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AIBridge

Lecture 2

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Lecture Outline

I/O

2

List Manipulation

What is a function?

Built-ins

Importing

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I/O

Input from console: input('prompt')

Open file: file_object=open(file, mode)

'r' is read and 'w' is write for the mode

read()

"""Here is a file.

This file has multiple lines.

This is the last line."""

, readline()

, readlines()

"Here is a file."

"This file has multiple lines."

"This is the last line."

["Here is a file.",

"This file has multiple lines.",

"This is the last line."]

Always close file: file_object.close()

Standard Input

First, we can take input from what’s called the “console.” **click** On Google Colab, the console is right below a cell where code runs, with this type of input being taken there. However, the location of the console is different for each environment, so just be aware of that. Let’s go over to Google Colab to test this out! (stretch boundary so that Colab is larger) (demonstrate over in Colab)

Now, we don’t always want to be inputting like this into our code. Imagine doing that for thousands or even millions of numbers. Even with copying and pasting, that would be quite tedious. So how should we do this? Well, we can use python to read from a file. (click) Now we would want to use files because they’re commonly how data is stored. For example, you might want to get data from an xlsx spreadsheet for your code. In Python, we read from a file using the (click) “open” function. You also need to declare which mode you want to access the file in, which in this case, is (click) read mode. What the “file” refers to here is the path to the file, and we’ll get into that in a moment. There are several ways to read out of the file. First, we can use the (click) read function. This will output the contents of the file into a single multiline string, as you can see (click) here. You can also use (click) readline(), which will read only the next line into a (click) single string, then the next line (click), then the next (click), or (click) readlines(), which will read each line into its own separate string and then store the lines into a list (click).

Just to prove the utility of files to you, we’re going to go ahead and do a name activity! I want everyone to come up and write the first name you think of in this input (doesn’t necessarily have to be your name). Don’t worry about what the code does, just write one name and hit enter. And we’re going to time this, so make sure everyone gets in line quickly!

Now we’re going to compare that to how quickly it takes us to read in from a file.

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I/O

Output to Console: print(object1, object2, ...)

Open file: file_object=open(file, mode)

write()

Always close file

Note: This removes any existing file with that name

print('a', 'b', 'c', 'd')

print('e', 'f', 'g')

Standard Output

Now with inputting comes outputting, and there are two main ways to do this as well. The first is the (click) print function, which like before corresponds to the console. You can add as many objects as you like to the print function, and all of them will be printed out one after another, separated by spaces.

For example, we might print something like this (click and describe, click again), or something like this (click and describe, click again).

As before, we can (click) write to files in a very similar way as we read from them, only instead of using the read() functions we will use the (click) write() function. There is only a single function to write luckily, and as always make sure you’re (click) closing the file! It’s also important to note that if you’re writing to a file that already exists, (click) any content previously in that file will be wiped out, so be careful with your file names.

Let’s go ahead and demo this in Colab!

strang = "Hello world"
file = open("file.txt", "w")
file.write(strang)
file.close()

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Lecture Outline

I/O

5

List Manipulation

What is a function?

Built-ins

Importing

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List Manipulation

Indexing

6

List Operations

Listcomp

String/list Interop

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List Manipulation

Single indexing

List slicing

[a, b, c, d, e]

0 1 2 3 4

-5 -4 -3 -2 -1

list_name[ ]

i

0

3

1

list_name[ : ]

list_name[-2]

i j

1 4

Indexing

The first and most important concept we’ll talk about is with list indexing. Indexing is essentially akin to telling the computer which drink you want from a vending machine, only there’s often only one dimension and the thing you’re retrieving is the string, character, integer, or other item stored at that index, which is decidedly less tasty than, say, a Mountain Dew Code Red.

Anyhow, there are two main ways we use indexing as you can see here. You can call a (click) single index like (click) this, which notably starts at 0 if you’re counting forward and -1 if you’re counting backward (as in 0 items from the first, or the first item coming backwards).

For example, (click) this is the 0^th index, (click) this is the 3^rd index, and (click) this is the 1^st. (click) You can index from the back of the list, like (click) this. (click)

You can also (click) slice the list (click) like this, which basically means you’re taking a continuous chunk out of the list. When you slice, you should remember that the first index is inclusive, while the second index isn’t (e.g. index 4 won’t be included in this slice here).

For example, (click and describe), (click and describe)

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Self-Test

What does the following code output?

arr = [4, 5, 6, 101, 102, 103, 104, 105]

new_arr = arr[2:6]

print(new_arr)

A. [5, 6, 7, 101, 102, 103, 104, 105]

B. [6, 7, 101, 102, 103, 104, 105]

C. [6, 101, 102, 103, 104]

D. [6, 101, 102, 103]

List Manipulation

Indexing

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Self-Test

What does the following code output?

arr = [4, 5, 6, 101, 102, 103, 104, 105]

new_arr = arr[2:6]

print(new_arr)

A. [5, 6, 7, 101, 102, 103, 104, 105]

B. [6, 7, 101, 102, 103, 104, 105]

C. [6, 101, 102, 103, 104]

D. [6, 101, 102, 103]

List Manipulation

Indexing

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List Manipulation

Indexing

10

List Operations

Listcomp

String/list Interop

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https://docs.python.org/3/tutorial/datastructures.html

List Manipulation

List Operations

1

2

Before we get into the actual functions themselves, we will point out that you can find a comprehensive list of all of the functions here: (click). If you’d like, you can also (click) search up “python list operations” and then (click) click on this link here.

