how to check if a string has matching values with a vector-CodePudding

I'm trying to use a for loop and an if statement to compare if any values in the string match with any in the vector. It only outputs "we have a match" if the first character in teststring is in the vector. Basically, it seems like the searching stops if the first character value in teststring isn't in the SpecialChars vector. Sorry if that is confusing, I can't think of a better way to explain it.

#include <iostream>
#include <vector>

void test()
{
    std::vector<char> SpecialChars = {'!', '@', '#', '$', '%', '^', '&', '8', '(', ')', '-', ' ', '='};
    std::string teststring = "loll!@";
    bool test = false;

    for (int i = 0; i < SpecialChars.size(); i  )
    {
        if (teststring[i] == SpecialChars[i])
        {
            std::cout << "We have a match \n";
        }
    }
}

CodePudding user response：

teststring has a different length than SpcialChars, and teststring is shorter, so your for loop will go out of bounds of teststring when i reaches 6, causing undefined behavior.

You need 2 loops, one to iterate teststring, and one to iterate SpecialChars, eg:

#include <iostream>
#include <vector>
#include <string>

void test()
{
    std::vector<char> SpecialChars = {'!', '@', '#', '$', '%', '^', '&', '8', '(', ')', '-', ' ', '='};
    std::string teststring = "loll!@";
    bool found = false;

    for (size_t i = 0; i < teststring.size();   i)
    {
        for (size_t j = 0; j < SpecialChars.size();   j)
        {
            if (teststring[i] == SpecialChars[j])
            {
                found = true;
                break;
            }
        }
        if (found) break;
    }

    if (found)
        std::cout << "We have a match\n";
}

You can simplify the outer loop by using a range-for loop, and eliminate the inner loop by using the standard std::find() algorithm, eg:

#include <iostream>
#include <vector>
#include <string>
#include <algorithm>

void test()
{
    std::vector<char> SpecialChars = {'!', '@', '#', '$', '%', '^', '&', '8', '(', ')', '-', ' ', '='};
    std::string teststring = "loll!@";
    bool found = false;

    for (char ch : teststring)
    {
        if (std::find(SpecialChars.begin(), SpecialChars.end(), ch) != SpecialChars.end())
        {
            found = true;
            break;
        }
    }

    if (found)
        std::cout << "We have a match\n";
}

Alternatively, you could change SpecialChars into a std::string and use std::string::find() on it, eg:

#include <iostream>
#include <string>

void test()
{
    std::string SpecialChars = "!@#$%^&8()- =";
    std::string teststring = "loll!@";
    bool found = false;

    for (char ch : teststring)
    {
        if (SpecialChars.find(ch) != std::string::npos)
        {
            found = true;
            break;
        }
    }

    if (found)
        std::cout << "We have a match\n";
}

Alternatively, you can use a lookup table, like @paddy suggested, eg:

#include <iostream>
#include <string>
#include <climits>

void test()
{
    std::string SpecialChars = "!@#$%^&8()- =";
    std::string teststring = "loll!@";
    bool found = false;

    bool isSpecial[1 << CHAR_BIT] = {};
    for(unsigned char ch : SpecialChars)
        isSpecial[ch] = true;

    for (unsigned char ch : teststring)
    {
        if (isSpecial[ch])
        {
            found = true;
            break;
        }
    }

    if (found)
        std::cout << "We have a match\n";
}