Basics

Question 1

what are 4 different types of data in python?

Answer 1

str = string

int = integer

float = number with decimal values

bool = boolean, True or False

Question 2

Can a list contain different types of data?

Answer 2

Yes

Question 3

With what starting number are lists indexed?

Answer 3

zero going forward

-1 starting from the end of the list

Question 4

List slicing syntax, e.g. 1:3. Is the 3 included?

Answer 4

start : end

No, the end is not included

Question 5

What happens if you don’t specify the start or end of a list when list slicing?

e.g. mylist[:4] or mylist[2:]

Answer 5

If don’t specify start, it starts at index zero

If don’t specify end, includes the start and rest of the list

Question 6

x = [2, 3, 4]

y = x

y[0] = 1

How do you prevent the first element changing to 1 in the original list x?

Answer 6

Instead of y=x, use y=list(x) or y=x[:]. Then changes to y will not affect x.

Question 7

Can a numpy array contain elements with different types?

Answer 7

No. If you try, some of the elements’ types are changed to end up with a homogenous list (type coersion)

Question 8

Create a 2D NumPy Array

How are rows and columns indexed in a 2D NumPy Array?

How do you subset a 2D NumPy Array?

Answer 8

np_2d = np.array([[1, 2, 3, 4],

[5, 6, 7, 8]])

Both rows and columns are indexed starting at 0 from the top left corner.

np_2d[0,2] or np_2d[0][2]: the first number is the row and the second the column

Question 9

np_heights = array([191, 184, 185])

np_positions = array([‘GK’, ‘M’, ‘A’])

gk_heights = np_heights[np_positions == ‘GK’]

What will the gk_heights array contain?

Answer 9

It will contain 191.

np_positions == ‘GK’ creates a boolean array. Using it to index np_heights causes the program to return heights that correspond to “True” values in the boolean array

Question 10

How do you make an x, y line plot?

Answer 10

Use matplotlib

import matplotlib.pyplot as plt

year = [1950, 1970, 1990, 2010]

pop = [2.5, 3.7, 5.3, 7.0]

plt. plot(year, pop)
plt. show()

Note that need plt.show() to see the chart

Question 11

How do you create a scatter plot?

Answer 11

import matplotlib.pyplot as plt

year = [1950, 1970, 1990, 2010]

pop = [2.5, 3.7, 5.3, 7.0]

plt. scatter(year, pop)
plt. show()

Note that need plt.show() to see the chart

Question 12

How do you make a histogram?

Answer 12

import matplotlib.pyplot as plt

values = [1, 2, 3, 4]

plt. hist(values, bins=2)
plt. show()

Note that bins is the number of bars you want the data to be divided into. The program automatically calculates appropriate boundaries for your data.

Question 13

How do you “clean up” a plot to start afresh?

Answer 13

plot.clf()

Question 14

How do you add names to a plot’s axes when using matplotlib.pyplot?

How do you add a title?

Answer 14

plt. xlabel(‘label1’)
plt. ylabel(‘label2’)
plt. title(‘title’)

Question 15

How do you specify the numbers or ticks to display on an axis?

How do you change the name of the ticks on an axis?

Answer 15

plt. yticks([0, 2, 4, 6, 8, 10])
plt. yticks([0, 2, 4, 6, 8, 10], [‘0’, ‘2B’, ‘4B’, ‘6B’, ‘8B’, ‘10B’])

note that the names have to correspond to the ticks listed

Question 16

How do you change an axis to logarithmic scale?

Answer 16

plt.scatter(x, y)

plt.xscale(‘log’)

Question 17

how do you change the size of the dots on a scatter plot to reflect a third variable?

Answer 17

plt.scatter(x, y, s=z)

Question 18

How do you add gridlines?

Answer 18

plt.grid(True)

Question 19

How do you create a dictionary?

Two facts about keys

Answer 19

dictionaryname = {key1:value1, key2:value2, … }

When you type in dictionaryname[key2], you get value2

Keys:

• Keys in a dictionary must be unique. If state same key twice, dictionary will just retain the last value stated
• Keys have to be “immutable” objects, e.g. cannot be changed after they’re created. Strings, booleans, integers, and floats are immutable.

Question 20

how do you access the “keys” in a dictionary?

Answer 20

dictionaryname.keys()

Note that it takes no arguments

This will list all the keys in the dictionary

Question 21

eHow do you add a key:value pair to a dictionary?

How do you change a key:value pair in a dictionary?

How do you delete a key:value pair?

