Loops

Loops#

`for` loops#

Suppose we have a list of numbers, and we want to square each of them.

numbers_to_square = [1, 3, 5, 6, 7, 9]

We can use a for loop to iterate over each item in the list.
Just like if statements, for loops require a colon at the end of the line and an indented block underneath.

for n in numbers_to_square:
    squared_value = n * n
    print(squared_value)

We use the in operator to iterate over a list.

Those two lines of code — squared_value = n * n and print(squared_value) — will run for every item in the list and then the loop will end.

n is a new variable that we define. You can think of it as the looping variable. You can name it whatever you like.

Now let’s loop over our list of competitors. This will just print out a string for each competitor in the list:

all_competitors = ['AAPL','MSFT','TSLA','AMZN']

for ticker in all_competitors:
    print('The ticker is ---', ticker)

The ticker is --- AAPL
The ticker is --- MSFT
The ticker is --- TSLA
The ticker is --- AMZN

Suppose that, instead of looping over a list, we know how many times we want to run a loop (e.g., 5 times).
If that is the case, we can use therange() function instead.
In this example, our loop will run 5 times.
But remember, indexes in Python start at zero, so the first loop iteration will be 0 and the last loop iteration will be 4.

for i in range(5):
    print(i)

Again, i is just a variable name that we get to choose.
The following is equivalent:

for my_cool_but_long_variable_name in range(5):
    print(my_cool_but_long_variable_name)

We can also incorporate the value of i as it iterates through the loop:

for i in range(5):
    print(i * 5)

Or, we could use i to index into a list:

for i in range(3):
    print('Competitor', i, 'is:', all_competitors[i])

Competitor 0 is: AAPL
Competitor 1 is: MSFT
Competitor 2 is: TSLA

Let’s use if statements and for loops together.
But first let’s define some new (and longer) lists.

ticker_list is the list of stocks we currently have in our portfolio.
sp500 contains all of the tickers in the S&P 500.

Naturally, these lists are pretty long.

ticker_list = ['AOS', 'APA', 'AIV', 'AAPL', 'AMAT', 'APTV', 'ADM', 'ARNC', 'ANET', 'SAM', 'AJG', 'AIZ', 'ATO', 'T', 'ADSK', 'ADP', 'AZO', 'AVB', 'AVY', 'BHGE', 'BLL', 'BAC', 'BK', 'BAX', 'BBT', 'BDX', 'BRK', 'BBY', 'BIIB', 'BLK', 'HRB', 'BA', 'BKNG', 'BWA', 'BXP', 'BSX', 'BMY', 'AVGO', 'BR', 'BF', 'CHRW', 'COG', 'CDNS', 'CPB', 'COF', 'CPRI', 'CAH', 'KMX', 'CCL', 'CAT', 'CBOE', 'CBRE', 'CBS', 'CE', 'CELG', 'CNC', 'CNP', 'CTL', 'CERN', 'CF', 'SCHW', 'UBER', 'CHTR', 'CVX', 'CMG', 'CB', 'CHD', 'CI', 'XEC', 'CINF', 'CTAS', 'CSCO', 'C', 'CFG', 'CTXS', 'CLX', 'CME', 'CMS', 'KO', 'CTSH', 'JNPR', 'KSU', 'K', 'KEY', 'KEYS', 'KMB', 'KIM', 'KMI', 'KLAC', 'KSS', 'KHC', 'KR', 'LB', 'LHX', 'LH', 'LRCX', 'LW', 'LEG', 'LDOS', 'LEN', 'LLY', 'LNC', 'LIN', 'LKQ', 'LMT', 'L', 'LOW', 'LYB', 'MTB', 'MAC', 'M', 'MMM', 'MRO', 'MPC', 'MKTX', 'MAR', 'MMC', 'MLM', 'MAS', 'MA', 'MKC', 'MXIM', 'MCD', 'MCK', 'MDT', 'MRK', 'MET', 'MTD', 'MGM', 'MCHP', 'MU', 'MSFT', 'MAA', 'MHK', 'TAP', 'MDLZ', 'MNST', 'MCO', 'MS', 'MOS', 'MSI', 'MSCI', 'MYL', 'NDAQ', 'NOV', 'NKTR', 'NTAP', 'NFLX', 'NWL', 'NEM', 'SNAP', 'NWSA', 'NWS', 'NEE', 'NLSN', 'NKE', 'NI', 'NBL', 'JWN', 'NSC', 'NTRS', 'NOC', 'NCLH', 'NRG', 'NUE', 'NVDA', 'ORLY', 'OXY', 'OMC', 'OKE', 'ORCL', 'PCAR', 'PKG', 'PH', 'PAYX', 'PYPL', 'PNR', 'PBCT', 'PEP', 'PKI', 'PRGO', 'PFE', 'PM', 'PSX', 'PNW', 'PXD', 'PNC', 'PPG', 'PPL', 'PFG', 'PG', 'PGR', 'PLD', 'PRU', 'PEG', 'PSA', 'PHM', 'PVH', 'QRVO', 'PWR', 'QCOM', 'DGX', 'RL', 'RJF', 'RTN', 'O', 'REG', 'REGN', 'RF', 'RSG', 'RMD', 'RHI', 'ROK', 'ROL', 'ROP', 'ROST']

