无法弄清楚如何在C ++中打印字母序列

Question

I am working on a project where I will be able to read in a file that contains any text, like the sample text below. Then, character by character, it will be able to output n-character long sequences (represented below as a read-in value given by the user along the lines of 1, 2, 3, 4...) along the whole length of the text. So, for example:

As Gregor Samsa awoke one morning from uneasy dreams he found himself transformed in his bed into a gigantic insect.

If the user provided 2 as the sequence length, the program should spit out: "As" "s " " G" "Gr" "re" "eg" "go" "or" "r " and so on...

I have written this code but don't know why it won't work. Right now, it doesn't spit out every possible variation of the sequence. Any suggestions would be very helpful. Thanks.

#include "genlib.h"
#include <iostream>
#include <fstream>
#include "simpio.h"
#include "random.h"
#include "vector.h"
#include "map.h"

/* Private Instance Variables */
int seed_length;
string line;
string seed_string;
string next_string;
char ch;

/* Function Prototypes */
string promptUserForFile(ifstream & infile);

int main() {

ifstream infile;
promptUserForFile(infile);

// Ask what order of Markov model to use.
cout << "What order of Markov model should we use? ";
cin >> seed_length;

while (infile.eof() == false) {

    ch = infile.get();

    for (int i = 0; i < seed_length - 1; i++) {

        cout << "ch up here is " << ch << endl;

        if (isspace(ch) && i == 0) {
            seed_string += ch;

        } else {

            seed_string += ch;
            ch = infile.get();

        }
    }

    next_string = ch;

    if (isspace(ch)) {
        next_string = " ";
    } else {
        char trythis = infile.get();
        next_string += trythis;
    }


    cout << seed_string << endl;
    cout << next_string << endl;

    seed_string = "";
    next_string = "";

}

cout << "TEST" << endl;

// Close the file when you're done storing all of the scores.
infile.close();


return 0;
}


string promptUserForFile(ifstream & infile) {

string prompt = "Please input your filename: ";

while(true) {

    cout << prompt;
    string filename;
    getline (cin, filename);
    infile.open(filename.c_str());
    if(!infile.fail()) return filename;
    infile.clear();
    cout << "Unable to open that file. Try again." << endl;
    if (prompt == "") prompt == "Input file: ";

}

return 0;
}

Answer 1

The code has two problems.

1. The special handling for isspace is broken:

    if (isspace(ch) && i == 0) { seed_string += ch; } else { seed_string += ch; ch = infile.get(); } 

   This essentially means that if the first character in this loop is a space, it will be added twice.

2. Every character received from infile.get() is only added to seed_string once (with the exception of isspace characters).

A better way to code this is to recognize that:

1. You have to ignore consecutive isspace characters.
2. Every sequence can be obtained by removing the first character of the preceding sequnce and appending the next character from the file.

Here is a better implementation; it takes the order of the Markov model in the first command line parameter and takes the text from standard input. By encapsulating the skipping of duplicate spaces in a separate function, you don't have to deal with it in the main body of the algorithm.

#include <iostream>
#include <cstdlib>

char next_character() {
    static bool was_space = false;
    char ret = 0;

    do {
        ret = std::cin.get();
    } while (was_space && std::isspace(ret));

    if (std::isspace(ret)) {
        was_space = true;
        ret = ' ';
    } else {
        was_space = false;
    }
    return ret;
}

int main(int argc, char **argv) {

    if (argc != 2) return 0;
    int mlen = std::atoi(argv[1]);

    std::string seq;
    for (unsigned i = 0; i < mlen; ++i) {
        seq += next_character();
    }
    std::cout << seq << '\n';

    while (true) {
        seq.erase(0, 1);
        char c = next_character();
        if (std::cin.eof()) break;
        seq += c;
        std::cout << seq << '\n';
    }
    return 0;
}

Example input:

This  is a    test

Example output:

This 
his i
is is
s is 
 is a
is a 
s a t
 a te
a tes
 test