You can loop through a dictionary by using a for loop. When looping through a dictionary, the return value are the keys of the dictionary, but there are methods to return the values as well.
You can iterate through a Python dictionary using the keys(), items(), and values() methods. keys() returns an iterable list of dictionary keys. items() returns the key-value pairs in a dictionary. values() returns the dictionary values.
Iterate keys in dictionary ( dict ): keys() As mentioned above, you can iterate keys by using the dictionary object directly, but you can also use keys() . The result is the same, but keys() may clarify the intent to the reader of the code. The keys() method returns dict_keys .
There are multiple ways to iterate through list of dictionaries in python. Basically, you need to use a loop inside another loop. The outer loop, iterates through each dictionary, while the inner loop iterates through the individual elements of each dictionary.
Yes, NSDictionary
supports fast enumeration. With Objective-C 2.0, you can do this:
// To print out all key-value pairs in the NSDictionary myDict
for(id key in myDict)
NSLog(@"key=%@ value=%@", key, [myDict objectForKey:key]);
The alternate method (which you have to use if you're targeting Mac OS X pre-10.5, but you can still use on 10.5 and iPhone) is to use an NSEnumerator
:
NSEnumerator *enumerator = [myDict keyEnumerator];
id key;
// extra parens to suppress warning about using = instead of ==
while((key = [enumerator nextObject]))
NSLog(@"key=%@ value=%@", key, [myDict objectForKey:key]);
The block approach avoids running the lookup algorithm for every key:
[dict enumerateKeysAndObjectsUsingBlock:^(id key, id value, BOOL* stop) {
NSLog(@"%@ => %@", key, value);
}];
Even though NSDictionary
is implemented as a hashtable (which means that the cost of looking up an element is O(1)
), lookups still slow down your iteration by a constant factor.
My measurements show that for a dictionary d
of numbers ...
NSMutableDictionary* dict = [NSMutableDictionary dictionary];
for (int i = 0; i < 5000000; ++i) {
NSNumber* value = @(i);
dict[value.stringValue] = value;
}
... summing up the numbers with the block approach ...
__block int sum = 0;
[dict enumerateKeysAndObjectsUsingBlock:^(NSString* key, NSNumber* value, BOOL* stop) {
sum += value.intValue;
}];
... rather than the loop approach ...
int sum = 0;
for (NSString* key in dict)
sum += [dict[key] intValue];
... is about 40% faster.
EDIT: The new SDK (6.1+) appears to optimise loop iteration, so the loop approach is now about 20% faster than the block approach, at least for the simple case above.
This is iteration using block approach:
NSDictionary *dict = @{@"key1":@1, @"key2":@2, @"key3":@3};
[dict enumerateKeysAndObjectsUsingBlock:^(id key, id obj, BOOL *stop) {
NSLog(@"%@->%@",key,obj);
// Set stop to YES when you wanted to break the iteration.
}];
With autocompletion is very fast to set, and you do not have to worry about writing iteration envelope.
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With