成员引用基类型不是结构或联合

Question

I need some help with a problem I'm having with my homework assignment.

//finds frequency of a sequence in an array (including mutations)
int find_freq_with_mutations(string target,string sequences[],int sequences_length) {
    for (int i = 0; i < sequences_length; i++) { //goes through each sequence in array
        string current_sequence = sequences[i]; //sets current sequence as a string
        for (int j = 0; j < current_sequence.length(); j++) { //iterate for every character in sequence
            if (current_sequence[j] == current_sequence[j+1]) {
                current_sequence[j].erase();
            }
        }
    }
    int target_frequency = find_frequency(target, sequences, sequences_length);

    return target_frequency;

}

The error message is:

DNA_sequencing.cpp:70:24: error: member reference base type
      'std::__1::basic_string<char, std::__1::char_traits<char>,
      std::__1::allocator<char> >::value_type' (aka 'char') is not a structure or
      union
                                current_sequence[j].erase();
                                ~~~~~~~~~~~~~~~~~~~^~~~~~

Any help is much appreciated!

Answer 1

Reading your code shows indicates that you are trying to call erase() on a character in an std::string . If you wish to erase a character in a string, you need to call the erase() function in std::string . I recommend using iterators to loop through your string, since using the erase() function on a string that you're iterating through with integer indices would be difficult, since the index of each character in the string changes every time you erase a character. Taking this into account, the inner for loop becomes:

for (auto it = current_sequence.begin(); it != current_sequence; ) {
      if (*it == *(it + 1)) {
           it = current_sequence.erase(it);
      } else {
           it++;
      }
}

Another note is that if you plan on mutating the strings in the array you should probably be using a pointer or reference, since current_sequence is just a copy of the string in the array. Since you're already using an array, I recommend looping through the string pointers themselves, like so:

for (std::string *current_sequence = sequences; current_sequence < sequences + sequences_length; current_sequence++) {
    /* Do something with current_sequence */
}