sp500 = ['ABT', 'ABBV', 'ABMD', 'ACN', 'ATVI', 'ADBE', 'AMD', 'AAP', 'AES', 'AMG', 'AFL', 'A', 'APD', 'AKAM', 'ALK', 'ALB', 'ARE', 'ALXN', 'ALGN', 'ALLE', 'AGN', 'ADS', 'LNT', 'ALL', 'GOOGL', 'GOOG', 'MO', 'AMZN', 'AMCR', 'AEE', 'AAL', 'AEP', 'AXP', 'AIG', 'AMT', 'AWK', 'AMP', 'ABC', 'AME', 'AMGN', 'APH', 'ADI', 'ANSS', 'ANTM', 'AON', 'AOS', 'APA', 'AIV', 'AAPL', 'AMAT', 'APTV', 'ADM', 'ARNC', 'ANET', 'AJG', 'AIZ', 'ATO', 'T', 'ADSK', 'ADP', 'AZO', 'AVB', 'AVY', 'BHGE', 'BLL', 'BAC', 'BK', 'BAX', 'BBT', 'BDX', 'BRK', 'BBY', 'BIIB', 'BLK', 'HRB', 'BA', 'BKNG', 'BWA', 'BXP', 'BSX', 'BMY', 'AVGO', 'BR', 'BF', 'CHRW', 'COG', 'CDNS', 'CPB', 'COF', 'CPRI', 'CAH', 'KMX', 'CCL', 'CAT', 'CBOE', 'CBRE', 'CBS', 'CE', 'CELG', 'CNC', 'CNP', 'CTL', 'CERN', 'CF', 'SCHW', 'CHTR', 'CVX', 'CMG', 'CB', 'CHD', 'CI', 'XEC', 'CINF', 'CTAS', 'CSCO', 'C', 'CFG', 'CTXS', 'CLX', 'CME', 'CMS', 'KO', 'CTSH', 'CL', 'CMCSA', 'CMA', 'CAG', 'CXO', 'COP', 'ED', 'STZ', 'COO', 'CPRT', 'GLW', 'CTVA', 'COST', 'COTY', 'CCI', 'CSX', 'CMI', 'CVS', 'DHI', 'DHR', 'DRI', 'DVA', 'DE', 'DAL', 'XRAY', 'DVN', 'FANG', 'DLR', 'DFS', 'DISCA', 'DISCK', 'DISH', 'DG', 'DLTR', 'D', 'DOV', 'DOW', 'DTE', 'DUK', 'DRE', 'DD', 'DXC', 'ETFC', 'EMN', 'ETN', 'EBAY', 'ECL', 'EIX', 'EW', 'EA', 'EMR', 'ETR', 'EOG', 'EFX', 'EQIX', 'EQR', 'ESS', 'EL', 'EVRG', 'ES', 'RE', 'EXC', 'EXPE', 'EXPD', 'EXR', 'XOM', 'FFIV', 'FB', 'FAST', 'FRT', 'FDX', 'FIS', 'FITB', 'FE', 'FRC', 'FISV', 'FLT', 'FLIR', 'FLS', 'FMC', 'F', 'FTNT', 'FTV', 'FBHS', 'FOXA', 'FOX', 'BEN', 'FCX', 'GPS', 'GRMN', 'IT', 'GD', 'GE', 'GIS', 'GM', 'GPC', 'GILD', 'GL', 'GPN', 'GS', 'GWW', 'HAL', 'HBI', 'HOG', 'HIG', 'HAS', 'HCA', 'HCP', 'HP', 'HSIC', 'HSY', 'HES', 'HPE', 'HLT', 'HFC', 'HOLX', 'HD', 'HON', 'HRL', 'HST', 'HPQ', 'HUM', 'HBAN', 'HII', 'IEX', 'IDXX', 'INFO', 'ITW', 'ILMN', 'IR', 'INTC', 'ICE', 'IBM', 'INCY', 'IP', 'IPG', 'IFF', 'INTU', 'ISRG', 'IVZ', 'IPGP', 'IQV', 'IRM', 'JKHY', 'JEC', 'JBHT', 'JEF', 'SJM', 'JNJ', 'JCI', 'JPM', 'JNPR', 'KSU', 'K', 'KEY', 'KEYS', 'KMB', 'KIM', 'KMI', 'KLAC', 'KSS', 'KHC', 'KR', 'LB', 'LHX', 'LH', 'LRCX', 'LW', 'LEG', 'LDOS', 'LEN', 'LLY', 'LNC', 'LIN', 'LKQ', 'LMT', 'L', 'LOW', 'LYB', 'MTB', 'MAC', 'M', 'MMM', 'MRO', 'MPC', 'MKTX', 'MAR', 'MMC', 'MLM', 'MAS', 'MA', 'MKC', 'MXIM', 'MCD', 'MCK', 'MDT', 'MRK', 'MET', 'MTD', 'MGM', 'MCHP', 'MU', 'MSFT', 'MAA', 'MHK', 'TAP', 'MDLZ', 'MNST', 'MCO', 'MS', 'MOS', 'MSI', 'MSCI', 'MYL', 'NDAQ', 'NOV', 'NKTR', 'NTAP', 'NFLX', 'NWL', 'NEM', 'NWSA', 'NWS', 'NEE', 'NLSN', 'NKE', 'NI', 'NBL', 'JWN', 'NSC', 'NTRS', 'NOC', 'NCLH', 'NRG', 'NUE', 'NVDA', 'ORLY', 'OXY', 'OMC', 'OKE', 'ORCL', 'PCAR', 'PKG', 'PH', 'PAYX', 'PYPL', 'PNR', 'PBCT', 'PEP', 'PKI', 'PRGO', 'PFE', 'PM', 'PSX', 'PNW', 'PXD', 'PNC', 'PPG', 'PPL', 'PFG', 'PG', 'PGR', 'PLD', 'PRU', 'PEG', 'PSA', 'PHM', 'PVH', 'QRVO', 'PWR', 'QCOM', 'DGX', 'RL', 'RJF', 'RTN', 'O', 'REG', 'REGN', 'RF', 'RSG', 'RMD', 'RHI', 'ROK', 'ROL', 'ROP', 'ROST', 'RCL', 'CRM', 'SBAC', 'SLB', 'STX', 'SEE', 'SRE', 'SHW', 'SPG', 'SWKS', 'SLG', 'SNA', 'SO', 'LUV', 'SPGI', 'SWK', 'SBUX', 'STT', 'SYK', 'STI', 'SIVB', 'SYMC', 'SYF', 'SNPS', 'SYY', 'TMUS', 'TROW', 'TTWO', 'TPR', 'TGT', 'TEL', 'FTI', 'TFX', 'TXN', 'TXT', 'TMO', 'TIF', 'TWTR', 'TJX', 'TSS', 'TSCO', 'TDG', 'TRV', 'TRIP', 'TSN', 'UDR', 'ULTA', 'USB', 'UAA', 'UA', 'UNP', 'UAL', 'UNH', 'UPS', 'URI', 'UTX', 'UHS', 'UNM', 'VFC', 'VLO', 'VAR', 'VTR', 'VRSN', 'VRSK', 'VZ', 'VRTX', 'VIAB', 'V', 'VNO', 'VMC', 'WAB', 'WMT', 'WBA', 'DIS', 'WM', 'WAT', 'WEC', 'WCG', 'WFC', 'WELL', 'WDC', 'WU', 'WRK', 'WY', 'WHR', 'WMB', 'WLTW', 'WYNN', 'XEL', 'XRX', 'XLNX', 'XYL', 'YUM', 'ZBH', 'ZION', 'ZTS']

