To start off, I'm four weeks into a C++ course and I don't even know loops yet, so please speak baby talk?
Okay, so I'm supposed to read a twelve character string (plus NULL makes thirteen) from a file, and then shift the letters backwards three, and then print my results to screen and file. I'm okay with everything except the shifting letters. I don't want to write miles of code to take each character individually, subtract three, and re-assemble the string, but I'm not sure how to work with the whole string at once. Can someone recommend a really simple method of doing this?
If you are dealing with simple letters (A to Z or a to z), then you can assume that the internals codes are linear.
Letters are coded as numbers, between 0 and 127. A is coded as 65, B as 66, C as 67, Z as 90.
In order to shift letters, you just have to change the internal letter code as if it were a number, so basically just substracting 3 from the character. Beware of edge cases though, because substracting 3 to 'A' will give you '>' (code 62) and not 'X' (code 88). You can deal with them using "if" statements or the modulo operator ("%").
Here is an ASCII characters table to help you
Once you've loaded your string in, you can use the modulous operator to rotate while keeping within the confines of A-Z space.
I'd keep track of whether the letter was a capital to start with:
bool isCaps = ( letter >= 'A' ) && ( letter <= 'Z' );
if( isCaps )
letter -= 'A'-'a';
and then just do the cipher shift like this:
int shift = -3;
letter -= 'a'; // to make it a number from 0-25
letter = ( letter + shift + 26 ) % 26;
// add 26 in case the shift is negative
letter += 'a'; // back to ascii code
finally finish off with
if( isCaps )
letter += 'A'-'a';
so, putting all this together we get:
char *mystring; // ciphertext
int shift = -3; // ciphershift
for( char *letter = mystring; letter; ++letter )
{
bool isCaps = ( *letter >= 'A' ) && ( *letter <= 'Z' );
if( isCaps )
*letter -= 'A'-'a';
letter -= 'a';
letter = ( letter + shift + 26 ) % 26;
letter += 'a';
if( isCaps )
letter += 'A'-'a';
}
You're going to have to learn loops. They will allow you to repeat some code over the characters of a string, which is exactly what you need here. You'll keep an integer variable that will be your index into the string, and inside the loop do your letter-shifting on the character at that index and increment the index variable by one until you reach NULL.
Edit: If you're not expected to know about loops yet in your course, maybe they want you to do this:
string[0] -= 3; // this is short for "string[0] = string[0] - 3;"
string[1] -= 3;
string[2] -= 3;
...
It will only result in 12 lines of code rather than miles. You don't have to "reassemble" the string this way, you can just edit each character in-place. Then I bet after making you do that, they'll show you the fast way of doing it using loops.
Iterate over the characters with a for loop. And do what you want with the char*. Then put the new char back.
for(int i=0; i<12; i++){
string[i] = string[i] - 3;
}
Where string is your character array (string). There is a bit more involved if you want to make it periodic (I.E. have A wrap round to Z, but the above code should help you get started)
I'm a little unclear what you mean by "shift the letters backwards 3"?
Does that mean D ==> A?
If so, here's a simple loop.
(I didn't do reading from the file, or writing to the file... Thats your part)
#include <string.h>
int main(void)
{
char input[13] = "ABCDEFGHIJKL";
int i;
int len = strlen(input);
for(i=0; i<len; ++i)
{
input[i] = input[i]-3;
}
printf("%s", input); // OUTPUT is: ">?#ABCDEFGHI"
}
Related
Hi, I don't know how to solve this. I am having problems with the 'cycle' that should occur when z is exceeded or a negative offset is placed that exceeds a. I think I would have to rethink the whole program. Sorry for my english and I hope you can help me or guide me :)
**Write a C++ program (with only if and else) to encode a letter. The encoding consists of returning the letter of the alphabet that is offset places to the right or to the left of the entered letter. The location to the right is given when the offset value is positive. On the other hand, if the offset value is negative the offset will be to the left. An offset value of 0 is considered invalid.
In the right path, after the letter z, the letter a is considered to be coming. The same for the uppercase alphabet, if the entered letter is uppercase.
On the left path, if the letter a is exceeded, the letter z is considered as coming. The same for the uppercase alphabet, if the letter entered is uppercase.
It is required to validate that the values entered are valid:
The value of the offset must be in the range (-26;26) and different from 0.**
let's consider all your letters are not capital which will make things easier.
char* encodeMessage(const char* input, int arraySize, int offset) {
char* encoded = (char*) malloc(arraySize);
for(int i=0;i<sizeof(input) / sizeof(char);i++) {
if(input[i] >= 'a' && input[i] <= 'z') {
char newLetter = input[i] - 'a';
newLetter += offset;
newLetter = newLetter % 26;
if(newLetter < 0) {
newLetter += 26;
}
encoded[i] = newLetter + 'a';
// added this condition to avoid encoding null characters
} else {
encoded[i] = input[i];
}
}
return encoded;
}
you can easily modify it to encode capital letters.
