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1. It's a small table and is likely to be cached in the processor
2. You are running things in a quite tight loop and/or the processor can pre-load the data

Background and why

Pfew, so what the hell is that supposed to mean?

From a processor perspective, your memory is slow. To compensate for the difference in speed, they build in a couple of caches in your processor (L1/L2 cache) that compensate for that. So imagine that you're doing your nice calculations and figure out that you need a piece of memory. The processor will get its 'load' operation and loads the piece of memory into cache - and then uses the cache to do the rest of the calculations. Because memory is relatively slow, this 'load' will slow down your program.

for (int i=0; i<array.Length; ++i)
   // Use array[i]

In this case, it's obvious to the compiler that the boundary condition will never be hit. At least the Microsoft JIT compiler (but I expect Java does similar things) will notice this and remove the check altogether. WOW - that means no branch. Similarly, it will deal with other obvious cases.

If you run into trouble with lookups on managed languages - the key is to add a & 0x[something]FFF to your lookup function to make the boundary check predictable - and watch it going faster.

// Generate data
int arraySize = 32768;
int[] data = new int[arraySize];

Random random = new Random(0);
for (int c = 0; c < arraySize; ++c)
{
    data[c] = random.Next(256);
}

/*To keep the spirit of the code in-tact I'll make a separate lookup table
(I assume we cannot modify 'data' or the number of loops)*/

int[] lookup = new int[256];

for (int c = 0; c < 256; ++c)
    lookup[c] = (c >= 128) ? c : 0;

// Test
DateTime startTime = System.DateTime.Now();
long sum = 0;

for (int i = 0; i < 100000; ++i)
{
    // Primary loop
    for (int j = 0; j < arraySize; ++j)
    {
        /* Here you basically want to use simple operations - so no
        random branches, but things like &, |, *, -, +, etc. are fine. */
        sum += lookup[data[j]];
    }
}

DateTime endTime = System.DateTime.Now;
Console.WriteLine(endTime - startTime);
Console.WriteLine("sum = " + sum);
Console.ReadLine();

1. it's a small table and is likely to be cached in the processor, and
2. you are running things in a quite tight loop and/or the processor can preload the data.

Background and why

From a processor perspective, your memory is slow. To compensate for the difference in speed, a couple of caches are built into your processor (L1/L2 cache). So imagine that you're doing your nice calculations and figure out that you need a piece of memory. The processor will get its 'load' operation and loads the piece of memory into cache -- and then uses the cache to do the rest of the calculations. Because memory is relatively slow, this 'load' will slow down your program.

for (int i = 0; i < array.Length; ++i)
{
   // Use array[i]
}

In this case, it's obvious to the compiler that the boundary condition will never be hit. At least the Microsoft JIT compiler (but I expect Java does similar things) will notice this and remove the check altogether. WOW, that means no branch. Similarly, it will deal with other obvious cases.

If you run into trouble with lookups in managed languages -- the key is to add a & 0x[something]FFF to your lookup function to make the boundary check predictable -- and watch it going faster.

// Generate data
int arraySize = 32768;
int[] data = new int[arraySize];

Random random = new Random(0);
for (int c = 0; c < arraySize; ++c)
{
    data[c] = random.Next(256);
}

/*To keep the spirit of the code intact, I'll make a separate lookup table
(I assume we cannot modify 'data' or the number of loops)*/

int[] lookup = new int[256];

for (int c = 0; c < 256; ++c)
{
    lookup[c] = (c >= 128) ? c : 0;
}

// Test
DateTime startTime = System.DateTime.Now;
long sum = 0;

for (int i = 0; i < 100000; ++i)
{
    // Primary loop
    for (int j = 0; j < arraySize; ++j)
    {
        /* Here you basically want to use simple operations - so no
        random branches, but things like &, |, *, -, +, etc. are fine. */
        sum += lookup[data[j]];
    }
}

DateTime endTime = System.DateTime.Now;
Console.WriteLine(endTime - startTime);
Console.WriteLine("sum = " + sum);
Console.ReadLine();

// Generate data
int arraySize = 32768;
int[] data = new int[arraySize];

Random rnd = new Random(0);
for (int c = 0; c < arraySize; ++c)
    data[c] = rnd.Next(256);

//To keep the spirit of the code in-tact I'll make a separate lookup table
// (I assume we cannot modify 'data' or the number of loops)
int[] lookup = new int[256];

for (int c = 0; c < 256; ++c)
    lookup[c] = (c >= 128) ? c : 0;

