CS50 (cs50x) pset2 Substitution Solution 2020

substitution.c

#include <stdio.h>
#include <cs50.h> // to get plain text
#include <string.h> // for strlen()
#include <ctype.h> // for string operation upper, lower

void Do_substitute(); // for further substitute
void alpha_arr_val(char pos, string key);
// to get the alphabet array value of each plain text element (ex:plaintext = h|alphabet array = 8 {a=0, b=1 .....z=26})

int main(int argc, string argv[])
{
    if (argc == 2) // number of commands in terminal
    {
        if (strlen(argv[1]) == 26)
        {
            for (int i = 0; i < strlen(argv[1]) ; i++) // checking each element of string
            {
    
                if (! isalpha(argv[1][i])) // if contain non-alphabet
                {
                    printf("Key must contain 26 characters.\n");
                    return 1;
                }
    
                for (int j = i + 1 ; j < strlen(argv[1]) ; j++) // checking to the next element of arg[i]
                {
    
                    if (toupper(argv[1][j]) == toupper(argv[1][i])) // checking repeated element
                    {
                        printf("Key must not contain repeated alphabets.\n");
                        return 1;
                    }
                }
            }
            
            Do_substitute(argv[1]);
            
        }
        else
        {
            printf("Key must contain 26 characters.\n");
            return 1;   
        }
        
    }
    else
    {
        printf("Usage: ./substitution key\n");
        return 1;
    }

    return 0;
}


void Do_substitute(string key)
{
    string p = get_string("plaintext: ");

    printf("ciphertext: ");

    for (int i = 0; i < strlen(p); i++)
    {
        if (isalpha(p[i])) // checking p[i] is alphabet
        {
            char x = p[i];
            if (islower(p[i]))
            {
                alpha_arr_val(tolower(x), key); // passing p[i] in lower-case
            }
            else
            {
                alpha_arr_val(toupper(x), key); // passing p[i] in UPPER-CASE
            }


        }
        else
        {
            printf("%c", p[i]); // print the element as it is (such as space | , | special characters | ? | etc...)
        }
    }

    printf("\n");

}
void alpha_arr_val(char pos, string key) // passing p[i] and upper/lower(alpha array)
{
    string alpha = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"; // alphabet array

    for (int i = 0; i < strlen(alpha); i++) // accessing each element in alpha array
    {
        if (islower(pos)) // p[i] is lower
        {
            if (pos == tolower(alpha[i]))
            {
                printf("%c", tolower(key[i]));
            }
        }
        else // for UPPERCASE // p[i] is upper
        {
            if (pos == toupper(alpha[i]))
            {
                printf("%c", toupper(key[i]));
            }
        }
    }
}

amazing

Why on line 29, you are using toupper functions ?

Why on line 29, you are using toupper functions ?

because, principle to Substitution is "The Key must not contain repeated alphabets". But user may accidentally enter repeated alphabets with different cases(like A,a) .so while checking(comparing )the repeated alphabets i make sure that both the alphabets should be of same cases, And if i didn't add "toupper()" the answer will always be false because of their ASCII values ex: A: 063, a: 097. And When "C-compiler" compares two character(alphabets), it uses their ASCII values to compare them internally . That's why i used toupper function to compare the two characters of same case to identify if user entered the same character, regardless of their cases and to maintain the rule of substitution cipher technique. hope you understand the problem.

Why in the for loop did you set j = i+ 1? where did the plus one come from?

also what does char, pos stand for

sorry for all the questions im new and really trying to understand this problem if you answer any ps answer this one. im extremely confused as to what is happening in the alphabet array value method

also what does char, pos stand for

"char" is the keyword in c language to define the type of value or a variable and "char" is the acronym of character, it means it represents in only character of an string (like string is "Hello" then char will be any single alphabet of given string ex:- char maybe "H") just as "int" represents any integer value like 1,2,74 and so on.

Here "pos" holds each character user has entered in the form of string as an input , and matching with the string it already has in it's memory (variable) as shown in line 90 (alphabet array), if match found then it replaces(substitute) the value with given character in the key (key is the non-repeating string given at the time of input.)

Hope this will satisfy your curiosity😉

Why in the for loop did you set j = i+ 1? where did the plus one come from?

suppose string = "Hello"(string(key) which will be given to the system as an input)

Take a look at line 17, i=1; which means i will be holding second character of our input string(e because string[1] is e),
now a rule of substitution is that it's key's" character must not be repeated.

So now we need to check the if there is any repeated character on the string, to compare the repeating characters we need the current character(e) and next character (l).
To write in equation if i=1 which is "e" then i+1(1+1 = 2), i=2 which is "l", hence we get the next character and on the line 29 we are comparing the two character by arranging them in same type (uppercase)
And i write it loop inside loop because in outer loop it holds one character and in the second it compares it all the way up to the last element.
if it find any repeated character it simply abort the program.

I recommend you to debug this program and go through the steps. It will help you for better understanding.

wooow

Just finished this problem on my own. Man, your solution looks waaaaay better than mine. But Im pretty happy knowing I had some of the right instincts. The output function you wrote was especially slick. Nice work, I wanted to se what someone with a better handle on coding would do with it.

