Udayan Das; Aubrey Lawson; Chris Mayfield; Narges Norouzi

Learning objectives

By the end of this section you should be able to

Write a conditional statement to check for a key/value.
Write a for loop to iterate over elements of a dictionary.

Conditionals for dictionary

Conditional statements can be used with dictionaries to check if certain keys, values, or dictionary items exist in the dictionary or if a value satisfies a particular condition.

Example 10.2

Templates and examples of a conditional statement on a dictionary

dict.items()
# (key, value) in dictionary.items() movies = {"The godfather": 1974, "Interstellar": 2014} print(("Interstellar", 2014) in movies.items())
`True`

Table 10.1

Conditionals on values or keys
# dictionary[key] operand test_value movies = {"The godfather": 1974, "Interstellar": 2014} print(movies["The godfather"] < 2000)
`True`

Table 10.2

dict.keys()
# key in dictionary.keys() movies = {"The godfather": 1974, "Interstellar": 2014} print("Interstellar" in movies.keys())
`True`

Table 10.3

dict.values()
# value in dictionary.values() movies = {"The godfather": 1974, "Interstellar": 2014} print(2014 in movies.values())
`True`

Table 10.4

Concepts in Practice

Conditionals on dictionaries

Given the dictionary fruit_count = {"apple": 2, "orange": 5, "pomegranate": 1}, answer the following questions.

1.

What is the output of "apple" in fruit_count.keys()?

True
False

2.

What is the output of ("orange", 5) in fruit_count.items()?

SyntaxError
True
False

3.

Which conditional statement checks if the value associated with the key "pomegranate" is greater than 0?

fruit_count("pomegranate") > 0
fruit_count["pomegranate"] > 0
fruit_count.get("pomegranate" > 0)

Looping on a dictionary

Looping over a Python dictionary is a way to iterate through key-value pairs in the dictionary. Looping in a dictionary can be done by iterating over keys or items. When looping using keys, keys are obtained using the keys() function and are passed to the loop variable one at a time. When looping over items using the items() function, both the key and value for each item are passed to the loop variable.

A for loop over a dictionary retrieves each key in the dictionary

    for key in dictionary: # Loop expression
      # Statements to execute in the loop

    # Statements to execute after the loop

Example 10.3

Iterating over a dictionary

dict.items()
zip_codes = {"Berkeley": 94709, "Santa Cruz": 95064, "Mountain View": 94030} for key, value in zip_codes.items(): print(key, value)
Berkeley 94709 Santa Cruz 95064 Mountain View 94030

Table 10.5

dict.keys()
zip_codes = {"Berkeley": 94709, "Santa Cruz": 95064, "Mountain View": 94030} for key in zip_codes.keys(): print(key)
Berkeley Santa Cruz Mountain View

Table 10.6

dict.values()
zip_codes = {"Berkeley": 94709, "Santa Cruz": 95064, "Mountain View": 94030} for value in zip_codes.values(): print(value)
94709 95064 94030

Table 10.7

Concepts in Practice

Loops on dictionaries

4.

Which method is used to loop over the values in a Python dictionary?

keys()
values()
items()

5.

What is the output of the following code?

fruit_count = {"apple": 2, "orange": 5, "banana": 1}
for key in fruit_count.keys():
  print(key, end = " ")

apple orange banana
2 5 1
("apple", 2) ("orange", 5) ("banana", 1)

6.

What is the output of the following code?

fruit_count = {"apple": 2, "orange": 5, "banana": 1}
for value in fruit_count.values():
  print(value * 2, end = " ")

apple orange banana
2 5 1
4 10 2

Try It

Character count in a string

Given a string value, calculate and print the number of occurrences of all characters using a dictionary.

    Input:
    string_value = "This is a string"

    Prints {"T": 1, "h": 1, "i": 3, "s": 3, " ": 3, "a": 1, "t": 1, "r": 1, "n": 1, 
    "g": 1}

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