Q28
2 marksShort AnswerSection B

The function given below is written to accept a string s as a parameter and return the number of vowels appearing in the string. The code has certain errors. Observe the code carefully and rewrite it after removing all the logical and syntax errors. Underline all the corrections made.

``python

def CountVowels(s):

c=0

for ch in range(s):

if 'aeiouAEIOU' in ch:

c=+1

return(ch)

``

Functions in Python
Debugging Python Functions (String Traversal)
Official Answer

The corrected function is shown below with every fix underlined/highlighted:


``python

def CountVowels(s):

c=0

for ch in s: # corrected: iterate over string s, not range(s)

if ch in 'aeiouAEIOU': # corrected: check ch in vowel string (reversed logic)

c+=1 # corrected: increment c, not c=+1 (unary plus)

return(c) # corrected: return count c, not ch

`


Four corrections made:


  • for ch in range(s): → for ch in s: — range() cannot take a string; must loop directly over the string.
  • if 'aeiouAEIOU' in ch: → if ch in 'aeiouAEIOU': — the membership check was reversed; it must test whether the single character ch is one of the vowels.
  • c=+1 → c+=1 — c=+1 is unary-plus assignment (always sets c to 1), not increment; c+=1 correctly adds 1 each time.
  • return(ch) → return(c) — the function must return the vowel count c, not the last character ch`.
debugginglogical errorsyntax errorrange()membership operatorincrement operatorstring traversal

Marking Scheme

  • 10.5 mark: correcting `for ch in range(s):` to `for ch in s:`.
  • 20.5 mark: correcting `if 'aeiouAEIOU' in ch:` to `if ch in 'aeiouAEIOU':`.
  • 30.5 mark: correcting `c=+1` to `c+=1` (or `c=c+1`).
  • 40.5 mark: correcting `return(ch)` to `return(c)`; deduct marks if corrections are not underlined/highlighted as instructed.

Hint

Check: can range() take a string? Is the 'in' check in the right direction? Does c=+1 actually increment? What should the function return?

Quick Oral Answer

I fixed four bugs: looped directly over the string instead of range(s), reversed the 'in' check to test each character against the vowel string, changed c=+1 to c+=1 so it actually accumulates, and returned c instead of ch.

Analysis & Explanation

This is a classic 'find and fix' debugging question involving both a syntax error and three logical errors.


Syntax error

range(s) raises a TypeError because range() requires integer arguments, not a string — this alone would crash the program before any logic could even execute.


Logical errors

  • The membership test direction was inverted: 'aeiouAEIOU' in ch checks if the entire vowel string appears inside a single character ch, which is always False; it must be ch in 'aeiouAEIOU'.
  • c=+1 looks like an increment but is parsed as c = (+1), resetting c to 1 every time a vowel is found, rather than accumulating a count.
  • Returning ch instead of c returns the last character processed by the loop instead of the tally of vowels.

Exam trap

CBSE explicitly asks students to underline corrections — missing this formatting instruction, even with correct code, can cost presentation marks. Also, c=+1 vs c+=1 is one of the most frequently tested 'silent' bugs since it does not raise any error.


Real-world relevance

This exact pattern — looping over characters and using membership tests — underlies many text-processing utilities like password validators and text analyzers.

Common Mistakes

  1. 1Not noticing that c=+1 is valid Python syntax (unary plus) and hence not an error the interpreter flags, but is still logically wrong — students often skip this fix.
  2. 2Fixing only the range(s) TypeError and missing the reversed 'in' condition or the wrong return variable.
  3. 3Forgetting to underline/highlight the corrections as explicitly instructed by the question, losing presentation marks even with a fully correct answer.

Interesting Facts

`c=+1` is a frequently exploited 'gotcha' in Python exams because `+1` is valid as a unary-plus expression, so the line runs without any error — making it a purely logical bug rather than a syntax bug.

`range()` only accepts integers because it was designed purely as a sequence generator for indices/counters — for arbitrary iterables Python encourages direct iteration (`for ch in s`), which is both simpler and slightly faster than index-based looping.

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Frequently Asked Questions

Why does for ch in range(s) cause an error?

range() requires integer arguments (start, stop, step); passing a string s directly to range() raises a TypeError because strings are not valid arguments for range().

Why is c=+1 wrong even though it doesn't crash the program?

c=+1 is parsed as c = (+1), which resets c to 1 every time it executes, rather than adding 1 to the existing value of c. The correct increment is c+=1 or c=c+1.