Arithmetic expressions ($add, $multiply, $divide) in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this MongoDB aggregation?

Given a collection with documents like {"value1": 10, "value2": 5}, what will be the result of this aggregation pipeline stage?

{ $project: { result: { $add: ["$value1", "$value2"] } } }

MongoDB

{ $project: { result: { $add: ["$value1", "$value2"] } } }

A [{ "result": 5 }]

B [{ "result": 50 }]

C [{ "result": 15 }]

D [{ "result": 2 }]

💡 Hint

Think about what $add does with two numbers.

The $add operator sums the values of value1 and value2, so 10 + 5 = 15.

❓ query_result

intermediate

What does this $multiply expression return?

For documents with fields {"a": 4, "b": 3}, what is the output of this aggregation stage?

{ $project: { product: { $multiply: ["$a", "$b"] } } }

MongoDB

{ $project: { product: { $multiply: ["$a", "$b"] } } }

A [{ "product": 0 }]

B [{ "product": 7 }]

C [{ "product": 1.33 }]

D [{ "product": 12 }]

💡 Hint

Multiply the two numbers.

4 multiplied by 3 equals 12.

📝 Syntax

advanced

Which option causes a syntax error in this $divide expression?

Identify which aggregation stage will cause a syntax error when trying to divide two fields:

{ $project: { ratio: { $divide: ["$num", "$den"] } } }

MongoDB

{ $project: { ratio: { $divide: ["$num", "$den"] } } }

A { $project: { ratio: { $divide: "$num" } } }

B { $project: { ratio: { $divide: ["$num", "$den"] } } }

C { $project: { ratio: { $divide: ["$num"] } } }

D { $project: { ratio: { $divide: ["$num", "$den", "$extra"] } } }

💡 Hint

Check the number of arguments $divide expects.

$divide requires exactly two arguments in an array. Option A provides a single string, causing a syntax error.

❓ query_result

advanced

What is the output of this nested arithmetic expression?

Given documents with {"x": 8, "y": 2, "z": 4}, what is the result of this aggregation stage?

{ $project: { result: { $add: [ { $multiply: ["$x", "$y"] }, { $divide: ["$z", 2] } ] } } }

MongoDB

{ $project: { result: { $add: [ { $multiply: ["$x", "$y"] }, { $divide: ["$z", 2] } ] } } }

A [{ "result": 18 }]

B [{ "result": 20 }]

C [{ "result": 16 }]

D [{ "result": 12 }]

💡 Hint

Calculate multiplication and division first, then add.

Multiply 8*2=16, divide 4/2=2, then add 16+2=18.

🧠 Conceptual

expert

Why does this $divide operation return null?

Consider documents with {"a": 10, "b": 0}. What is the output of this aggregation stage?

{ $project: { result: { $divide: ["$a", "$b"] } } }

Why does it return null instead of an error?

MongoDB

{ $project: { result: { $divide: ["$a", "$b"] } } }

A MongoDB throws a syntax error for division by zero.

B MongoDB returns null to avoid runtime errors when dividing by zero.

C MongoDB returns Infinity for division by zero.

D MongoDB returns zero when dividing by zero.

💡 Hint

Think about how MongoDB handles invalid math operations.

MongoDB returns null for division by zero to prevent pipeline failure.

Practice

1. What does the MongoDB expression { $add: [5, 3] } do?

easy

A. Multiplies 5 and 3 to get 15

B. Adds 5 and 3 to get 8

C. Divides 5 by 3 to get 1.67

D. Subtracts 3 from 5 to get 2

Solution

1. Step 1: Understand the $add operator

   The $add operator sums the numbers inside the array.

2. Step 2: Calculate the sum of 5 and 3

   5 + 3 equals 8.

