10.4 Conditionals and looping in dictionaries

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 1 Templates and examples of a conditional statement on a dictionary

**Table 10.1**
dict.items
`# (key, value) in dictionary.items movies = {"The godfather": 1974, "Interstellar": 2014} print(("Interstellar", 2014) in movies.items)`
`True`

**Table 10.2**
Conditionals on values or keys
`# dictionary[key] operand test_value movies = {"The godfather": 1974, "Interstellar": 2014} print(movies["The godfather"] < 2000)`
`True`

**Table 10.3**
dict.keys
`# key in dictionary.keys movies = {"The godfather": 1974, "Interstellar": 2014} print("Interstellar" in movies.keys)`
`True`

**Table 10.4**
dict.values
`# value in dictionary.values movies = {"The godfather": 1974, "Interstellar": 2014} print(2014 in movies.values)`
`True`

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.

Note

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 2 Iterating over a dictionary

**Table 10.5**
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 10.6**
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 10.7**
dict.values
`zip_codes = {"Berkeley": 94709, "Santa Cruz": 95064, "Mountain View": 94030} for value in zip_codes.values: print(value)`
`94709 95064 94030`

Note

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}

string_value = "This is a string"

# Create a dictionary and count the number of occurrences of characters

Note

Calculate the total number of fruits

Given a fruit_count dictionary that contains information about fruits and the count of each fruit, calculate the total number of fruits across all fruit types.


    Input:
    fruit_count = {"banana": 2, "orange": 5, "peach": 5}


    Prints 12

fruit_count = {"banana": 2, "orange": 5, "peach": 5}

# Calculate and print the total number of fruits

Adapted from Introduction to Python Programming by OpenStax (openstax.org), licensed under CC BY-NC-SA 4.0. Changes were made. License: CC-BY-NC-SA-4.0.