How to put multiple string into vector?

[sangkun]

Hi,
The program I'm trying to make is to transcribe DNA to RNA and separate into codons.
So far I was able to transcribe DNA to RNA and store it in string variable.
Next part is to separate RNA strand into groups of 3 (codon) and store each group in vector.
Below is part of my code.

void GeneLoader::setCodon ()
{
	string s = rna.getRnaStrand ();
	
	for (int i=0; i < s.length() - 2; i+3)
	{
	temp = s[i] + s[i+1] + s[i+2];
		s.push_back (temp);	
	}
}

The string s has the RNA sequence stored inside. I'm trying to concatenate s[i] + s[i+1] + s[i+2] into a one string and store into the vector.
For example, if s[i] == 'a', s[i+1] == 'c', s[i+2] == 'g' then s[i] + s[i+1] + s[i+2] would equal to "acg". The "acg" would be stored into the vector[i] and so forth.
Any advice on how to tackle this problem?

[vijayan121]

s[i] + s[i+1] + s[i+2]; would just do an integral addition of the three char values; instead you need to make astd::string of three chars.

I suppose for (int i=0; i < s.length() - 2; i+3) is a typo;
should be for( int i=0 ; i < s.length() - 2 ; i += 3 ) .

Something like:

std::vector< std::string > strand_to_codons( const std::string& strand )
{
    std::vector< std::string > codons ;

    for( std::string::size_type i = 0 ; i < strand.size() - 2 ; i += 3 )
    {
        const char temp[] = { strand[i], strand[i+1], strand[i+2], 0 } ;
        codons.emplace_back(temp) ;
    }

    return codons ;
}

Another option would be to use thesubstr() member function of std::string:

std::vector< std::string > strand_to_codons( const std::string& strand )
{
    std::vector< std::string > codons ;

    for( std::string::size_type i = 0 ; i < strand.size() - 2 ; i += 3 )
            codons.push_back( strand.substr( i, 3 ) ) ;

    return codons ;
}

Yet another way is to construct the substring in situ into thestd::vector<> via an emplace_back() (requires compiler support for variadic templates).

std::vector< std::string > strand_to_codons( const std::string& strand )
{

    std::vector< std::string > codons ;

    for( auto iter = strand.begin() ; iter < strand.end()-2 ; iter += 3 )
            codons.emplace_back( iter, iter+3 ) ;

    return codons ;
}

[sangkun]

I'm a beginner in c++.
I have tried your code but it didn't work.
Also I don't understand the need for "std::string::size_type i = 0".
Please explain.

[vijayan121]

> I don't understand the need for "std::string::size_type i = 0".
> Please explain.

std::string::size_type is an alias (a typedef) for some unsigned integral type that is sufficiently large to represent the number of chars in a std::string, no matter how long the string is. An int need not be able to do that on every implementation.

std::string::size_type eliminates the guesswork involved in choosing a suitable integral type to represent the position of a char in a std::string; using it allows us to write code that is portable across implementations.

> I have tried your code but it didn't work.

That came as a surprise to me; so I've just tried compiling and running the code. All three versions of strand_to_codons() work as expected. (With a conforming compiler, of course.)