如何仅用Amazon States Language获取Amazon Step Functions中数组长度

Nice question! You don't need Lambda or any external AWS services to calculate an array's length in Step Functions—Amazon States Language (ASL) has built-in intrinsic functions that let you do this directly within a Pass state (a state type designed for in-line data transformation).

Complete ASL State Machine Definition #

Here's a ready-to-use state machine that takes your sample input and returns exactly the output you're looking for:

{
  "Comment": "Calculate length of SampleArray using ASL intrinsic functions",
  "StartAt": "CalculateArrayLength",
  "States": {
    "CalculateArrayLength": {
      "Type": "Pass",
      "Parameters": {
        "LengthOfSampleArray.$": "States.ArrayLength($.SampleArray)"
      },
      "End": true
    }
  }
}

How This Works #

Let's break down the key components:

• Pass State: This state runs entirely within Step Functions' runtime, no external calls required. It's purpose-built for shaping or transforming input data.
• Parameters Field: We use this to construct our desired output structure. The .$ suffix tells Step Functions that the value is dynamically calculated, not static text.
• States.ArrayLength(): This is ASL's native function for getting an array's length. We pass it the JSONPath to our target array ($.SampleArray) from the input.
• End: true: Marks this as the final state in the workflow, so execution stops here and returns the constructed output.

Input/Output Example #

Sample Input:

{ 
  "SampleField" : "SampleString", 
  "SampleField2" : "SampleString2", 
  "SampleArray": [ 
    { "Name": "Jack", "Age": 10 }, 
    { "Name": "John", "Age": 18 }, 
    { "Name": "Mary", "Age": 15 } 
  ] 
}

Resulting Output:

{ "LengthOfSampleArray": 3 }

Bonus: Retain Original Input + Add Length Field #

If you ever want to keep the original input data while adding the length field, adjust the ResultPath to merge the calculated value into the input:

{
  "Comment": "Add array length to original input",
  "StartAt": "AddArrayLength",
  "States": {
    "AddArrayLength": {
      "Type": "Pass",
      "Parameters": {
        "LengthOfSampleArray.$": "States.ArrayLength($.SampleArray)"
      },
      "ResultPath": "$",
      "End": true
    }
  }
}

This will output the original input with the LengthOfSampleArray field added to the top level.

内容的提问来源于stack exchange，提问作者Prithvi Sreenivasan