Check out example codes for "round double to n decimal places c++". It will help you in understanding the concepts better.

Code Example 1

float roundoff(float value, unsigned char prec)
{
  float pow_10 = pow(10.0f, (float)prec);
  return round(value * pow_10) / pow_10;
}

auto rounded = roundoff(100.123456, 3);
// rounded = 100.123;

Code Example 2

value = round( value * 100.0 ) / 100.0; // 2 decimal places
value = round( value * 1000.0 ) / 1000.0; // 3 decimal places

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