I'm watching a python course and I saw a line of code that I don't understand
books_dict[title] = [author,subject,year]
what I see from this line is the key of books_dict is the title and there are multiple values for it?
You can print the type of books_dict[title] with type() function. It tells you that it's a list(so there is only one object). List is a container so it can contain other objects. In your dictionary there is only one value for that key. Whenever you access to that key you will get that one list not individual items inside it. That would be problematic then!
If you have:
d = {}
d["key1"] = [1, 2, 3]
There is only one value, and that value is a list. The list is [author, subject, year].
In addition to what others have already stated, a dictionary holds key, value pairs. One key to one value, however the data types used to create the key and value can both be containers holding more than one element
for example
books_dict[title] = [author,subject,year]
is the same as
temp = [author, subject, year]
books_dict[title] = temp
The key can also hold an iterable, however it must be hashable and immutable.
books_dict[(title, author)] = [subject, year]
which is the same as
key = (title, author)
value = [subject, year]
books_dict[key] = value
Related
I am trying to find a way to remove duplicates from a dict list. I don't have to test the entire object contents because the "name" value in a given object is enough to identify duplication (i.e., duplicate name = duplicate object). My current attempt is this;
newResultArray = []
for i in range(0, len(resultArray)):
for j in range(0, len(resultArray)):
if(i != j):
keyI = resultArray[i]['name']
keyJ = resultArray[j]['name']
if(keyI != keyJ):
newResultArray.append(resultArray[i])
, which is wildly incorrect. Grateful for any suggestions. Thank you.
If name is unique, you should just use a dictionary to store your inner dictionaries, with name being the key. Then you won't even have the issue of duplicates, and you can remove from the list in O(1) time.
Since I don't have access to the code that populates resultArray, I'll simply show how you can convert it into a dictionary in linear time. Although the best option would be to use a dictionary instead of resultArray in the first place, if possible.
new_dictionary = {}
for item in resultArray:
new_dictionary[item['name']] = item
If you must have a list in the end, then you can convert back into a dictionary as such:
new_list = [v for k,v in new_dictionary.items()]
Since "name" provides uniqueness... and assuming "name" is a hashable object, you can build an intermediate dictionary keyed by "name". Any like-named dicts will simply overwrite their predecessor in the dict, giving you a list of unique dictionaries.
tmpDict = {result["name"]:result for result in resultArray}
newArray = list(tmpDict.values())
del tmpDict
You could shrink that down to
newArray = list({result["name"]:result for result in resultArray}.values())
which may be a bit obscure.
I was wondering if there's a way for me to pull a value at a specific index. Let's say I have a key with multiple values associated with it. But in my dictionary I have multiple keys, each key with multiple values. I want to iterate through the keys and then each respective value associated with that key. I want to be able to pull the value at the first index and subtract it from the value at the second index.
d= {108572791: [200356.77, 200358], 108577388: [19168.7, 19169]}
output for key 108572791 would be -1.33
output for key 108577388 would be -.03
I've try reading up on dict and how it works apparently you can't really index it. I just wanted to know if there's a way to get around that.
for key, values in total_iteritems():
for value in values:
value[0]-value[1]:
Edit:
Since the question is way different now, I'll address the new subject:
d= {108572791: [200356.77, 200358], 108577388: [19168.7, 19169]}
for i in d:
print("Output for key ",str(i), "would be ",(d[i][1]-d[i][0]))
Output:
Output for key 108572791 would be 1.2300000000104774
Output for key 108577388 would be 0.2999999999992724
Original answer
Yes. When you have a dict containing a list as value if you want to obtain a specific value, then you need to address the index in the list. An example is:
a = {'Name':['John','Max','Robert']}
This means that:
print(a['Name'])
Output:
['John','Max','Robert']
Since ['Name'] is a list:
for i in range(len(a['Name'])):
print(a['Name'][i]
Output:
John #(Because it's the index 0)
Max #(Index = 1)
Robert #(Index = 2)
If you want a specific value (for instance 'Max' which is index = 1)
print(a['Name'][1]
Output:
Max
Depends on how many values in key obvious but this does the trick:
for x in d:
print(x)
print(d[x][0]-d[x][1])
You can use list of tuples if you want to use indexing.
d= [(108572791,[200356.77, 200358]), (108577388,[19168.7, 19169)]
for tuple in my_list:
print(tuple[0])
for value in tuple[1]:
print(value)
the current code I have is category1[name]=(number) however if the same name comes up the value in the dictionary is replaced by the new number how would I make it so instead of the value being replaced the original value is kept and the new value is also added, giving the key two values now, thanks.
You would have to make the dictionary point to lists instead of numbers, for example if you had two numbers for category cat1:
categories["cat1"] = [21, 78]
To make sure you add the new numbers to the list rather than replacing them, check it's in there first before adding it:
cat_val = # Some value
if cat_key in categories:
categories[cat_key].append(cat_val)
else:
# Initialise it to a list containing one item
categories[cat_key] = [cat_val]
To access the values, you simply use categories[cat_key] which would return [12] if there was one key with the value 12, and [12, 95] if there were two values for that key.
