如何有效地遍历字典？

Question

I'm new to Python and programming.

My textbook says I have to do the following problem set:

Create a second purchase summary that which accumulates total investment by ticker symbol. In the above sample data, there are two blocks of CAT.

These can easily be combined by creating a dict where the key is the ticker and the value is the list of blocks purchased. The program makes one pass through the data to create the dict. A pass through the dict can then create a report showing each ticker symbol and all blocks of stock.

I cannot think of a way, apart from hard-coding, to add the two entries of the 'CAT' stock.

## Stock Reports

stockDict = {"GM":"General Motors", "CAT":"Caterpillar", "EK":"Eastman Kodak",
             "FB":"Facebook"}
# symbol,prices,dates,shares
purchases = [("GM",100,"10-sep-2001",48), ("CAT",100,"01-apr-1999",24),
             ("FB",200,"01-jul-2013",56), ("CAT", 200,"02-may-1999",53)]

# purchase history:
print "Company", "\t\tPrice", "\tDate\n"
for stock in purchases:
    price = stock[1] * stock[3]
    name = stockDict[stock[0]]
    print name, "\t\t", price, "\t", stock[2]
print "\n"

# THIS IS THE PROBLEM SET I NEED HELP WITH:
# accumulate total investment by ticker symbol
byTicker = {}
# create dict
for stock in purchases:
    ticker = stock[0]
    block = [stock]
    if ticker in byTicker:
        byTicker[ticker] += block
    else:
        byTicker[ticker] = block

for i in byTicker.values():
    shares = i[0][3]
    price = i[0][1]
    investment = shares * price
    print investment

Right now, the output is:

4800
11200
2400

It's not good because it does not calculate the two CAT stocks. Right now it only calculates one. The code should be flexible enough that I could add more CAT stocks.

Answer 1

Maybe something like this:

## Stock Reports

stockDict = {"GM":"General Motors", "CAT":"Caterpillar", "EK":"Eastman Kodak",
             "FB":"Facebook"}
# symbol,prices,dates,shares
purchases = [("GM",100,"10-sep-2001",48), ("CAT",100,"01-apr-1999",24),
             ("FB",200,"01-jul-2013",56), ("CAT", 200,"02-may-1999",53)]

# purchase history:
print "Company", "\t\tPrice", "\tDate\n"
for stock in purchases:
    price = stock[1] * stock[3]
    name = stockDict[stock[0]]
    print name, "\t\t", price, "\t", stock[2]
print "\n"

# THIS IS THE PROBLEM SET I NEED HELP WITH:
# accumulate total investment by ticker symbol
byTicker = {}
# create dict
for stock in purchases:
    ticker = stock[0]
    price = stock[1] * stock[3]
    if ticker in byTicker:
        byTicker[ticker] += price
    else:
        byTicker[ticker] = price

for ticker, price in byTicker.iteritems():
    print ticker, price

The output I get is:

Company                 Price   Date

General Motors          4800    10-sep-2001
Caterpillar             2400    01-apr-1999
Facebook                11200   01-jul-2013
Caterpillar             10600   02-may-1999


GM 4800
FB 11200
CAT 13000

which appears to be correct.

Testing whether or not a ticker is in the byTicker dict tells you whether or not there's already been a purchase recorded for that stock. If there is, you just add to it, if not, you start fresh. This is basically what you were doing, except for some reason you were collecting all of the purchase records for a given stock in that dict, when all you really cared about was the price of the purchase.

You could build the dict the same way you were originally, and then iterate over the items stored under each key, and add them up. Something like this:

totals = []
for ticker in byTicker:
    total = 0
    for purchase in byTicker[ticker]:
        total += purchase[1] * purchase[3]
    totals.append((ticker, total))

for ticker, total in totals:
    print ticker, total

And just for kicks, you could compress it all into one line with generator statements:

 print "\n".join("%s: %d" % (ticker, sum(purchase[1]*purchase[3] for purchase in byTicker[ticker])) for ticker in byTicker)

Either of these last two are completely unnecessary to do though, since you're already iterating through every purchase, you may as well just accumulate the total price for each stock as you go, as I showed in the first example.

Answer 2

Your problem is in the last part of your code, the penultimate bit creates a list of all stocks against each ticker, which is fine:

for i in byTicker.values():
    shares = i[0][3]
    price = i[0][1]
    investment = shares * price
    print investment

Here you only use the zeroth stock for each ticker. Instead, try:

for name, purchases in byTicker.items():
    investment = sum(shares * price for _, shares, _, price in purchases)
    print name, investment

This will add up all of the stocks for each ticker, and for your example gives me:

CAT 13000
FB 11200
GM 4800

Answer 3

The problem with your code is that you are not iterating over the purchaes, but just getting the first element from each ticker value. That is, byTicker looks something like:

byTicker: {
  "GM": [("GM",100,"10-sep-2001",48)],
  "CAT": [("CAT",100,"01-apr-1999",24), ("CAT", 200,"02-may-1999",53)],
  "FB": [("FB",200,"01-jul-2013",56)]
}

so when you iterate over the values, you actually get three lists. But when you process these lists, you are actually accessing only the first of them:

price = i[0][1]

for the value corresponding to "CAT", i[0] is ("CAT",100,"01-apr-1999",24). You should look into i[1] as well! Consider iterating over the different purchases:

for company, purchases in byTicker.items():
  investment = 0
  for purchase in purchases:
      investment += purchase[1] * purchase[3]
  print(company, investment)