Let $mathbb{F}_{q}$ be the finite field of $q$ elements and let $D_{2n}=langle x,ymid x^n=1, y^2=1, yxy=x^{n-1}rangle$ be the dihedral group of order $n$. Left ideals of the group algebra $mathbb{F}_{q}[D_{2n}]$ are known as left dihedral codes over $mathbb{F}_{q}$ of length $2n$, and abbreviated as left $D_{2n}$-codes. Let ${rm gcd}(n,q)=1$. In this paper, we give an explicit representation for the Euclidean hull of every left $D_{2n}$-code over $mathbb{F}_{q}$. On this basis, we determine all distinct Euclidean LCD codes and Euclidean self-orthogonal codes which are left $D_{2n}$-codes over $mathbb{F}_{q}$. In particular, we provide an explicit representation and a precise enumeration for these two subclasses of left $D_{2n}$-codes and self-dual left $D_{2n}$-codes, respectively. Moreover, we give a direct and simple method for determining the encoder (generator matrix) of any left $D_{2n}$-code over $mathbb{F}_{q}$, and present several numerical examples to illustrative our applications.
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