Modify Program 8.3, letters.cpp, so that a vector of 26 elements, indexed from 0 to 25, is used to track how many times each letter in the range ’a’–’z’ occurs. To do this, map the character ’a’ to 0,...

Modify Program 8.3, letters.cpp, so that a vector of 26 elements, indexed from 0 to 25, is used to track how many times each letter in the range ’a’–’z’ occurs. To do this, map the character ’a’ to 0, ’b’ to 1,…, and ’z’ to 25. Isolate this mapping in a function CharToIndex whose header is

int CharToIndex(char ch)

// pre: ’a’ <=>

// post: returns 0 for ’a’, 1 for ’b’, ... 25 for ’z’

Note that ’a’ - ’a’ == 0, ’b’ - ’a’ == 1, and ’z’ - ’a’ == 25.)