsi',
  'Potato_chip',
  'Cake',
]

url = 'https://en.wikipedia.org/w/api.php?action=parse&format=json&page='

for food in junk_foods:
  contents = urllib.request.urlopen(f'{url}{food}').read()
  print(f"{food}: {json.loads(contents)['parse']['properties'][0]['*']}")

const https = require('https')

junk_foods = ['Pizza', 'Popcorn', 'Hamburger', 'Pepsi', 'Potato_chip', 'Cake']

url = 'https://en.wikipedia.org/w/api.php?action=parse&format=json&page='

junk_foods.forEach(food => {
  https.get(
    {
      host: 'en.wikipedia.org',
      path: '/w/api.php?action=parse&format=json&page=' + food,
    },
    function (res) {
      let body = ''
      res.on('data', function (d) {
        body += d
      })
      res.on('end', function () {
        console.log(`${food}: ${JSON.parse(body).parse.properties[0]['*']}`)
      })
    },
  )
})