L1 · FoundationsText basics~2 min · 4 tests

Count Uppercase and Lowercase Letters

Count uppercase and lowercase letters in a string with isupper() and islower(), ignoring digits and symbols. A beginner Python loop, run live.

The problem

Return a tuple (uppercase_count, lowercase_count) for the letters in s. Ignore digits, spaces and symbols.

Examples

Example 1
Input
```
case_counts('Hello World')
```
Expected output
```
(2, 8)
```
Example 2
Input
```
case_counts('PYTHON')
```
Expected output
```
(6, 0)
```

+ 2 hidden tests on Submit.

Edge cases to ask about

Digits and symbols
Empty

Hints

0/3

How an interviewer scores this

0/9

1. Understand the problemRestate it, ask about edge cases and input size.
2. Write a brute-force solutionCorrect first, clever later.
3. Get the correct outputautoAll visible examples pass.
4. Analyse time complexityautoCommit to a Big-O in the Complexity Lab.
5. Analyse space complexityautoExtra memory, not the input.
6. Identify the bottleneckautoMeasure your code — where does it bend?
7. OptimiseUsually: a set or dict instead of a scan.
8. Explain why it worksSay it out loud to Solvi.
9. Test the edge casesautoSubmit passes every hidden test.

Python 3.13 · case_counts

def case_counts(s):
    """Return (number of uppercase, number of lowercase)."""
    # Write your solution here
    pass

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Complexity Lab

What does this cost as n grows?

Interviewers score the analysis as much as the code. Commit to an answer first — then check it, and read why.

Time complexity of the optimal solution

Space complexity (extra memory)

Pick both to reveal the answer.

From brute force to optimal

The progression an interviewer wants to hear, one step at a time.

Approach	Time	Space	Idea
One loop, two counters	O(n)	O(1)

Walkthrough of the optimal approach (try it yourself first)

Loop once and increment the right counter. isupper()/islower() return False for digits and symbols, so they are ignored automatically.

Complexity: O(n) time, O(1) space. One pass, two integer counters.

Reveal the reference solution

def case_counts(s):
    upper = lower = 0
    for ch in s:
        if ch.isupper():
            upper += 1
        elif ch.islower():
            lower += 1
    return upper, lower

Follow-ups interviewers ask

Swap the case of every letter without swapcase().

Frequently asked interview questions

Core interview concepts, complexities, and follow-ups scored by hiring teams.

What is the time complexity of Count Uppercase and Lowercase Letters in Python?

The optimal solution runs in O(n) time and O(1) auxiliary space. One pass, two integer counters.

What follow-up questions do interviewers ask about Count Uppercase and Lowercase Letters?

Swap the case of every letter without swapcase().