Flow operators (map, filter, transform) in Kotlin - Step-by-Step Execution

Concept Flow - Flow operators (map, filter, transform)

Start Flow

↓

Apply filter

↓

Apply map

↓

Apply transform

↓

Collect/Output

↓

End

The flow starts by emitting values, then filters them, maps each value, transforms with custom logic, and finally collects the results.

Execution Sample

Kotlin

flowOf(1, 2, 3, 4)
  .filter { it % 2 == 0 }
  .map { it * 10 }
  .transform { emit(it + 1) }
  .collect { println(it) }

This code creates a flow of numbers, filters even numbers, multiplies them by 10, adds 1 to each, and prints the results.

Execution Table

Step	Input Value	Filter Condition (even?)	After Filter	Map Operation (*10)	Transform Operation (+1)	Output (emit)
1	1	1 % 2 == 0 ? False	Filtered out	-	-	-
2	2	2 % 2 == 0 ? True	2	2 * 10 = 20	20 + 1 = 21	21
3	3	3 % 2 == 0 ? False	Filtered out	-	-	-
4	4	4 % 2 == 0 ? True	4	4 * 10 = 40	40 + 1 = 41	41
-	-	-	-	-	-	Flow ends, no more values

💡 All input values processed; flow completes after last value.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
inputValue	N/A	1	2	3	4
filteredValue	N/A	null	2	null	4
mappedValue	N/A	N/A	20	N/A	40
transformedValue	N/A	N/A	21	N/A	41

Key Moments - 3 Insights

Why does the value 1 not appear in the output?

Because in the execution_table row 1, the filter condition is false for 1, so it is filtered out and does not proceed to map or transform.

What does the transform operator do differently than map?

Transform allows emitting zero, one, or multiple values per input and can contain custom logic, while map strictly transforms one input to one output. See execution_table rows 2 and 4 for transform emitting values.

Why is there a '-' in map and transform columns for filtered out values?

Because those values never reach map or transform operators after being filtered out, so no operations are performed on them.

Visual Quiz - 3 Questions

Look at the execution_table at step 2. What is the output emitted after transform?

A 21

B 20

C 2

D 41

💡 Hint

Check the 'Transform Operation (+1)' and 'Output (emit)' columns in row 2 of execution_table.

At which step does the filter condition become false for the first time?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look at the 'Filter Condition (even?)' column in execution_table rows 1 and 3.

If the filter condition was changed to allow all numbers, what would be the output at step 1?

A 1

B 11

C 10

D 21

💡 Hint

Consider that map multiplies by 10 and transform adds 1, so for input 1: (1*10)+1=11.

Concept Snapshot

Flow operators in Kotlin:
- filter: keeps values matching condition
- map: transforms each value
- transform: custom emit logic, can emit multiple values
Use chaining: flow.filter{}.map{}.transform{}.collect{}

Full Transcript

This example shows a Kotlin flow starting with values 1 to 4. The filter operator removes odd numbers (1 and 3). The map operator multiplies remaining numbers by 10. The transform operator adds 1 and emits the result. Finally, collect prints emitted values. Step-by-step, values 2 and 4 pass filter, become 20 and 40 after map, then 21 and 41 after transform, and are printed. Values 1 and 3 are filtered out early and do not appear in output.