我在for循环中收到错误

I am having errors in my for loop and I wonder why is that:

for (int i = 0; char a = 'a'; i < n; i++; a++)

substitution.c:24:25: error: expected expression
        for (int i = 0; char a = 'a'; i < n; i++; a++)
                        ^
substitution.c:24:44: error: expected ')'
        for (int i = 0; char a = 'a'; i < n; i++; a++)
                                           ^
substitution.c:24:13: note: to match this '('
        for (int i = 0; char a = 'a'; i < n; i++; a++)
            ^
substitution.c:24:46: error: use of undeclared identifier 'i'
        for (int i = 0; char a = 'a'; i < n; i++; a++)
                                             ^
substitution.c:24:51: error: use of undeclared identifier 'a'
        for (int i = 0; char a = 'a'; i < n; i++; a++)