Answer 21

dictionaryname[keytobeadded] = valuetobeadded

dictionaryname[key] = newvalueforkey

del(dictionaryname[key])

Question 22

How do you get a value that is in a nested dictionary?

e.g.

countries = {‘spain’:{‘capital’:’madrid’, ‘population’:46.77}

‘france’:{‘capital’:’paris’, ‘population’: 66.04}}

extract population of france

Answer 22

dictionaryname[key1][key2]

countries[‘france’][‘population’]

Question 23

How do you create a Dataframe from a Dictionary?

How do you change the default row index numbers?

Answer 23

Make a dictionary where the keys will be the column labels (variables) of the Data frame, and the values are in list form:

dict ={‘country’:[‘Brazil’, ‘Russia’], ‘capital’:[‘Brasilia’, ‘Moscow’]}

Then,

import pandas as pd

dataframename = pd.DataFrame(dict)

For index change:

dataframename.index = [‘row1’, ‘row2’, ‘row3’}

Question 24

How do you create a DataFrame from a CSV file?

Answer 24

dataframename = pandas.read_csv(“filepath.csv”, index_col = 0)

Note that the index_col is not necessary but use if first column in file contains the row labels

Question 25

How do you output values of an entire column of a DataFrame? What type of values do you get? How do you keep the data in a DataFrame? How do you output values of two columns?

Answer 25

dataframename['columnname'] panda series - it's like a 1D labelled array use double brackets: dataframename[['columnname']] dataframename[['column1', 'column2']]

Question 26

How do you output the certain rows from a DataFrame?

Answer 26

dataframename[startrow:endrow] \*remember to use index NOT row label \*remember rows are indexed starting at zero

Question 27

How do you output a row from a DataFrame using loc (label-based)? How do you select multiple rows?

Answer 27

dataframename. loc[['rowname']] dataframename. loc[['row1', 'row2']] \*double brackets to keep type as DataFrame

Question 28

how do you output specific rows and specific columns from a DataFrame? How do you select all rows but only a couple of columns?

Answer 28

dataframename. loc[['row1', 'row2'], ['column1', 'column2']] dataframename. loc[:, ['column1', 'column2']]

Question 29

How do you output a row with iloc? How do you select specific rows and specific columns with iloc?

Answer 29

dataframename. iloc[[indexposition]] dataframename. iloc[[rowindex1, rowindex2], [columnindex1, columnindex2]]

Question 30

String operations my\_string = 'thisStringisAwesome' What will the following yield? my\_string\*2 my\_string + 'Innit' 'm' in my\_string

Answer 30

my\_string\*2 = 'thisStringisAwesomethisStringisAwesome' my\_string + 'Innit' = 'thisStringisAwesomeInnit' 'm' in my\_string = True

Question 31

How do you subset lists of lists? e.g. my\_list= [[4,5,6,7], [3,4,5,6]] Select 7

Answer 31

my\_list[indexlist1][indexlist2] To select 7: my\_list[1][3]

Question 32

List Operations my\_list = ['my', 'list', 'is', 'nice'] my\_list2 = [[4,5,6,7], [3,4,5,6]] What do the following yield? my\_list + my\_list my\_list \* 2 my\_list2 \> 4

Answer 32

my\_list + my\_list = ['my', 'list', 'is', 'nice', 'my', 'list', 'is', 'nice'] my\_list \* 2 = ['my', 'list', 'is', 'nice', 'my', 'list', 'is', 'nice'] my\_list2 \> 4 = True

Question 33

**List Methods** How do you... Get the index of an item in a list? Count an item? Reverse the list? Sort the list?

Answer 33

my\_list.index(item) my\_list.count(item) my\_list.reverse() my\_list.sort()

Question 34

**List Methods** How do you... Append an item at a time? Append an item? 3 ways to remove an item? Insert an item?

Answer 34

my\_list.append(item) my\_list.extend(item) my\_list.remove(item) my\_list.pop(indexofitem) del(my\_list[0:1]) my\_list.insert(index\*, item) \*index of the item *before* which to insert

Question 35

**String Methods** How do you... Change string to uppercase? Change string to lowercase? Count string elements?

Answer 35

my\_string.upper() my\_string.lower() my\_string.count('element')

Question 36

How do you create a numpy array? Use my\_list = [1, 2, 3, 4]

Answer 36

my\_array=numpy.array(my\_list)

Question 37

**Numpy array operations** my\_array = numpy.array([1, 2, 3, 4]) What is the output of... my\_array \> 3 my\_array \* 2 my\_array + numpy.array([5, 6, 7, 8])

Answer 37

my\_array \> 3 : array([False, False, False, True] my\_array \* 2 : array([2, 4, 6, 8]) my\_array + numpy.array([5, 6, 7, 8]) : array([6, 8, 10, 12])

Question 38

Numpy Array Functions How do you... Get the dimensions of an array? Get the mean of the array? Get the median of the array? Get the correlation coefficient of the array? Get the standard deviation of the array?

