Debugging – Common Mistakes not Producing an Error Message « PD4CS - Python programming, common student misconceptions, student projects

Tracing code and printing the values of variables often identifies errors in short programs. Below are examples of short code segments without compile-time or run-time errors generating the wrong output. The illustrated errors are quite common and the reasons are explained for each case.

Example 1

a = 4
b = 6
average = a + b / 2
print average

The code prints 7, but the student knows it should be 5.

Cause of error: The statement computing the average has no parenthesis. Thus, b / 2 = 3 was computed and was added to a (resulting in 4+ 3 = 7).
Correction:

a = 4
b = 6
average = (a + b) / 2
print average

Example 2

# Calculate the area of a triangle
base = 10
height = 5
area = (1 / 2) * base * height
print area

The code prints 0, but the student knows it should be 25.

Cause of error: In Python 2.x, the type of an operand in an expression defines the type of the output. If both operands are integers, the result is an integer. Hence, 1/2 evaluates to 0 and not 0.5. The statement area = (1 / 2) * base * height assigns 0 to variable area. We need to change one of the operands used in the division to a float.
Correction:

# Calculate the area of a triangle
base = 10
height = 5
area = (1.0 / 2) * base * height
print area

Example 3

# Case Sensitive
name_list = ['Tom', 'John', 'Mike']
if "john" in name_list:
    print 'Yes'
else:
    print 'No'

The code prints ‘No’, but the student knows it should be ‘Yes’.

Cause of error: In the statement if “john” in name_list: we typed ‘john’ rather than ‘John’. Python is a case-sensitive language and thus ‘john’ is different from ‘John’. The code does not generate a compile-time error (it just says “john” is not in the list). Students also make this mistake in naming variables (where it often results in a compile-time error).
Correction:

name_list = ['Tom', 'John', 'Mike']
if "John" in name_list:
    print 'Yes'
else:
    print 'No'

Example 4

# Misunderstanding about list index
name_list = ['Tom', 'John', 'Mike']
if name_list[2] == 'John':
    print 'Yes'
else:
    print 'No'

The code prints ‘No’, but the student knows it should be ‘Yes’.

Cause of error: In the statement if name_list[2] == ‘John’:, the student thought that name_list[2] would give the value ‘John’ which is the second item in the list. However, indices start at 0 and the second item of the list is name_list[1].
Correction:

name_list = ['Tom', 'John', 'Mike']
if name_list[1] == 'John':
    print 'Yes'
else:
    print 'No'

Example 5

# No output generated
# This program is designed to compare two numbers and print out the larger one.
num1 = int(raw_input('Please input the first number: '))
num2 = int(raw_input('Please input the second number: '))

if num1 > num2:
    print num1
if num2 > num1:
    print num2

The code has no output if the two numbers input by the user is equal.

Cause of error: The program did not consider the situation with equal input. This error appears very often when students start learning if statement.
Correction:

# This program is designed to compare two numbers and print out the larger one.
num1 = int(raw_input('Please input the first number: '))
num2 = int(raw_input('Please input the second number: '))

if num1 > num2:
    print num1
else:
    print num2

Example 6

# Nested Loops. Example 5: Find prime numbers.
# Example from http://www.pd4cs.org/example-5-find-prime-numbers/
n = 5
for i in range(2,n+1):
    count = 0
    for j in range(2,i):
        if(i % j ==0):
            count = count + 1
        if(count == 0):
            print i

The code prints 3 5 5 5, but the student knows it should be 2 3 5.

Cause of error: Confusing the action of the inner loop and the outer loop in a nested loop is common. In this example, the second if-statement if(count == 0): was meant to belong to the outer loop, but the student did not use the correct indentation. The indentation lines up with the first if-statement and thus the second if-statement is executed in the inner loop.
Correction:

n = 5
for i in range(2,n+1):
    count = 0
    for j in range(2,i):
        if(i % j ==0):
            count = count + 1
    if(count == 0):
        print i