I have a assingment were I need to code and decode txt files, for example: hello how are you? has to be coded as hel2o how are you? and aaaaaaaaaajkle as a10jkle.
while ( ! invoer.eof ( ) ) {
if (kar >= '0' && kar <= '9') {
counter = kar-48;
while (counter > 1){
uitvoer.put(vorigeKar);
counter--;
}
}else if (kar == '/'){
kar = invoer.get();
uitvoer.put(kar);
}else{
uitvoer.put(kar);
}
vorigeKar = kar;
kar = invoer.get ( );
}
but the problem I have is if need to decode a12bhr, the answer is aaaaaaaaaaaabhr but I can't seem to get the 12 as number without problems, I also can't use any strings or array's.
c++
I believe that you are making following mistake: imagine you give a32, then you read the character a and save it as vorigeKar (previous character, I am , Flemish so I understand Dutch :-) ).
Then you read 3, you understand that it is a number and you repeat vorigeKar three times, which leads to aaa. Then you read 2 and repeat vorigeKar two times, leading to aaaaa (five times, five equals 3 + 2).
You need to learn how to keep on reading numeric characters, and translate them into complete numbers (like 32, or 12 in your case).
Like #Dominique said in his answers, You're doing it wrong.
Let me tell you my logic, you can try it.
Pesudo Code + Logic:
Store word as a char array or string, so that it'll be easy to print at last
Loop{
Read - a //check if it's number by subtracting from '0'
Read - 1 //check if number = true. Store it in int res[] = res*10 + 1
//Also store the previous index in an index array(ie) index of char 'a' if you encounter a number first time.
Read - 2 //check if number = true. Store it in res = res*10 + 2
Read - b , h and so on till "space" character
If you encounter another number, then store it's previous character's index in index array and then store the number in a res[] array.
Now using index array you can get the index of your repeating character to be printed and print it for it's corresponding times which we have stored in the result array.
This goes for the second, third...etc:- numbers in your word till the end of the word
}
First, even though you say you can't use strings, you still need to know the basic principle behind how to turn a stream of digit characters into an integer.
Assuming the number is positive, here is a simple function that turns a series of digits into a number:
#include <iostream>
#include <cctype>
int runningTotal(char ch, int lastNum)
{
return lastNum * 10 + (ch -'0');
}
int main()
{
// As a test
char s[] = "a123b23cd1/";
int totalNumber = 0;
for (size_t i = 0; s[i] != '/'; ++i)
{
char digit = s[i]; // This is the character "read from the file"
if ( isdigit( digit) )
totalNumber = runningTotal(digit, totalNumber);
else
{
if ( totalNumber > 0 )
std::cout << totalNumber << "\n";
totalNumber = 0;
}
}
std::cout << totalNumber;
}
Output:
123
23
1
So what was done? The character array is the "file". I then loop for each character, building up the number. The runningTotal is a function that builds the integer from each digit character encountered. When a non-digit is found, we output that number and start the total from 0 again.
The code does not save the letter to "multiply" -- I leave that to you as homework. But the code above illustrates how to take digits and create the number from them. For using a file, you would simply replace the for loop with the reading of each character from the file.
Well currently I am re creating my own version of enigma as a little project but if you understand how the enigma machine works it has rotors which connect a character to a completely different character for example A might be connected to F or U may be connected to C and this is done three times. Currently I am getting the char for the rotor by using this function:
char getRotorOne(char i) {
if(i == 'a') {
return 'g';
}if(i == 'b') {
return 'A';
}if(i == 'c') {
return 'o';
}
The main problem with this is it takes a long time to write and it seems inefficient and I think there must be a better way. The other problem with this is on the original enigma machine there were only the 26 letters of the alphabet on this there are the 94 tapeable characters of ascii (32-126) is there any other simpler way that this can be done? If you think this question is unclear or you don't understand please tell me instead of just flagging my post, then you can help me improve my question.
Use tables! Conveniently, C string literals are arrays of characters. So you can do this:
// abc
const char* lower_mapping = "gAo";
// ABC
const char* upper_mapping = "xyz";
char getRotorOne(char i) {
if (i >= 'a' && i <= 'z') return lower_mapping[i - 'a'];
if (i >= 'A' && i <= 'Z') return upper_mapping[i - 'A'];
assert(false && "Unknown character cannot be mapped!");
}
Since chars are really just small integers, and ASCII guarantees contiguous ranges for a-z and A-Z (and 0-9) you can subtract from a given character the first one in its range (so, 'a' or 'A') to get an index into that range. That index can then be used to look up the corresponding character via a table, which in this case is just a simple hardcoded string literal.
