Use SCAN when you need the running result at every item in an array; use REDUCE when you need only the final accumulated result. Both apply a LAMBDA across an array, carrying an accumulator from one value to the next. The difference is what Excel returns: SCAN returns each intermediate state, while REDUCE returns the last one.
What is the difference between SCAN and REDUCE?
| Function | What it returns | Use it when |
|---|---|---|
| SCAN | An array containing the accumulator after each value is processed. | You need a running total, running product, cumulative text, or another view of how a result changes step by step. |
| REDUCE | One final accumulated value after all values are processed. | You need a summary or result and do not need to display intermediate states. |
A quick decision rule: need every step? Use SCAN. Need only the finished accumulator? Use REDUCE. Microsoft describes SCAN as returning an array of intermediate values; REDUCE applies a LAMBDA over an array and returns the accumulated result. See Microsoft’s SCAN function documentation and REDUCE function documentation.
How the accumulator and LAMBDA work
Both functions use the same general pattern. The LAMBDA takes two inputs: the current accumulator and the current array value. It calculates the next accumulator state. SCAN returns the successive states as an array; REDUCE returns only the final state.
=SCAN([initial_value], array, LAMBDA(accumulator, value, calculation))
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=REDUCE([initial_value], array, LAMBDA(accumulator, value, calculation))
- initial_value seeds the accumulator.
- array is the set of values to process.
- accumulator is the state carried from the preceding calculation.
- value is the current item being processed.
- calculation returns the next accumulator state.
When to use SCAN
Choose SCAN when the sequence of intermediate results is useful, not just the final one. For example, it can show the total after each entry, successive products, or text as it is built.
Running products
Microsoft illustrates a running product with:
=SCAN(1, A1:C2, LAMBDA(a,b,a*b))
The initial value of 1 is the multiplicative identity, so it does not change the first product. SCAN returns the successive products as an array rather than collapsing them into one value.
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Cumulative text
To build text by concatenating values in an array, Microsoft’s example is:
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For this text accumulation pattern, Microsoft recommends an empty string ("") as the initial value.
When to use REDUCE
Use REDUCE if the intermediate states are not needed and one accumulated result is the goal. Its LAMBDA still processes each array item in turn, but Excel returns only the final accumulator.
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Sum squared values
Microsoft’s example adds the square of each value into a single result:
=REDUCE(, A1:C2, LAMBDA(a,b,a+b^2))
This example omits the initial value. Microsoft documents that REDUCE then uses the first value in the array as the starting accumulator.
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Multiply values that pass a condition
This example multiplies only values greater than 50 in the table’s nums column:
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=REDUCE(1,Table3[nums],LAMBDA(a,b,IF(b>50,a*b,a)))
The seed is 1 so the multiplication is not initialized with zero; values that do not meet the condition leave the accumulator unchanged.
Count even values
REDUCE can also update a count as it processes an array:
=REDUCE(0,Table4[Nums],LAMBDA(a,n,IF(ISEVEN(n),1+a,a)))
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Here, the accumulator starts at 0 and increases by one whenever the current value is even. The formula returns one count, not a running list of counts.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Choose the right initial value
The initial value defines the accumulator’s starting state, so it must fit the operation. For multiplication, 1 avoids forcing the product to zero. For counting, 0 provides a natural starting count. For text concatenation with SCAN, use "", as Microsoft recommends.
REDUCE has a documented omission behavior: if initial_value is left out, the first array value becomes the starting accumulator. That can change the result compared with seeding explicitly, so consider the operation before omitting it. Microsoft’s examples and argument details are in its REDUCE function documentation.
Availability in Excel
Microsoft’s alphabetical function index marks both SCAN and REDUCE as introduced in Excel 2024 and explains that its version markers identify when functions were introduced. The individual support pages list different applicable products: SCAN lists Excel for Microsoft 365, Excel for Microsoft 365 for Mac, Excel for the web, Excel 2024, and Excel 2024 for Mac; REDUCE lists Excel for Microsoft 365 and Excel for Microsoft 365 for Mac. Because the index and function pages do not provide identical scopes, check your own Excel release and update channel if a function is unavailable. Microsoft’s alphabetical function index provides the version markers.
Fixing an “Incorrect Parameters” error
Microsoft says an invalid LAMBDA or incorrect number of parameters returns #VALUE!, identified as “Incorrect Parameters.” Check the function’s LAMBDA arguments and calculation before changing the formula structure.
- Confirm the LAMBDA has the expected accumulator and current-value parameters.
- Check that the calculation returns the next accumulator state you intend.
- Verify that the initial value is appropriate for the operation.
For function-specific syntax and error details, consult Microsoft’s SCAN documentation or REDUCE documentation.
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