3. Final Answer:

   Adds 5 and 3 to get 8 -> Option B

4. Quick Check:

   $add: [5, 3] = 8 [OK]

Hint: Remember $add sums all values inside the array [OK]

Common Mistakes:

• Confusing $add with $multiply
• Thinking $add subtracts values
• Assuming $add divides numbers

2. Which of the following is the correct syntax to multiply 4 by 7 using MongoDB arithmetic expressions?

easy

A. { $multiply: [4, 7] }

B. { $multiply: 4, 7 }

C. { $multiply: (4, 7) }

D. { multiply: [4, 7] }

Solution

1. Step 1: Recall correct operator syntax

   MongoDB arithmetic operators use $ followed by the operator name and an array of values.

2. Step 2: Check each option

   { $multiply: [4, 7] } uses $multiply with an array [4, 7], which is correct syntax.

3. Final Answer:

   { $multiply: [4, 7] } -> Option A

4. Quick Check:

   Correct syntax uses $multiply with array [OK]

Hint: Use square brackets [] for values in $multiply [OK]

Common Mistakes:

• Omitting $ before multiply
• Using parentheses instead of brackets
• Passing values without array brackets

3. Given the MongoDB aggregation expression { $divide: [ { $multiply: [10, 5] }, 2 ] }, what is the result?

medium

A. 25

B. 100

C. 2.5

D. 50

Solution

1. Step 1: Calculate the inner $multiply expression

   $multiply: [10, 5] equals 50.

2. Step 2: Divide the result by 2

   $divide: [50, 2] equals 25.

3. Final Answer:

   25 -> Option A

4. Quick Check:

   (10 * 5) / 2 = 25 [OK]

Hint: Calculate inner expressions first, then outer [OK]

Common Mistakes:

• Dividing 10 by 5 instead of multiplying
• Forgetting to divide after multiplication
• Mixing order of operations

4. Identify the error in this MongoDB expression: { $divide: [10, { $add: 2, 3 }] }

medium

A. There is no error; the expression is correct

B. The $divide operator cannot divide numbers

C. The numbers 10, 2, and 3 must be strings

D. The $add operator must have an array of values, not separate arguments

Solution

1. Step 1: Check $add operator usage

   $add requires an array of values, but { $add: 2, 3 } uses separate arguments incorrectly.

2. Step 2: Confirm correct syntax

   The correct syntax is { $add: [2, 3] } with square brackets.

3. Final Answer:

   The $add operator must have an array of values, not separate arguments -> Option D

4. Quick Check:

   $add needs array syntax [OK]

Hint: Always use square brackets [] for $add values [OK]

Common Mistakes:

• Using commas instead of array brackets
• Assuming $divide cannot divide numbers
• Passing numbers as strings

5. You want to calculate the average price of items stored in a MongoDB collection using aggregation. Which expression correctly calculates the average price if you have total price as { $add: ["$price1", "$price2", "$price3"] } and want to divide by 3?

hard

A. { $multiply: [ { $add: ["$price1", "$price2", "$price3"] }, 3 ] }

B. { $add: [ { $divide: ["$price1", 3] }, { $divide: ["$price2", 3] }, { $divide: ["$price3", 3] } ] }

C. { $divide: [ { $add: ["$price1", "$price2", "$price3"] }, 3 ] }

D. { $divide: [ 3, { $add: ["$price1", "$price2", "$price3"] } ] }

Solution

1. Step 1: Understand the average formula

   Average = (sum of values) / number of values.

2. Step 2: Check each option

   { $divide: [ { $add: ["$price1", "$price2", "$price3"] }, 3 ] } sums prices then divides by 3, matching the average formula.

3. Final Answer:

   { $divide: [ { $add: ["$price1", "$price2", "$price3"] }, 3 ] } -> Option C

4. Quick Check:

   Sum prices then divide by count = average [OK]

Hint: Sum first, then divide by count for average [OK]

Common Mistakes:

• Dividing each price separately then adding
• Multiplying sum by count instead of dividing
• Dividing count by sum instead of sum by count