Answer 38

my\_array.shape numpy. mean(my\_array) numpy. median(my\_array) my\_array.corrcoef() numpy.std(my\_array)

Question 39

Numpy Array Functions How do you... Append items to an array? Insert items in an array? Delete subarray in an array?

Answer 39

numpy. append(other\_array) numpy. insert(my\_array, index\*, valuestoinsert) \*index *before* which value is inserted numpy.delete(my\_array, indexofarraytodelete)

Question 40

What happens when you do 'carl' \< 'chris'

Answer 40

'carl' comes first in alphabet, you get True

Question 41

Less than or equal to Greater than or equal to equal to not equal to

Answer 41

\<= \>= \*Note =\< is not correct syntax == !=

Question 42

What are the numpy array equivalents of the boolean operators and, or, and not?

Answer 42

numpy. logical\_and() numpy. logical\_or() numpy. logical\_not() eg. numpy.logical\_and(arrayname \> 21, arrayname \< 22) \*note that these also work on Pandas DataFrame

Question 43

if statement

Answer 43

if condition: expression1 espression2 \*The indentation of the expression lets python know that the expression is part of the if statement. To exit if statement, simply write code without indentation.

Question 44

if else statement

Answer 44

if condition: expression else: expression

Question 45

elif statement What happens with the elif statement if the condition of the if statement is true?

Answer 45

if condition: expression elif condition: expression else: expression If condition for if is true, the output is the if's expression. elif is never reached

Question 46

while loop statement

Answer 46

while condition: expression

Question 47

fofor loop statement

Answer 47

for var in seq: expression \*var is a variable you create \*seq is a list of values

Question 48

enumerate function

Answer 48

enumerate(seq) Returns an enumerate object that produces a sequence of tuples, and each of the tuples is an index-value pair e.g. fam = [1, 2, 3, 4] for index, height in enumerate(fam): print('index " + str(index) + ":" + str(height)) Output: index 0: 1 index 1: 2 index 2: 3 index 3: 4

Question 49

Loop over string What is the output of the following? for c in "family": print (c.capitalize())

Answer 49

F A M I L Y

Question 50

Using the following dictionary, print out each key and value using the for loop. world = {'afghanistan':30.55, 'albania':2.77, 'algeria':39.21}

Answer 50

for key, value in world.items(): print (key + str(value)) \*Note that need the .items() to use for loop with the dictionary

Question 51

How do you print out every element of a 2D Numpy Array (using for loop)? np\_height = np.array([1, 2, 3, 4]) np\_weight = np.array([1, 2, 3, 4]) meas = np.array([np\_height, np\_weight])

Answer 51

for val in **np.nditer**(meas): print(val) \*This will print out all the heights, then all the weights

Question 52

For loop with Pandas Dataframe How do you print out every single element of a DataFrame? Use dataset "countries"

Answer 52

for rowlabel, rowvalues in countries.**iterrows**(): print(rowlabel) print(rowvalues) This will print the rowlabel followed by all the column names with row values. e.g. BR country Brazil capital Brasilia area 8.516 population 200.4 etc

Question 53

For loops with Pandas DataFrame How do you print out row labels with only the capital cities for the dataset countries?

Answer 53

for rowlabel, rowvalues in countries.iterrows(): print(rowlabel + ": " + rowvalues["capital"]) This will print: BR: Brasilia RU: Moscow etc

Question 54

What function gives you the length of a string?

Answer 54

len('string')

Question 55

Using the apply function, how do you create a new column with the countries' name length in the DataFrame countries?

Answer 55

countries["name\_length"] = countries["country"].apply(len)

Question 56

Which numpy function generates random numbers? How do you manually choose a seed?

Answer 56

numpy.random.rand() This generates psedurandom numbers between 0 and 1 starting from a seed numpy.random.seed(*number*) \*seed can be any number. If don't specify, python will automatically pick a seed. You can set the seed so that results are reproducible between simulations (same seed will generate same random numbers)

Question 57

Simulate a coin toss

Answer 57

coin = numpy.random.randint(lowest, high) \*lowest is the lowest number and high is one above the highest number coin = numpy.random.randint(0, 2) if coin == 0: print('heads') else: print('tails')