索引到函数数组:表达式解析为未使用的l值。

I am trying to index into an array of functions but I get the error: "Expression resolves to an unused l-value". I have tried to google what this means but information is scarce and what I find seems unrelated. Does anyone know what I am doing wrong here? Any help would be much appreciated ! Thank you.I am trying to index into an array of functions