#include <iostream>
#include <vector>
#include <sstream>
#include <iterator>
#include <algorithm>

std::string getString();
std::vector<std::string> tokenize(const std::string & str);
std::vector<int> getPattern();

int main(){
    std::string input;
    std::vector<std::string> words;
    std::vector<int> pattern;
    do{
        if(words.size() != pattern.size()){
            std::cout << "\nError: Number of words must be equal to number of integers in pattern.";
        }
        input = getString();
        words = tokenize(input);
        pattern = getPattern();
    } while(words.size() != pattern.size());

    for(int i = 0; i < words.size(); ++i){
        if(words[i].length() > pattern[i]){
            std::string str = words[i];
            std::cout << str[pattern[i]];
        }
    }

    std::cout << '\n';
    return 0;
}

std::string getString(){
    std::cout << "Seperate words with spaces.\nInput string: ";
    std::string input;
    std::getline(std::cin, input);
    return input;
}

std::vector<std::string> tokenize(const std::string & str){
    std::stringstream ss(str);
    std::istream_iterator<std::string> begin(ss);
    std::istream_iterator<std::string> end;
    std::vector<std::string> words(begin, end);
    return words;
}

std::vector<int> getPattern(){
    std::string pattern;
    std::vector<int> intTokenizedPattern;
    bool validPattern;
    do{
        std::cout << "Seperate positive integers with spaces: ";
        std::getline(std::cin, pattern);
        std::vector<std::string> tokenizedPattern = tokenize(pattern);
        try{
            std::transform(tokenizedPattern.begin(), tokenizedPattern.end(), 
                std::back_inserter(intTokenizedPattern), 
                [](const std::string & str){ return std::abs(std::stoi(str) - 1);});
            validPattern = true;
        } catch(...){
            validPattern = false;
        }
    } while(!validPattern);

    return intTokenizedPattern;
}

In the output above, it is deciphered to say "sail at seven" (null ciphers are not used in the modern age of cryptography).

One problem I came across was with integers in the pattern less than 1. For example, if the number 0 was entered, it would be come -1 in the lambda, and that would create many problems. I patched it haphazardly with an std::abs(), any help there would be useful.

#include <iostream>
#include <vector>
#include <sstream>
#include <iterator>
#include <algorithm>

std::string getString();
std::vector<std::string> tokenize(const std::string & str);
std::vector<int> getPattern();

int main(){
    std::string input;
    std::vector<std::string> words;
    std::vector<int> pattern;
    do{
        if(words.size() != pattern.size()){
            std::cout << "\nError: Number of words must be equal to number of integers in pattern.";
        }
        input = getString();
        words = tokenize(input);
        pattern = getPattern();
    } while(words.size() != pattern.size());

    for(int i = 0; i < words.size(); ++i){
        if(words[i].length() > pattern[i]){
            std::string str = words[i];
            std::cout << str[pattern[i]];
        }
    }

    std::cout << '\n';
    return 0;
}

std::string getString(){
    std::cout << "Seperate words with spaces.\nInput string: ";
    std::string input;
    std::getline(std::cin, input);
    return input;
}

std::vector<std::string> tokenize(const std::string & str){
    std::stringstream ss(str);
    std::istream_iterator<std::string> begin(ss);
    std::istream_iterator<std::string> end;
    std::vector<std::string> words(begin, end);
    return words;
}

std::vector<int> getPattern(){
    std::string pattern;
    std::vector<int> intTokenizedPattern;
    bool validPattern;
    do{
        std::cout << "Seperate positive integers with spaces: ";
        std::getline(std::cin, pattern);
        std::vector<std::string> tokenizedPattern = tokenize(pattern);
        try{
            std::transform(tokenizedPattern.begin(), tokenizedPattern.end(), 
                std::back_inserter(intTokenizedPattern), 
                [](const std::string & str){ return (std::stoi(str) > 0 ? std::stoi(str) - 1 : throw "Input less than one");});
            validPattern = true;
        } catch(...){
            tokenizedPattern.clear();
            intTokenizedPattern.clear();
            validPattern = false;
        }
    } while(!validPattern);

    return intTokenizedPattern;
}

In the output above, it is deciphered to say "sail at seven" (null ciphers are not used in the modern age of cryptography).