As you can see when you go into the link, there exists a rather large number of functions that you can use with lists. This is perhaps one of the most relevant examples of the importance of documentation, as memorizing this extremely large list would be very tedious and difficult. Instead, it’s much easier to simply look through the documentation to find the function that you need and to make sure that you’re using it right. With that being said, let’s go ahead and take a look at a couple of the most relevant list operations.

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List Manipulation

List Operations

my_list =

[3, 14, 0, -2, 5

]

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List Manipulation

List Operations

append()

[3, 14, 0, -2, 5

]

my_list.append(19)

19

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[3, 14, 0, -2, 5

[3, 14, 0, -2, 5, 19

my_list.append(19)

List Manipulation

List Operations

append()

]

19

,

my_list.append( )

8

my_list.append(8)

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[3, 14, 0, -2, 5, 19

List Manipulation

List Operations

append()

my_list.append(19)

]

8

,

my_list.append(8)

[3, 14, 0, -2, 5, 19, 8]

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[3, 14, 0, -2, 5, 19, 8]

[3, 14, 0,

List Manipulation

List Operations

remove()

my_list.remove(-2)

5, 19, 8]

-2,

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[3, 14, 0, 5,

8]

19,

List Manipulation

List Operations

remove()

my_list.remove(-2)

5, 19, 8]

my_list.remove(19)

[3, 14, 0,

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List Manipulation

List Operations

remove()

my_list.remove(-2)

my_list.remove(19)

[3, 14, 0, 5,

8]

[3, 14, 0, 5, 8]

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1,

1

,

14, 5, 8]

List Manipulation

List Operations

pop()

my_list.pop(3)

[3, 14, 0,

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14, 5, 8]

14

5, 8]

my_list.pop(3)

1

List Manipulation

List Operations

pop()

[3, 14, 0,

,

my_list.pop(3)

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14

5, 8]

my_list.pop(3)

1

List Manipulation

List Operations

pop()

[3, 14, 0,

my_list.pop(3)

[3, 14, 0, 5, 8]

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[3, 14, 0, 5, 8

]

[3, 14, 0, 5, 8]

my_list_2 = [

List Manipulation

List Operations

+

my_list = my_list + my_list_2

10, 9, 8, 7]

my_list_2 = [10, 9, 8, 7]

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[3, 14, 0, 5, 8

List Manipulation

List Operations

+

my_list = my_list + my_list_2

10, 9, 8, 7]

,

my_list_2 = [10, 9, 8, 7]

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[0, 3, 5, 7, 8, 8, 9, 10, 14]

List Manipulation

List Operations

len()

len(my_list)

9

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[0, 3, 5, 7, 8, 8, 9, 10, 14]

List Manipulation

List Operations

max()

max(my_list)

14

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[0, 3, 5, 7, 8, 8, 9, 10, 14]

List Manipulation

List Operations

min()

min(my_list)

0

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List Manipulation

Indexing

27

List Operations

Listcomp

String/list Interop

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Shorthand for “for” loops

new_list = [expression for object in iteration]

List Manipulation

Listcomp

[obj1, obj2, obj3, obj4, obj5, obj6, obj7 ...]

[new1, new2, new3, new4, new5, new6, new7 ...]

expression

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new_list = [(i+1)/2 for i in range(7)]

List Manipulation

Listcomp

[0, 1, 2, 3, 4, 5, 6]

[0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5]

(i+1)/2

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List Manipulation

Indexing

30

List Operations

Listcomp

String/list Interop

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separator

List Manipulation

String/list Interop

join()

separator.join(my_list)

List of strings

my_list = [str1, str2, str3]

str1

str3

str2

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List Manipulation

String/list Interop

join()

separator.join(my_list)

List of strings

my_list = [str1, str2, str3]

str1

str3

str2

separator

Final String

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my_list = ["Hello,", "my", "name", "is", "Bob!"]

List Manipulation

String/list Interop

join()

' '.join(my_list)

"Hello,"

"my"

"name"

"is"

"Bob!"

' '

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"Bob!"

"is"

"Hello, my name is Bob!"

' '

"name"

' '

"my"

' '

"Hello,"

my_list = ["Hello,", "my", "name", "is", "Bob!"]

List Manipulation

String/list Interop

join()

' '.join(my_list)

' '

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Lecture Outline

I/O

35

List Manipulation

What is a function?

Built-ins

Importing

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What is a function?�

Input

Output

Function

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What is a function?�

Args and Kwargs

Normal arguments are based on position (nth input mapped to nth argument)
Kwargs (keyword arguments) are assigned

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What is a function?�

Arguments

They are positional
Can have defaults

def my_function(arg_1=-1, arg_2=-1, arg_3=-1, arg_4=-1):

print(arg_1, arg_2, arg_3, arg_4)

my_function(1,2,3,4)

my_function(3,4)

�

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What is a function?�

Keyword Arguments

You can specify a specific argument instead of relying on positions (helpful when there are a lot of arguments and you only need a few)

def my_function(a,b,c,d):

print(a,b,c,d)

my_function(b=2,c=3,d=4,a=1)

�

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What is a function?��

"Hello world!"

print()

Print

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What is a function?�

Args and Kwargs

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Lecture Outline

I/O

42

List Manipulation

What is a function?

Built-ins

Importing

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Built-ins

https://docs.python.org/3/library/functions.html

1

2

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Lecture Outline

I/O

44

List Manipulation

What is a function?

Built-ins

Importing

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45

Importing

Module

Python

Burp

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46

Importing

import sklearn

from sklearn import linear_model

import pandas as pd

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Importing

import sklearn

from sklearn import linear_model

import pandas as pd

To find functions we might want to use, we need to look at the documentation, for example:

https://pandas.pydata.org/docs/

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48

That was a lot!

Let’s get to the lab!