Arithmetic expressions ($add, $multiply, $divide) in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add two fields 'price' and 'tax' in a MongoDB aggregation pipeline.

MongoDB

{ $project: { totalCost: { $add: ["$price", [1] ] } } }

Drag options to blanks, or click blank then click option'

A"$tax"

B"tax"

C"$total"

D"cost"

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting the $ before the field name

Using the field name as a string without $

In MongoDB aggregation, to refer to a field, you use "$" before the field name. So "$tax" correctly refers to the 'tax' field.

2fill in blank

medium

Complete the code to multiply 'quantity' by 'price' to get 'total' in a MongoDB aggregation pipeline.

MongoDB

{ $project: { total: { $multiply: ["$quantity", [1] ] } } }

Drag options to blanks, or click blank then click option'

A"price"

B"$price"

C"cost"

D"amount"

Attempts:

3 left

💡 Hint

Common Mistakes

Using field names without $

Using incorrect field names

Use "$price" to correctly reference the 'price' field in the aggregation pipeline.

3fill in blank

hard

Fix the error in the code to divide 'total' by 'count' in a MongoDB aggregation pipeline.

MongoDB

{ $project: { average: { $divide: ["[1]", "$count" ] } } }

Drag options to blanks, or click blank then click option'

A"$total"

B"total"

C$total

Dtotal

Attempts:

3 left

💡 Hint

Common Mistakes

Using unquoted field names

Omitting the $ prefix

To refer to a field inside an aggregation expression, you must use "$" before the field name and wrap it in quotes.

4fill in blank

hard

Fill both blanks to calculate the total price including tax by adding 'price' and the product of 'price' and 'taxRate'.

MongoDB

{ $project: { totalPrice: { $add: ["$price", { $multiply: ["$price", [1] ] } ] } } }

Drag options to blanks, or click blank then click option'

A"$tax"

B"taxRate"

C"$taxRate"

D"tax"

Attempts:

3 left

💡 Hint

Common Mistakes

Not using $ before 'taxRate'

Using incorrect field names

To multiply 'price' by 'taxRate', both fields must be referenced with a $ prefix inside strings.

5fill in blank

hard

Fill all three blanks to calculate the average price per item after discount: subtract 'discount' from 'price', then divide by 'quantity'.

MongoDB

{ $project: { avgPrice: { $divide: [ { $subtract: ["[1]", "[2]" ] }, "[3]" ] } } }

Drag options to blanks, or click blank then click option'

A"$price"

B"$discount"

C"$quantity"

D"price"

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting $ before field names

Using field names without quotes

Each field must be referenced with a $ prefix inside quotes to be correctly used in aggregation expressions.

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