Find the missing value from the given equation a + b = c in Python

In Python, we can solve equations of the form a + b = c where one value is missing (represented by ?). This involves parsing the equation string and applying basic arithmetic to find the unknown value.

For example, if the input is ? + 4 = 9, the output will be 5.

Algorithm

To solve this problem, we follow these steps:

• Remove all spaces from the string and replace + and = with commas

• Split the string by commas to get individual elements

• Find the position of the missing value (?)

• Apply the appropriate arithmetic operation based on position:

  • If c is missing: return a + b

  • If b is missing: return c - a

  • If a is missing: return c - b

Implementation

def find_missing(equation):
    # Remove spaces and replace operators with commas
    equation = equation.strip().replace(' ', '')
    equation = equation.replace('=', ',')
    equation = equation.replace('+', ',')
    
    # Split into elements
    elements = equation.split(',')
    
    # Find position of missing value
    missing_idx = 0
    for i in range(len(elements)):
        if not elements[i].isnumeric():
            missing_idx = i
            break
    
    # Calculate missing value based on position
    if missing_idx == 2:  # c is missing (a + b = ?)
        return int(elements[0]) + int(elements[1])
    elif missing_idx == 1:  # b is missing (a + ? = c)
        return int(elements[2]) - int(elements[0])
    elif missing_idx == 0:  # a is missing (? + b = c)
        return int(elements[2]) - int(elements[1])

# Test cases
print(find_missing('6 + 8 = ?'))
print(find_missing('? + 8 = 20'))
print(find_missing('5 + ? = 15'))

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How It Works

The solution works by transforming the equation string into a list of elements:

• '6 + 8 = ?' becomes ['6', '8', '?']

• We identify which position contains the ?

• Based on the position, we apply the correct arithmetic operation

Alternative Approach Using eval()

def find_missing_eval(equation):
    # Split by '=' to get left and right parts
    left, right = equation.split('=')
    left, right = left.strip(), right.strip()
    
    if '?' in left:
        # Unknown is on the left side
        if '+' in left:
            parts = left.split('+')
            if '?' in parts[0].strip():
                return int(right) - int(parts[1].strip())
            else:
                return int(right) - int(parts[0].strip())
    else:
        # Unknown is on the right side (c = ?)
        parts = left.split('+')
        return int(parts[0].strip()) + int(parts[1].strip())

# Test cases
print(find_missing_eval('6 + 8 = ?'))
print(find_missing_eval('? + 8 = 20'))
print(find_missing_eval('5 + ? = 15'))

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Conclusion

Both approaches effectively solve missing value problems in simple addition equations. The first method uses string parsing and position-based logic, while the second approach splits by the equals sign for cleaner handling of equation sides.