C ++不确定如何让我的程序输出空间的位置

Question

Alright, so I have two questions and if anyone can help me out I would greatly appreciate it! This is my first programming class, so it would also be my first C++ class and I'm a bit stuck.

So I created a Caesar cipher that shifts the string that the user inputs to the right by a pseudo random number between 8-15. What the complete program needs to do is give the number it is shifted by at the beginning, followed by the encrypted string. If there are spaces in the string that are inputed, they need to take the letter/number that is before the space and shift it by 4. I need to terminate the encryption with an '&' character and that is followed by a '#' character and then the number location of the first space followed by another '#' character and another location of a second space if there is one and so on.

So for example, if I were encrypting a string that was being shifted by 9 and said:

Hello World 123

It should look like this when encrypted:

9qnuuxsfxaumh012&#6#12

1. My first and more important question. I can't figure out how to make the program output the '#' character followed by the number that tells the location of the space. I've thought of maybe doing some kind of loop that reads the string but I'm coming up blank. If I could get some advice that would be great as this is the only part holding me up from turning this in.

2. My second question comes from a little confusion within my own code that I would love an English interpretation on how it works since I don't understand it myself. I was first using just for loops to make it so that the character 'z' would wrap back around to 'a' but no matter what I did, I kept getting it to only wrap around after a '{' character which is the next character after 'z' on the ascii table. So I decided to change my method and I read on wikipedia under "Caesar cipher" that you could use a modulus. So I used the equation they gave me which was E(x) = (a + b) mod 26. Well it didn't work. So I started to do a google search and saw 2 different posts where people subtracted the character 'a' and then added the chracter 'a' back on at the end as well as added the variable to itself with +=. So I put it in and it worked.
   It looks like this:

    output += ((input[count] - 'a' + n) % 26) + 'a'; 

and I thought it would look like this after reading the wiki and it not working when i put this in

    output = ((input[count] + n) % 26)

Same goes for wrapping the numbers as well:

output += ((input[count] - '0' + n) % 10) + '0';

So if someone could explain to me why I am adding output to itself as well as subtracting 'a' in the beginning and then re-adding 'a' at the end so I could understand what's going on. I really don't like having code in a program that I'm going to turn in that I don't even understand myself.

Anyways, I'm sorry for the long read, I just thought I would explain what's going on and what I need clearly so that anyone willing to help would completely understand what I'm saying without me having to follow up with a second post explaining.

And finally here's the full program that I have written:

#include <ctime>
#include <iostream>
#include <string>
#include <cstdlib>
#include <fstream>
using namespace std;

//random number generator between 8 and 15
int random()
{
    int value;
    value = rand() % (18 - 10) + 8;
    return value;
}

int main()

{
    //Give random() a seed
    srand(time(NULL));

    //Declare variables
    string input;
    int length;
    int n = random();
    string output;

    //Program
    cout << "Enter your secret message: " << endl;
    getline (cin, input);
    cout << n;

    length = input.length();

    for (int count = 0; count < length; count++)
    {
        input[count] = tolower(input[count]);
        if (input[count] >= 'a' && input[count] <= 'z')
        {
            output += ((input[count] - 'a' + n) % 26) + 'a';
        }
        if (input[count] >= '0' && input[count] <= '9')
        {
            output += ((input[count] - '0' + n) % 10) + '0';
        }
        else if(input[count] == ' ')
        {
            if (input[count - 1] >= 'a' && input[count - 1] <= 'z')
            {
                output += ((input[count - 1] - 'a' + 4) % 26) + 'a';
            }
            else if (input[count - 1] >= '0' && input[count - 1] <= '9')
            {
                output += ((input[count - 1] - '0' + 4) % 10) + '0';
            }
            cout << output;
        }
    }
            cout << output << endl;
            return 0;   
}

Thanks so much for anyone willing to help!

Answer 1

Two answer the second question:

input[count] - 'a'

This gives you 0 for the letter a, 1 for the letter b, ... 25 for the letter z.

input[count] - 'a' + n

Then you add the number n. Having "a" as an input and being n==2 you will get a 3. But for a "z" as input you will get a 27.

To solve the problem you use the modulus:

(input[count] - 'a' + n) % 26

The result is a 1 for the "z".

((input[count] - 'a' + n) % 26) + 'a'

Now you transfer the number from 0 to 25 back to the corresponding ASCII code.

Answer 2

The point of the seemingly odd expansion is to do the following:

• Create a number from 0..25: input[count] - 'a'
• Adjust that number by adding your shift amount: + n
• Modulo the result with 26 to wrap overflow of 26+ back into 0..25: % 26
• And finally, add that result back to the base character: + a``

Your idea of a shortcut:

output = ((input[count] + n) % 26)

simply takes the ascii value of the input char, adds the shift, then modulo 26. The result is a value in 0..25, nowhere near the range of 'a'..'z' .

And before you think just adding 'a' would work, it isn't that simple. For example, suppose you had a shift of 9 and in input char of 'z'

The presented formula that works: (ch - 'a' + n) % 26 + 'a'

(('z' - 'a' + 9) % 26 + 'a'
((122 - 97 + 9) % 26 + 97
34 % 26 + 97
8 + 97
105, the ascii value of 'i'

Your formula, with 'a' adjustment: (ch + n) % 26 + 'a'

('z' + 9) % 26 + 'a'
(122 + 9) % 26 + 97
131 % 26 + 97
1 + 97
98, the ascii value for 'b'.

The problem is the distance from the beginning of the char sequence that is being modulo-adjusted is never accounted for in the modulo reduction. Thus the reason for the formula you find odd.

---

Regarding how to accumulate a list of space locations. a ostringstream will make that trivial , as would a std::vector<int> . An example of the former looks like this:

#include <ctime>
#include <iostream>
#include <string>
#include <cstdlib>
#include <fstream>
#include <sstream> // for std::ostringstream
using namespace std;

int main()

{
    //Give random() a seed
    srand(static_cast<unsigned>(time(NULL)));

    //Declare variables
    string input;
    int length;
    int n = rand() % (18 - 10) + 8;
    string output;

    //Program
    cout << "Enter your secret message: " << endl;
    getline (cin, input);
    cout << n;

    // string stream to hold the list of space locations.
    std::ostringstream oss;

    length = input.length();

    for (int count = 0; count < length; count++)
    {
        input[count] = tolower(input[count]);
        if (input[count] >= 'a' && input[count] <= 'z')
        {
            output += ((input[count] - 'a' + n) % 26) + 'a';
        }
        if (input[count] >= '0' && input[count] <= '9')
        {
            output += ((input[count] - '0' + n) % 10) + '0';
        }
        else if(input[count] == ' ')
        {
            if (input[count - 1] >= 'a' && input[count - 1] <= 'z')
            {
                output += ((input[count - 1] - 'a' + 4) % 26) + 'a';
            }
            else if (input[count - 1] >= '0' && input[count - 1] <= '9')
            {
                output += ((input[count - 1] - '0' + 4) % 10) + '0';
            }

            // add space location with preamble to string stream
            oss << '#' << count;
        }
    }

    // append space accumulated list string to the end after '&'
    cout << output << '&' << oss.str() << endl;
    return 0;
}