Python list and for loop

Question

I'm expecting this code to print spade:A spade:2 and so on until heart:K.
But it only does heart:A to heart:K.
How should I do it?

symbols = ["spade", "clover", "diamond", "heart"]
numbers = ["A", "2", "3", "4", "5", "6", "7", "8", "9", "10", "J", "Q",       "K"]
cards = {}

for num in numbers:
    for symbol in symbols:
        cards[num] = symbol

print cards

You are replacing the values for cards. You can only have one value for each numbers key, so the last cards[num] = symbol is going to win. — Martijn Pieters
– Martijn Pieters, Commented Jan 26, 2015 at 17:39
Also, your code would print {'A': 'heart', '10': 'heart', 'K': 'heart', 'J': 'heart', ...}, because 'heart' is the last entry in symbols. — Martijn Pieters
– Martijn Pieters, Commented Jan 26, 2015 at 17:40
@MartijnPieters I didn't understand ur first comment... can u pls elaborate? — Yubin Lee
– Yubin Lee, Commented Jan 26, 2015 at 17:40
Dictionaries map one key to one value. You are replacing that value each time. First you set cards['A'] to 'spade', then to 'clover', then 'diamond' then finally 'heart'. You cannot add multiple 'A' keys to a dictionary. — Martijn Pieters
– Martijn Pieters, Commented Jan 26, 2015 at 17:52

Adam Smith · Accepted Answer · 2015-01-27 18:58:37Z

Use your itertools toolbox

import itertools

symbols = ["spade", "clover", "diamond", "heart"]
numbers = ["A", "2", "3", "4", "5", "6", "7", "8", "9", "10", "J", "Q",       "K"]

combinations = itertools.product(symbols, numbers)

cards = ["{}:{}".format(suit, rank) for suit,rank in combinations]

This will give you the list:

['spade:A',
 'spade:2',
 'spade:3',
 'spade:4',
 'spade:5',
 'spade:6',
 'spade:7',
 'spade:8',
 'spade:9',
 'spade:10',
 'spade:J',
 'spade:Q',
 'spade:K',
 'clover:A',
 'clover:2',
 'clover:3',
 'clover:4',
 'clover:5',
 'clover:6',
 'clover:7',
 'clover:8',
 'clover:9',
 'clover:10',
 'clover:J',
 'clover:Q',
 'clover:K',
 'diamond:A',
 'diamond:2',
 'diamond:3',
 'diamond:4',
 'diamond:5',
 'diamond:6',
 'diamond:7',
 'diamond:8',
 'diamond:9',
 'diamond:10',
 'diamond:J',
 'diamond:Q',
 'diamond:K',
 'heart:A',
 'heart:2',
 'heart:3',
 'heart:4',
 'heart:5',
 'heart:6',
 'heart:7',
 'heart:8',
 'heart:9',
 'heart:10',
 'heart:J',
 'heart:Q',
 'heart:K']

Panagiotis Barkoutsos · Accepted Answer · 2015-01-27 12:25:53Z

1

The problem is that you are not iterating the right way and thus you are not appending in the list. The right way to do it is

symbols = ["spade", "clover", "diamond", "heart"]
numbers = ["A", "2", "3", "4", "5", "6", "7", "8", "9", "10", "J", "Q",         "K"]
cards = []

for j in range(len(symbols)):
    for i in range(len(numbers)):
       cards.append(str(symbols[j]+':'+str(numbers[i])))

print cards

with output:

['spade:A', 'spade:2', 'spade:3', 'spade:4', 'spade:5', 'spade:6', 'spade:7',  'spade:8', 
'spade:9', 'spade:10', 'spade:J', 'spade:Q', 'spade:K', 'clover:A', 'clover:2',
'clover:3', 'clover:4', 'clover:5', 'clover:6', 'clover:7', 'clover:8', 'clover:9',
'clover:10', 'clover:J', 'clover:Q', 'clover:K', 'diamond:A', 'diamond:2', 'diamond:3',
'diamond:4', 'diamond:5', 'diamond:6', 'diamond:7', 'diamond:8', 'diamond:9', 'diamond:10',
'diamond:J', 'diamond:Q', 'diamond:K', 'heart:A', 'heart:2', 'heart:3', 'heart:4',
'heart:5', 'heart:6', 'heart:7', 'heart:8', 'heart:9', 'heart:10', 'heart:J', 'heart:Q', 'heart:K']

Made with Ipython Notebook in python 2.7

Hope it helps.

edited Jan 27, 2015 at 12:25

answered Jan 27, 2015 at 12:10

Panagiotis Barkoutsos

114 bronze badges

6 Comments

Jon Clements Over a year ago

Also, in Python - you generally don't iterate over the indices of sequences, just the sequencing themselves. In this case, a list-comp can be used to simplify the code and can be reduced to: cards = ['{}:{}'.format(s, n) for s in symbols for n in numbers]

Panagiotis Barkoutsos Over a year ago

Seems to be an exotic way to show you are good in Python (which I am sure you do), but cannot be easily understood (or thought) by everyone. Anyway thanks for the information.

Jon Clements Over a year ago

I agree in some sense - I'm pointing it out because it's a very common thing in the world of Python - who knows - SO is meant to be a resource of information, this post will be read by future visitors - if something comes in handy - all the better!

PM 2Ring Over a year ago

@PanagiotisBarkoutsos: Is this one more readable, in your opinion? cards = [s + ':' + n for s in symbols for n in numbers]

Panagiotis Barkoutsos Over a year ago

I can easily get what you mean, but I am working on Python a lot. For someone asking such a question, means he is in beginner level and do not want to overcomplicate the structures. Everything is readable and can be understood, but the thing is that you need to explain why his idea is not working and not to provide a way that he will not be able to implement in future use. I wrote what I wrote based on the format the guy asking used. I could write what you propose but it would not be a correction, it would be just a different way to express it.

|

Noman Ur Rehman · Accepted Answer · 2015-01-26 17:46:21Z

0

You are iterating the symbols just fine but when you are going over the numbers in the second loop, you are actually replacing the values set by the previous loop hence you only have values from the last loop left and everything is replaced. This means cards["A"] value is set 4 times in the loop and the last for the "heart" is retained. The same thing is happening for all the other indexes.

answered Jan 26, 2015 at 17:46

Noman Ur Rehman

7,0073 gold badges26 silver badges41 bronze badges

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