Why does it return 1?

perfect bro

on line 26, why does j have to equal to i+1 to test for repeated letters? I don't get the logic there

you can make code smaller for the two functions Do_substitute && alpha_arr_val
void substitution (string key ,string plaintext)
{
printf("ciphertext: ");
for (int i = 0, len = strlen(plaintext); i < len; i++)
{
if (isalpha(plaintext[i]))
{
alpha_converter(key, plaintext[i]);
}
else
{
printf("%c",plaintext[i]);
}
}
printf("\n");
}

---

void alpha_converter (string key, char pi)
{
string alpha = "abcdefghijklmnopqrstuvwxyz";
for (int i = 0, len = strlen(alpha); i < len; i++)
{
if (tolower(pi) == alpha[i])
{
if (islower(pi))
{
printf("%c", tolower(key[i]));
}
else
{
printf("%c", toupper(key[i]));
}
}
}
}

Thanks a lot

How can I solve the error "could not find ciphertext in plaintext"? Any help?

beautiful

what is the point of this :
if (islower(p[i]))
{
alpha_arr_val(tolower(x), key); // passing p[i] in lower-case
}
else
{
alpha_arr_val(toupper(x), key); // passing p[i] in UPPER-CASE
}

on line 68
because we are doing the same thing on the line 94

#line 6 you might consider removing the check_key () function on line 6, seems redundant.
#line 61 you couldmerge the printf("ciphetext: ") statement on line 61 and merged it with that on line 81.

#line 6 you might consider removing the check_key () function on line 6, seems redundant. #line 61 you couldmerge the printf("ciphetext: ") statement on line 61 and merged it with that on line 81.

Dear @Tomi-3-0 ,
Thankyou for your suggestion on #line 6 , the checkey() function was declared but never initialized, I've updated the changes.

#line 61, if we try to merge the printf's on line 61 and line 81, since it is written inside the loop we see "ciphertext : " repeatedly which may be not look pleasing as per viewing experience. But I encourage you to try to merge the lines as per your convenience. :)

Why does it return 1?

return 1 means interrupting the execution of a code and aborting the program. Please notice line no 21,30, 41,48 these printf statements are indicating error, and to terminate the program so that it will not move further we use return 1 (because the main function is initialized with int return type).
return 0 for successfully exiting from the program.

this was posted like a billion years ago

…

On Mon, Nov 13, 2023 at 12:57 PM khoaoaoaoa ***@***.***> wrote: ***@***.**** commented on this gist. ------------------------------ for me, it is kinda simple `#include <cs50.h> #include <ctype.h> #include <stdio.h> #include <string.h> int check_CLIargs(int argc, string argv[]); string encrypt_plaintext(string plaintext, string key); int main(int argc, string argv[]) { int status = check_CLIargs(argc, argv); if (status == 1) { return 1; } string plaintext = get_string("plaintext: "); string ciphertext = encrypt_plaintext(plaintext, argv[1]); printf("ciphertext: %s\n", ciphertext); return 0; } int check_CLIargs(int argc, string argv[]) { if (argc != 2) { printf("Usage: ./substitution key\n"); return 1; } else if (strlen(argv[1]) != 26) { printf("Key must contain 26 characters.\n"); return 1; } for (int i = 0; i < 26; i++) { if (!isalpha(argv[1][i])) { printf("Key must contain 26 alphabetical letters.\n"); return 1; } for (int j = i + 1; j < 26; j++) { if (tolower(argv[1][j]) == tolower(argv[1][i])) { printf("Key must contain 26 distinct letters.\n"); return 1; } } } return 0; } string encrypt_plaintext(string plaintext, string key) { string ciphertext = plaintext; for (int i = 0, n = strlen(plaintext); i < n; i++) { if (islower(plaintext[i])) { ciphertext[i] = tolower(key[plaintext[i] - 97]); } else if (isupper(plaintext[i])) { ciphertext[i] = toupper(key[plaintext[i] - 65]); } } return ciphertext; } ` — Reply to this email directly, view it on GitHub <https://gist.github.com/Latishfaction/b76758b11105e25342c0aac201d6d7e6#gistcomment-4759431> or unsubscribe <https://github.com/notifications/unsubscribe-auth/AHPHDEVJH6QR2OSF53254HTYEJNRHBFKMF2HI4TJMJ2XIZLTSKBKK5TBNR2WLJDHNFZXJJDOMFWWLK3UNBZGKYLEL52HS4DFQKSXMYLMOVS2I5DSOVS2I3TBNVS3W5DIOJSWCZC7OBQXE5DJMNUXAYLOORPWCY3UNF3GS5DZVRZXKYTKMVRXIX3UPFYGLK2HNFZXIQ3PNVWWK3TUUZ2G64DJMNZZDAVEOR4XAZNEM5UXG5FFOZQWY5LFVEYTAMZUG42DMMRUU52HE2LHM5SXFJTDOJSWC5DF> . You are receiving this email because you commented on the thread. Triage notifications on the go with GitHub Mobile for iOS <https://apps.apple.com/app/apple-store/id1477376905?ct=notification-email&mt=8&pt=524675> or Android <https://play.google.com/store/apps/details?id=com.github.android&referrer=utm_campaign%3Dnotification-email%26utm_medium%3Demail%26utm_source%3Dgithub> .

Why does it return 1?

Return 1 is another thing for saying that if the input doesn't meet the requirement, it will assume it is false and stop the function and go the next function rather than repeating it, causing errors, and stopping it from going to the next function. Similarly, return 0 is the opposite, it means it will return true, stop the function, and move to the next function.