# Python – Cross mapping of Two dictionary value lists

• Last Updated : 21 Jun, 2022

GIven two dictionaries with list values, perform mapping of keys of first list with values of other list, by checking values-key linkage.

Input : test_dict1 = {“Gfg” : [4, 10], “Best” : [8, 6], “is” : [9, 3]}, test_dict2 = {6 : [15, 9], 8 : [6, 3], 7 : [9, 8], 9 : [10, 11]} Output : {‘Best’: [6, 3, 15, 9], ‘is’: [10, 11]} Explanation : “Best” has 8 and 6, which are mapped to 6, 3 and 15, 9 hence output for that key. Input : test_dict1 = {“Gfg” : [4, 10], “Best” : [18, 16], “is” : [9, 3]}, test_dict2 = {6 : [15, 9], 8 : [6, 3], 7 : [9, 8], 9 : [10, 11]} Output : {‘is’: [10, 11]} Explanation : Only 9 present as possible key.

Method #1 : Using loop + setdefault() + extend()

The combination of above functions can be used to solve this problem. In this, we perform the task of getting the matching keys with values using get() and setdefault is used to construct empty list for mapping.

## Python3

 `# Python3 code to demonstrate working of``# Cross mapping of Two dictionary value lists``# Using loop + setdefault() + extend()` `# initializing dictionaries``test_dict1 ``=` `{``"Gfg"` `: [``4``, ``7``], ``"Best"` `: [``8``, ``6``], ``"is"` `: [``9``, ``3``]}``test_dict2 ``=` `{``6` `: [``15``, ``9``], ``8` `: [``6``, ``3``], ``7` `: [``9``, ``8``], ``9` `: [``10``, ``11``]}` `# printing original lists``print``(``"The original dictionary 1 is : "` `+` `str``(test_dict1))``print``(``"The original dictionary 2 is : "` `+` `str``(test_dict2))` `res ``=` `{}` `# getting all values of first dictionary``for` `key, val ``in` `test_dict1.items():``    ``for` `key1 ``in` `val:``        ` `        ``# getting result with default value list and extending``        ``# according to value obtained from get()``        ``res.setdefault(key, []).extend(test_dict2.get(key1, []))` `# printing result``print``(``"The constructed dictionary : "` `+` `str``(res))`

Output

```The original dictionary 1 is : {'Gfg': [4, 7], 'Best': [8, 6], 'is': [9, 3]}
The original dictionary 2 is : {6: [15, 9], 8: [6, 3], 7: [9, 8], 9: [10, 11]}
The constructed dictionary : {'Gfg': [9, 8], 'Best': [6, 3, 15, 9], 'is': [10, 11]}```

Method #2 : Using list comprehension + dictionary comprehension

This is one more way in which this problem can be solved. In this, we extract all the mapping using list comprehension and then construct new dictionary by cross-mapping the extracted values.

## Python3

 `# Python3 code to demonstrate working of``# Cross mapping of Two dictionary value lists``# Using list comprehension + dictionary comprehension` `# initializing dictionaries``test_dict1 ``=` `{``"Gfg"` `: [``4``, ``7``], ``"Best"` `: [``8``, ``6``], ``"is"` `: [``9``, ``3``]}``test_dict2 ``=` `{``6` `: [``15``, ``9``], ``8` `: [``6``, ``3``], ``7` `: [``9``, ``8``], ``9` `: [``10``, ``11``]}` `# printing original lists``print``(``"The original dictionary 1 is : "` `+` `str``(test_dict1))``print``(``"The original dictionary 2 is : "` `+` `str``(test_dict2))` `# using internal and external comprehension to``# solve problem``res ``=` `{key: [j ``for` `i ``in` `val ``if` `i ``in` `test_dict2 ``for` `j ``in` `test_dict2[i]]``      ``for` `key, val ``in` `test_dict1.items()}` `# printing result``print``(``"The constructed dictionary : "` `+` `str``(res))`

Output

```The original dictionary 1 is : {'Gfg': [4, 7], 'Best': [8, 6], 'is': [9, 3]}
The original dictionary 2 is : {6: [15, 9], 8: [6, 3], 7: [9, 8], 9: [10, 11]}
The constructed dictionary : {'Gfg': [9, 8], 'Best': [6, 3, 15, 9], 'is': [10, 11]}```

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