Note that if you don't want to store duplicate keys you can use a set rather than a list:
cat_val = # Some value
if cat_key in categories:
categories[cat_key].add(cat_val)
else:
# Initialise it to a set containing one item
categories[cat_key] = set(cat_val)
a key only has one value, you would need to make the value a tuple or list etc
If you know you are going to have multiple values for a key then i suggest you make the values capable of handling this when they are created
It's a little hard to understand your question.
I think you want this:
>>> d[key] = [4]
>>> d[key].append(5)
>>> d[key]
[4, 5]
Depending on what you expect, you could check if name - a key in your dictionary - already exists. If so, you might be able to change its current value to a list, containing both the previous and the new value.
I didn't test this, but maybe you want something like this:
mydict = {'key_1' : 'value_1', 'key_2' : 'value_2'}
another_key = 'key_2'
another_value = 'value_3'
if another_key in mydict.keys():
# another_key does already exist in mydict
mydict[another_key] = [mydict[another_key], another_value]
else:
# another_key doesn't exist in mydict
mydict[another_key] = another_value
Be careful when doing this more than one time! If it could happen that you want to store more than two values, you might want to add another check - to see if mydict[another_key] already is a list. If so, use .append() to add the third, fourth, ... value to it.
Otherwise you would get a collection of nested lists.
You can create a dictionary in which you map a key to a list of values, in which you would want to append a new value to the lists of values stored at each key.
d = dict([])
d["name"] = 1
x = d["name"]
d["name"] = [1] + x
I guess this is the easiest way:
category1 = {}
category1['firstKey'] = [7]
category1['firstKey'] += [9]
category1['firstKey']
should give you:
[7, 9]
So, just use lists of numbers instead of numbers.
Python dictionaries really have me today. I've been pouring over stack, trying to find a way to do a simple append of a new value to an existing key in a python dictionary adn I'm failing at every attempt and using the same syntaxes I see on here.
This is what i am trying to do:
#cursor seach a xls file
definitionQuery_Dict = {}
for row in arcpy.SearchCursor(xls):
# set some source paths from strings in the xls file
dataSourcePath = str(row.getValue("workspace_path")) + "\\" + str(row.getValue("dataSource"))
dataSource = row.getValue("dataSource")
# add items to dictionary. The keys are the dayasource table and the values will be definition (SQL) queries. First test is to see if a defintion query exists in the row and if it does, we want to add the key,value pair to a dictionary.
if row.getValue("Definition_Query") <> None:
# if key already exists, then append a new value to the value list
if row.getValue("dataSource") in definitionQuery_Dict:
definitionQuery_Dict[row.getValue("dataSource")].append(row.getValue("Definition_Query"))
else:
# otherwise, add a new key, value pair
definitionQuery_Dict[row.getValue("dataSource")] = row.getValue("Definition_Query")
I get an attribute error:
AttributeError: 'unicode' object has no attribute 'append'
But I believe I am doing the same as the answer provided here
I've tried various other methods with no luck with various other error messages. i know this is probably simple and maybe I couldn't find the right source on the web, but I'm stuck. Anyone care to help?
Thanks,
Mike
The issue is that you're originally setting the value to be a string (ie the result of row.getValue) but then trying to append it if it already exists. You need to set the original value to a list containing a single string. Change the last line to this:
definitionQuery_Dict[row.getValue("dataSource")] = [row.getValue("Definition_Query")]
(notice the brackets round the value).
ndpu has a good point with the use of defaultdict: but if you're using that, you should always do append - ie replace the whole if/else statement with the append you're currently doing in the if clause.
Your dictionary has keys and values. If you want to add to the values as you go, then each value has to be a type that can be extended/expanded, like a list or another dictionary. Currently each value in your dictionary is a string, where what you want instead is a list containing strings. If you use lists, you can do something like:
mydict = {}
records = [('a', 2), ('b', 3), ('a', 4)]
for key, data in records:
# If this is a new key, create a list to store
# the values
if not key in mydict:
mydict[key] = []
mydict[key].append(data)
Output:
mydict
Out[4]: {'a': [2, 4], 'b': [3]}
Note that even though 'b' only has one value, that single value still has to be put in a list, so that it can be added to later on.
Use collections.defaultdict:
from collections import defaultdict
definitionQuery_Dict = defaultdict(list)
# ...
In the below Python Code, am dynamically create Lists.
g['quest_{0}'.format(random(x))] = []
where random(x) is a random number, how to print the List(get the name of the dynamically created List name?)
To get a list of all the keys of your dictionary :
list(g.keys())
There is nothing different with a regular dictionary because you generate the key dynamically.
Note that you can also put any type of hashable object as a key, such as a tuple :
g[('quest', random(x))] = []
Which will let you get a list of all your quest numbers easily :
[number for tag, number in g.keys() if tag == "quest"]
With this technic, you can actually loop through the tag ('quest'), the number and the value in one loop :
for (tag, number), value in g.items():
# do somthing
Unpacking is your best friend in Python.
You can iterate over the g dictionary with a for loop, like this
for key, value in g.items():
print key, value
This will print all the keys and their corresponding lists.