// Test
DateTime startTime = System.DateTime.Now;
long sum = 0;

for (int i = 0; i < 100000; ++i)
{
    // Primary loop
    for (int j = 0; j < arraySize; ++j)
    {
        // Here you basically want to use simple operations - so no
        // random branches, but things like &, |, *, -, +, etc. are fine.
        sum += lookup[data[j]];
    }
}

DateTime endTime = System.DateTime.Now;
Console.WriteLine(endTime - startTime);
Console.WriteLine("sum = " + sum);
 
Console.ReadLine();

// Generate data
int arraySize = 32768;
int[] data = new int[arraySize];

Random random = new Random(0);
for (int c = 0; c < arraySize; ++c)
{
    data[c] = random.Next(256);
}
 
/*To keep the spirit of the code in-tact I'll make a separate lookup table
(I assume we cannot modify 'data' or the number of loops)*/

int[] lookup = new int[256];

for (int c = 0; c < 256; ++c)
    lookup[c] = (c >= 128) ? c : 0;

// Test
DateTime startTime = System.DateTime.Now();
long sum = 0;

for (int i = 0; i < 100000; ++i)
{
    // Primary loop
    for (int j = 0; j < arraySize; ++j)
    {
        /* Here you basically want to use simple operations - so no
        random branches, but things like &, |, *, -, +, etc. are fine. */
        sum += lookup[data[j]];
    }
}

DateTime endTime = System.DateTime.Now;
Console.WriteLine(endTime - startTime);
Console.WriteLine("sum = " + sum);
Console.ReadLine();

So we've figured out that we can create a small table. Next thing to do is get a lookup function in place. Lookup functions are usually small functions that use a couple of basic integer operations (and, or, xor, shift, add, remove and perhaps a multiply). You want to have your input translated by the lookup function to some kind of 'unique key' in your table, which then simply gives you the answer of all the work you wanted it to do.

In this case it's obvious to the compiler that the boundary condition will never be hit. At least the Microsoft JIT compiler (but I expect Java does similar things) will notice this and remove the check all together. WOW - that means no branch. Similarly, it will deal with other obvious cases.

The result for this case

// Generate data
int arraySize = 32768;
int[] data = new int[arraySize];

Random rnd = new Random(0);
for (int c = 0; c < arraySize; ++c)
    data[c] = rnd.Next(256);

// Too keep the spirit of the code in-tact I'll make a separate lookup table
// (I assume we cannot modify 'data' or the number of loops)
int[] lookup = new int[256];

for (int c = 0; c < 256; ++c)
    lookup[c] = (c >= 128) ? c : 0;

// Test
DateTime startTime = System.DateTime.Now;
long sum = 0;

for (int i = 0; i < 100000; ++i)
{
    // Primary loop
    for (int j = 0; j < arraySize; ++j)
    {
        // Here you basically want to use simple operations - so no
        // random branches, but things like &, |, *, -, +, etc. are fine.
        sum += lookup[data[j]];
    }
}

DateTime endTime = System.DateTime.Now;
Console.WriteLine(endTime - startTime);
Console.WriteLine("sum = " + sum);

Console.ReadLine();

So we've figured out that we can create a small table. Next thing to do is get a lookup function in place. Lookup functions are usually small functions that use a couple of basic integer operations (and, or, xor, shift, add, remove and perhaps multiply). You want to have your input translated by the lookup function to some kind of 'unique key' in your table, which then simply gives you the answer of all the work you wanted it to do.

In this case, it's obvious to the compiler that the boundary condition will never be hit. At least the Microsoft JIT compiler (but I expect Java does similar things) will notice this and remove the check altogether. WOW - that means no branch. Similarly, it will deal with other obvious cases.

The result of this case

// Generate data
int arraySize = 32768;
int[] data = new int[arraySize];

Random rnd = new Random(0);
for (int c = 0; c < arraySize; ++c)
    data[c] = rnd.Next(256);

//To keep the spirit of the code in-tact I'll make a separate lookup table
// (I assume we cannot modify 'data' or the number of loops)
int[] lookup = new int[256];

for (int c = 0; c < 256; ++c)
    lookup[c] = (c >= 128) ? c : 0;

// Test
DateTime startTime = System.DateTime.Now;
long sum = 0;

for (int i = 0; i < 100000; ++i)
{
    // Primary loop
    for (int j = 0; j < arraySize; ++j)
    {
        // Here you basically want to use simple operations - so no
        // random branches, but things like &, |, *, -, +, etc. are fine.
        sum += lookup[data[j]];
    }
}

DateTime endTime = System.DateTime.Now;
Console.WriteLine(endTime - startTime);
Console.WriteLine("sum = " + sum);

Console.ReadLine();