Let’s loop over all of the tickers in our portfolio and check to see whether they are in the S&P 500.
If our stock is not in the S&P 500, then we want to print out the ticker.

Important

Notice how there are two levels of indentation. The first level is for the for loop, and the second level is for the if statement.

for ticker in ticker_list:
    if ticker not in sp500:
        print(ticker)

SAM
UBER
SNAP

That is much faster than checking hundreds of tickers by hand!

But let’s say we want to store those tickers in a new list that we can reference later instead of just printing them out.
Let’s begin by creating an empty list to hold the results, and then we can append each result to the new list.

# new empty list to hold values
stocks_not_in_sp500 = []

# loop over and check
for ticker in ticker_list:
    if ticker not in sp500:
        # append to list
        stocks_not_in_sp500.append(ticker)

# display the final list results
stocks_not_in_sp500

['SAM', 'UBER', 'SNAP']

list comprehensions#

Since it is very common to create a new list based on values in an existing list, list comprehensions were developed.

Instead of doing the following…

initial_numbers = [5, 10, 15, 20, 25]
numbers_plus_1 = []

# loop over the list
# "num" will hold each value from the list temporarily
for num in initial_numbers:
    # create a variable to hold the new value
    # this value will be the number from the list plus 1
    add_one = num + 1
    # append the new value to the new list
    numbers_plus_1.append(add_one)

numbers_plus_1

[6, 11, 16, 21, 26]

…we can use a list comprehension:

initial_numbers = [5, 10, 15, 20, 25]
numbers_plus_1 = [ num+1 for num in initial_numbers ]
numbers_plus_1

[6, 11, 16, 21, 26]

We could also add an if statement to this if we only want to keep the numbers that are less than 20, for example.

initial_numbers = [5, 10, 15, 20, 25]
numbers_plus_1 = [ num+1 for num in initial_numbers if num < 20 ]
numbers_plus_1

[6, 11, 16]

`while` loops#

A while loop continues forever until the condition has been met. While loops are useful when you don’t know how many times you need to run the loop.

And again, remember to use the colon and indent.

i = 0

while i < 7:
    print(i)
    i = i + 1

If we did not include the line i = i + 1, we would get stuck in an “infinite loop”.
i would always be 0, and i < 7 would always be True.

Loops

Contents

Loops#

for loops#

list comprehensions#

while loops#

`for` loops#

`while` loops#