53. Constexpr ADC Temperature Table

#include <iostream>
using namespace std;

// Compile-time temperature lookup table
constexpr int TEMP_TABLE[6] = { -40, -10, 20, 50, 80, 120 };

int main() {
    int idx;
    cin >> idx;

    cout << TEMP_TABLE[idx];

    return 0;
}

Explanation & Logic Summary

• TEMP_TABLE is declared as constexpr, so it is evaluated entirely at compile time
• The table is stored in Flash / ROM, not RAM
• Runtime execution performs a constant-time (O(1)) lookup
• No arithmetic or floating-point computation occurs at runtime
• This guarantees deterministic execution time

Firmware Relevance & Real Embedded Meaning

In memory-constrained embedded systems (8 KB–32 KB MCUs):

• constexpr tables save RAM by residing in Flash
• Lookup tables eliminate costly runtime math
• Immutable data prevents accidental modification
• Compilers can aggressively optimize constant access

This pattern is commonly used in:

• Thermistor temperature conversion
• Battery voltage-to-percentage mapping
• Display gamma correction
• Motor control profiles
• Sensor calibration data

Using a non-constexpr array would:

• Consume RAM
• Require runtime initialization
• Allow unintended modification
• Reduce optimization opportunities

For these reasons, constexpr lookup tables are standard practice in production embedded firmware.