This is an improvement on Cameron's answer. You should use a simple char array for each rotor, but as you said you want to process ASCII characters in the range 32-126, you should build each mapping as an array of 95 characters:
char rotor1[95] ="aXc;-0..."; // the 95 non control ascii characters in arbitrary order
Then you write your rotor function that way:
char getRotorOne(char i) {
if ((i < 32) || (i > 126)) return i; // do not change non processed characters
return rotor1[i - 32]; // i - 32 is in range 0 - 94: rotor1[i - 32] is defined
}
As the post title suggests, I'm working to strengthen my grasp on C++ and character manipulation, this time through creating a Vigenere Cipher. For those unfamiliar with it, it's a fairly simple way to encrypt a text file.
The basic way it works is that there exists a string "key", and each character (in my case at least) is a lowercase alphabetical character. These are stored into an array and are used to "shift" the value of the file being encoded. A character of 'a' will shift the target by 0, while 'z' will shift it by 25. The "shift" is cyclical, meaning that if 'z' is shifted by 'b' (1) it should result in an 'a'.
My current method is found below:
//Assume cipher[] contains "[a][b][c][x ][y ][z ]" Cipher is a <string> object
//Assume ptr[] contains "[0][1][2][23][24][25]
#A whole bunch of includes
char c;
ifstream is;
ofstream os;
is.open(argv[3]) //"myinput.txt"
os.open(argv[4]) //"myoutput.txt"
int i = 0;
while( is.good() ) {
c = is.get();
if( is.good() ) { //did we just hit the EoF?
c = tolower( c - 0 ); //just make sure it's lowercase
c = c + ptr[ i % cipher.size() ] % 26;
if( c> 122 )
c = ( c % 123 ) + 97;
i++;
os.put( c );
}
}
My problem lies in my modulo operations, I believe. Maybe it's because I've spent so much time hashing this out, but I spent hours last night writing this, and then another hour lying in bed trying to wrap my mind around how to effectively create what I want:
grab char.
check char. //let char = 'z'
check the cipher. //let the cipher = 'y'
eval cipher shift //'y' shift value = 24
shift z 24 places (cyclically) //'z'==25, 25+24=49, 49%26=23. 23='x'
HERE IS THE ISSUE: How to do this with ACSII? ('a'=97, z='121')
Imagine that you want to "shuffle" the "ones" digits 0-9 between 20 and 29 by two steps, such that 20 becomes 22, and 29 becomes 21,. How would you do that?
Well, I would subtract 20 [our base number], and then shuffle the remaining digit, and then add 20 back in again.
newnum = num - 20;
newnum %= 10;
newnum += 20;
The same principle would apply for ascii - just that of course the base isn't 20.
I need to store a letter a-z char to its corresponding slot in a size 0 - 25 array in c++. What's the best way to do this without a lot of if statements?
You can determine character index the following way:
int index = yourCharacter-'a';
And then use that index to store what you need
To work out the index just subtract 'a' from a char variable that holds the characters a-z. For example:
char c='x';
int index=(int)(c-'a');
Is this what are you looking for?
char c;
...
arrayName[c - 'a'] = value;
Edit
Not homework, so I'll clarify my answer.
const char * letters = "abcdefghijklmnopqrstuvwxyz";
Stores the letters from 'a' to 'z' in in an array. (Note that the array size is 27, not 26, because of the null character at the end of the string.)
Characters are just integers, so you can do arithmetic on them.
char c = ...;
array[c - 'a'] = c.
Note that upper case characters are distinct from lower case ones, so you'll need to handle them separately (if required).
if (c >= 'A' && c <= 'Z')
c += 'a' - 'A'; // make lower case
If you want to be portable, you can't use the solution proposed by most
others: 'a' to 'z' are not necessarily contiguous. The surest
solution is to look the letter up in a table, and use the index of that
table, e.g.:
char letters[] = "abcdefghijklmnopqrstuvwxyz";
int index = std::find( begin( letters ), end( letters ) - 1, ch )
- begin( letters );
if ( index < size( letters ) - 1 )
// it's good
else
// character wasn't a (lower case) letter.
Note the - 1 for end and size: this is because letters has an
additional '\0' at the end.
Note that in a lot of cases, on a modern machine, it may be just simpler
to use an array of 256 entries; the difference in space isn't likely to
cause you to run out of memory, and the code to manage it will be a lot
simpler.
char letters[26];
for (char c = 'a'; c <= 'z'; c++)
{
letters[c - 'a'] = c;
}