What It Means & How to Fix It in UE5

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🧠 Why You’re Seeing This Error

This error means you’re trying to access an element in an array using an index that goes beyond the array’s declared size — either:

• At compile time (e.g., hardcoded index too large), or
• At runtime (if index could vary but isn’t guaranteed safe)

Unreal’s HLSL shader compiler throws this to prevent undefined behavior or GPU crashes.

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💥 Example Error Message

error X3550: array index out of bounds

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🛠️ Common Shader Scenarios That Cause This

❌ Index is Hardcoded Beyond Array Size

float values[3];
float x = values[4]; // ❌ Index 4 is out of bounds (valid: 0–2)

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❌ Index is Dynamic but Not Properly Clamped

float4 myColors[5];
int index = CustomInputValue;
float4 color = myColors[index]; // ❌ Unsafe if index >= 5

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❌ Using a Loop That Exceeds Array Length

float weights[8];
for (int i = 0; i <= 8; i++) // ❌ Should be i < 8
{
    sum += weights[i];
}

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✅ How to Fix X3550 in UE5 – Step-by-Step

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✔️ 1. Clamp or Validate Your Array Index

Always make sure your index stays within bounds:

int safeIndex = clamp(index, 0, 4);
float value = myArray[safeIndex];

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✔️ 2. Double-Check Array Size vs Access

If your array has 8 elements:

float values[8];

Then only access:

values[0] to values[7]; // ✅
values[8]; // ❌ Index out of bounds

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✔️ 3. Fix Loop Conditions That Overrun

Bad loop:

for (int i = 0; i <= 7; i++)

Correct loop:

for (int i = 0; i < 8; i++)

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✔️ 4. Consider Using static const for Safe Iteration

static const int ArraySize = 4;
float values[ArraySize];

for (int i = 0; i < ArraySize; i++)
{
    // Safe loop
}

This makes your loops and bounds easier to maintain and less error-prone.

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✅ Summary: How to Fix “error X3550: array index out of bounds” in UE5

Cause	Fix
Hardcoded index too large	Check the array size and update the access
Dynamic index unsafe	Clamp the index with clamp() or bounds-check manually
Incorrect loop range	Change <= to <, or use a defined constant for array size
Logic mismatch between size and access	Double-check all indexing patterns