The ${\displaystyle a}$-weight of a string, for a letter ${\displaystyle a}$, is the number of times that letter occurs in the string. More precisely, let ${\displaystyle A}$ be a finite set (called the alphabet), ${\displaystyle a\in A}$ a letter of ${\displaystyle A}$, and ${\displaystyle c\in A^{*}}$ a string (where ${\displaystyle A^{*}}$ is the free monoid generated by the elements of ${\displaystyle A}$, equivalently the set of strings, including the empty string, whose letters are from ${\displaystyle A}$). Then the ${\displaystyle a}$-weight of ${\displaystyle c}$, denoted by ${\displaystyle \mathrm {wt} _{a}(c)}$, is the number of times the generator ${\displaystyle a}$ occurs in the unique expression for ${\displaystyle c}$ as a product (concatenation) of letters in ${\displaystyle A}$.

If ${\displaystyle A}$ is an abelian group, the Hamming weight ${\displaystyle \mathrm {wt} (c)}$ of ${\displaystyle c}$, often simply referred to as "weight", is the number of nonzero letters in ${\displaystyle c}$.

• Let ${\displaystyle A=\{x,y,z\}}$. In the string ${\displaystyle c=yxxzyyzxyzzyx}$, ${\displaystyle y}$ occurs 5 times, so the ${\displaystyle y}$-weight of ${\displaystyle c}$ is ${\displaystyle \mathrm {wt} _{y}(c)=5}$.
• Let ${\displaystyle A=\mathbf {Z} _{3}=\{0,1,2\}}$ (an abelian group) and ${\displaystyle c=002001200}$. Then ${\displaystyle \mathrm {wt} _{0}(c)=6}$, ${\displaystyle \mathrm {wt} _{1}(c)=1}$, ${\displaystyle \mathrm {wt} _{2}(c)=2}$ and ${\displaystyle \mathrm {wt} (c)=\mathrm {wt} _{1}(c)+\mathrm {wt} _{2}(c)=3}$.

This article incorporates material from Weight (strings) on PlanetMath, which is licensed under the Creative Commons Attribution/Share-Alike License.