Exercise 1.2: Statistics

Given: four values, a, b, c, and d, all real numbers;
       the formulas for the mean and standard deviation
            mean = (a+b+c+d)/4;
            standard deviation = (((a-mean)**2 + (b-mean)**2 +
                                   (c-mean)**2 + (d-mean)**2) / 3)**0.5

Compose: a let-in statement that returns the mean and standard deviation of the given set of numbers.

let mean := (a + b + c + d) / 4.0;
    a_diff := a - mean;
    b_diff := b - mean;
    c_diff := c - mean;
    d_diff := d - mean;

    a_diff_sq := a_diff * a_diff;
    b_diff_sq := b_diff * b_diff;
    c_diff_sq := c_diff * c_diff;
    d_diff_sq := d_diff * d_diff;

    diff_sq_sum := a_diff_sq + b_diff_sq +
                   c_diff_sq + d_diff_sq;

    sigma := sqrt( diff_sq_sum / 3.0 )

in  mean, sigma

end let

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