Replacing Characters in a String (still need help as of 20th)

[QK11nhu]

Relatively new to C++. I posted a previous question related to this before, and have since continued to work on the problem, to no avail.

Basically, I need a function that takes a string as input, then replaces the letters of the string based on another function.

This other function uses a string to define the characters that may be used in the input,

const string ALPHABET = "ABCDEFGHIJKLMNOPQRSTUVWXYZ.,' ";

then creates a permutation by randomly reorganising the letters.

string create_permutation(string permutation)
{
	unsigned seed = chrono::system_clock::now().time_since_epoch().count();
	shuffle(permutation.begin(), permutation.end(), default_random_engine(seed));
	return permutation;
}

string permu = create_permutation(ALPHABET);

How do I essentially encrypt the message input, by looking up the alphabet letter, and then replacing the input letter with the corresponding permutation letter?

E.G.

Input is 'HELLO'
const string ALPHABET = "ABCDEFGHIJKLMNOPQRSTUVWXYZ.,' ";
permutation of ALPHABET = "BUYWPLJDKIXQSZGAVCEFN'MR,OT .H";
Output should be 'DPFFG' (D replaces H's place, P replaces E's place, etc)

Below is the full code used. Note that the inverse permutation would then be used to decrypt the encrypted message.

#include <iostream>
#include <string>
#include <algorithm>
#include <random>

using namespace std;

const string ALPHABET = "ABCDEFGHIJKLMNOPQRSTUVWXYZ.,' ";

string create_permutation(string permutation)
{
	cout << "Input seed" << endl;
	unsigned seed;
	cin >> seed;
	shuffle(permutation.begin(), permutation.end(), default_random_engine(seed));
	return permutation;
}

string findPermu2(const string& permu1)
{
	string permu2(ALPHABET);
	for (int i = 0; i<ALPHABET.length(); ++i)
		for (int j = 0; j<ALPHABET.length(); ++j)
			if (permu1[i] == ALPHABET[j])
			{
				permu2[j] = ALPHABET[i];
				break;
			}
	return permu2;
} 

int main()
{
	string permu = create_permutation(ALPHABET);
	string invpermu = findPermu2(permu);

	cout << "Original ALPHABET:        - " << ALPHABET << endl;
	cout << "Permutation of ALPHABET:  - " << permu << endl;
	cout << "Inverse of Permutation:   - " << invpermu << endl;
	cout << endl << "Input any single character to begin" <<
		endl << "Encryption / decryption, or '\Q'\ to [Q]uit" << endl;

	char enterorquit;
	cin >> enterorquit;
	if (enterorquit == 'q' || enterorquit == 'Q')
	{
		return 0;
	}
	else
	{
		string inputmessage;
		cout << "Type your message below" << endl;
		cin >> inputmessage;
		cout << endl << "Input \'E\' to [E]ncrypt, any other single character to decrypt:" << endl;
	}
	char encryptordecrypt;
	if (encryptordecrypt == 'e' || encryptordecrypt == 'E')
		{
			cout << "Encrypted message: " << endl /*<< encrypted message here*/;
		}
		else
		{
			cout << "Decrypted message: " << endl /*<< decrypted message here*/;
		}
					return 0;
}

[keskiverto]

foo = "ABCDEFGHIJKLMNOPQRSTUVWXYZ.,' ";
bar = "BUYWPLJDKIXQSZGAVCEFN'MR,OT .H";

At what position x is 'H' in foo? 7
What is at position x in bar? bar[7] == 'D'

[QK11nhu]

That makes sense, but how would that be used to change a string's letters?

Something like this?

foo = "ABCDEFGHIJKLMNOPQRSTUVWXYZ.,' ";
bar = "BUYWPLJDKIXQSZGAVCEFN'MR,OT .H";
input = 'HELLO'

for (i = 0, i<input.length, ++i)
{
/* something to represent input[i] being replaced by (foo[i] == bar[i]) */
}

[fredk]

In a loop over the input array, use
int k = strchr( foo, input[j] );
to find the index of the replacement, and then set
input[j] = bar[k];
But be sure to check that k is >= 0 and decide what to do if it isn't (i.e., if the jth character of input is not in the foo array)

[QK11nhu]

Is this correct?

for (int i = 0; i < inputmessage.length; ++i)
			{
				int j;
				if (int k = strchr(ALPHABET, inputmessage[j]))
				{
					inputmessage[j] = permu[k];
				}	
				else 
				{
					cout << "Invalid characters detected in string" << endl;
				}
			}

strchr is stating an error - no instance of overloaded function "strchr" matches the argument list
argument types are: (const std::string, char)

[QK11nhu]

Still need assistance, if anyone could lend a hand.

[D80DSL3A]

The letter j in fredk's example corresponds to your letter i.
He said check if k>=0. That's what goes in the if().
The function strchr() needs to find the index in ALPHABET where the letter inputmessage[j] appears.
I don't know this function, but the findEle() function from the other thread does just this. Case 3 alone is fine, just brute force it.

for (int i = 0; i < inputmessage.length(); ++i)
{
	int k = strchr(ALPHABET, inputmessage[j]);
	if ( k >= 0 )
	{
		inputmessage[i] = permu[k];
	}	
	else 
	{
		cout << "Invalid characters detected in string" << endl;
	}
}

[QK11nhu]

Strchr is again producing the same error message.

Error: no instance of overloaded function "strchr" matches the argument list
argument types are: (const std::string, char)

if (enterorquit == 'q' || enterorquit == 'Q')
	{
		return 0;
	}
	else
	{
		string inputmessage;
		cout << "Type your message below" << endl;
		cin >> inputmessage;
		cout << endl << "Input \'E\' to [E]ncrypt, any other single character to decrypt:" << endl;
		char encryptordecrypt;
		cin >> encryptordecrypt;
		if (encryptordecrypt == 'e' || encryptordecrypt == 'E')
		{
			for (int i = 0; i < inputmessage.length; ++i)
			{
				int k = strchr(ALPHABET, inputmessage[i]);
				if ( k >= 0 )
				{
					inputmessage[i] = permu[k];
				}	
				else 
				{
					cout << "Invalid characters detected in string" << endl;
				}
			}
			cout << "Encrypted message: " << endl << inputmessage;
		}
		else
		{
			cout << "Decrypted message: " << endl /*<< decrypted message here*/;
		}
	}
		return 0;
}

[Moschops]

Here is the function strchr: http://www.cplusplus.com/reference/cstring/strchr/

Take a look at what input parameters it accepts. A char-pointer, and an int.

Take a look at how you're trying to use it: strchr(ALPHABET, inputmessage[i]);

ALPHABET is an object of type string. Is a string an object of type char-pointer? No, it is not.

Understand the functions you're trying to use. Look at the function documentation, see what kinds of parameters it accepts, and write code accordingly.

[QK11nhu]

Thanks, I understand why the error is showing now. I wonder if there is some way to manipulate ALPHABET so that it would be acceptable with strchr? Or if not, if you've encountered another function that would be better suited.

[keskiverto]

You do "wonder" what operations does a std::string support?

Read: http://www.cplusplus.com/reference/string/string/


@fredk: What made you suggest the strchr (a C function)? Did you notice that the ALPHABET is a std::string?

[fredk]

@keskiverto: I missed the fact that ALPHABET was a string. The OP can use ALPHABET.find_first_of(c,0) to find the index, if it exists.