Writing Conditional Statements

Selecting Entries by Value

Suppose a dictionary records how many times each name appears in a dataset. You want only the names whose counts are greater than 10. The important question is not merely how to write an if statement. It is how to connect that condition to the value belonging to the current dictionary key.

When a for loop iterates through a dictionary, Python gives the loop one key at a time. If the condition concerns the corresponding value, retrieve that value with the index operator before testing it. The general pattern is: iterate through a key, look up its value, and let an if statement decide whether the entry should be selected.

The Three-Step Filter

1. The for statement retrieves one dictionary key.
2. The index operator retrieves the value associated with that key.
3. The if statement compares the retrieved value with the threshold or other condition.

counts = {'Ana': 14, 'Bo': 7, 'Chuck': 1, 'Dina': 18} for name in counts: if counts[name] > 10: print(name, counts[name])

Storing the Filtered Result

Printing a passing entry is useful when inspecting a result immediately. If the selected entries will be used later, place them in a new dictionary instead. The condition remains the same; only the action taken when the condition is true changes.

counts = {'Ana': 14, 'Bo': 7, 'Chuck': 1, 'Dina': 18}
filtered = {}

for name in counts:
    if counts[name] > 10:
        filtered[name] = counts[name]

print(filtered)

{'Ana': 14, 'Dina': 18}

This loop does not alter the entries in counts. It selects qualifying key-value pairs and places them in filtered. After the loop, filtered contains only entries whose values exceeded 10. That new dictionary can be passed to other functions, iterated through again, or modified further.

Nested Branches

x = 3 y = 5 if x == y: print('x and y are equal') else: if x < y: print('x is less than y') else: print('x is greater than y')

Only one complete path executes. Python evaluates the outer condition first. If the selected outer branch contains the inner conditional, Python evaluates the inner condition next. If the other outer branch is selected, the inner conditional is never evaluated.

Indentation and Readability

Indentation shows which statements belong to each conditional block. In a nested structure, the inner if statement is indented farther than the outer if statement, and statements inside the inner branches are indented farther again. Reading the indentation lets you identify which branch controls each action.

• Comparing the dictionary key instead of its value

  The loop variable is a key. In the described pattern, the condition is supposed to test the value associated with that key.

  Fix: Use the index operator to retrieve the value: if counts[name] > 10:

• Using assignment where a comparison is required

  The conditional needs to test whether two values are equal.

  Fix: Use the comparison operator: if x == y:

• Modifying the dictionary being iterated through

  Changing the dictionary during iteration is identified as a common mistake in this filtering pattern.

  Fix: Collect passing entries in a separate dictionary such as filtered.

• Assuming the nested conditional always runs

  An inner conditional runs only when execution enters the outer branch that contains it.

  Fix: Trace the outer condition first, then decide whether the inner condition is reached.

Trace and Practice

Key Takeaways

1. A for loop over a dictionary supplies keys, so filtering by value requires an index lookup such as counts[key].
2. The if statement decides whether the current entry passes the filter; rejected entries are skipped.
3. Filtered entries can be printed immediately or collected in a separate dictionary for later use.
4. Filtering selects entries, while transforming modifies selected entries.
5. A nested conditional runs only when execution reaches the branch that contains it, and excessive nesting can make code difficult to read.