CS50x - PSET2- Substitution - c

Question

I am having troubles with this submission, I don't see a problem with my code and it works fine when I test it but for some reason I don't know when I use check50, it returns this result:
:) substitution.c exists
:) substitution.c compiles
:( encrypts "A" as "Z" using ZYXWVUTSRQPONMLKJIHGFEDCBA as key
Cause
output not valid ASCII text
:( encrypts "a" as "z" using ZYXWVUTSRQPONMLKJIHGFEDCBA as key
Cause
output not valid ASCII text
:( encrypts "ABC" as "NJQ" using NJQSUYBRXMOPFTHZVAWCGILKED as key
Cause
expected "ciphertext: NJ...", not ""
:( encrypts "XyZ" as "KeD" using NJQSUYBRXMOPFTHZVAWCGILKED as key
Cause
expected "ciphertext: Ke...", not ""
:( encrypts "This is CS50" as "Cbah ah KH50" using YUKFRNLBAVMWZTEOGXHCIPJSQD as key
Cause
expected "ciphertext: Cb...", not ""
:) encrypts "This is CS50" as "Cbah ah KH50" using yukfrnlbavmwzteogxhcipjsqd as key
:( encrypts "This is CS50" as "Cbah ah KH50" using YUKFRNLBAVMWZteogxhcipjsqd as key
Cause
expected "ciphertext: Cb...", not ""
:( encrypts all alphabetic characters using DWUSXNPQKEGCZFJBTLYROHIAVM as key
Cause
expected "ciphertext: Rq...", not ""
:) handles lack of key
:) handles invalid key length
:) handles invalid characters in key
:) handles duplicate characters in key
:) handles multiple duplicate characters in key
This results mean that my code outputs the wrong result but when I test the keys and input text myself , it works as it is supposed to, I spent hours trying to figure it out but I am clueless here is my code :
#include <cs50.h>
#include <stdio.h>
#include <string.h>
#include <ctype.h>
#include <math.h>
bool contains(char c, char arr[], int n)
{
for(int i = 0 ;i < n; i++)
{
if(c == arr[i])
{
return true;
}
}
return false;
}
bool validkey(string key)
{
if (strlen(key) != 26 )
{
return false;
}
char l[26];
for (int i = 0; i < 26; i++)
{
if(isalpha(key[i]))
{
if(contains(key[i],l,i))
{
return false;
}
else
{
l[i] = key[i];
}
}
else
{
return false;
}
}
return true;
}
void cypher(string key, string s)
{
int n = strlen(s);
char r[n+1];
char t;
char a = 'a';
int pos = 0;
char w;
for(int i = 0; i < n; i++)
{
if(isalpha(s[i])){
t = tolower(s[i]);
pos = t - a;
if (islower(s[i]))
{
r[i]= tolower(key[pos]);
}
else if (isupper(s[i]))
{
r[i] = toupper(key[pos]);
}
}
else
{
r[i] = s[i];
}
}
r[n]='\0';
printf("ciphertext: %s\n",r);
}
int main(int argc, string argv[])
{
if(argc != 2)
{
return 1;
}
string key = argv[1];
if(!validkey(key))
{
printf("Invalid");
return 1;
}
string q = get_string("plain text: ");
cypher(key,q);
return 0;
}

Answer 1

Your actual problem looks to be the fact that you have overlooked the simple output required if no-input is provided, or if an invalid key is provided, e.g. from Pset2 - Substitution
And what if a user doesn’t provide a valid key?
$ ./substitution ABC
Key must contain 26 characters.
Or really doesn’t cooperate?
$ ./substitution
Usage: ./substitution key
You fail to provide the correct output in each circumstance.
Now your code is a bit rough as pointed out by #EugeneSh in the comments. Your contains() is a bit awkward and superfluous. What you need instead is a simple frequency-array. An array of 26-integers, initialized all zero, where you map the characters in key from 0-25 (by converting the char tolower() and subtracting 'a' so a-z maps to 0-25. Then for each character in key, you simply check if array[tolower(key[i]) - 'a'] then it is a duplicate char and return false. If not, increment that element and check the next char, e.g.:
#define KEYSZ 26
bool validkey (string key)
{
int keychk[KEYSZ] = {0};
if (strlen (key) != KEYSZ) {
return false;
}
for (int i = 0; key[i]; i++) {
int lowerk = tolower(key[i]) - 'a';
if (!isalpha (key[i]) || keychk[lowerk])
return false;
keychk[lowerk]++;
}
return true;
}
Where the keychk[] array is used for that purpose.
Your cypher() function should really return type string so the cipher text is available to the calling function, not simply output. (that isn't an error, just a practical consideration). You can rewrite and simplify your cypher() function (renamed: encipher()) as:
string encipher (string key, string s, string cipher)
{
int i = 0;
for (; s[i]; i++) {
if (isalpha (s[i])) {
int pos = tolower (s[i]) - 'a';
cipher[i] = islower(s[i]) ? tolower(key[pos]) : toupper(key[pos]);
}
else
cipher[i] = s[i];
}
cipher[i] = 0;
return cipher;
}
Those are really the only two functions needed. If you put it altogether you would have:
#include <cs50.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#define KEYSZ 26
bool validkey (string key)
{
int keychk[KEYSZ] = {0};
if (strlen (key) != KEYSZ) {
return false;
}
for (int i = 0; key[i]; i++) {
int lowerk = tolower(key[i]) - 'a';
if (!isalpha (key[i]) || keychk[lowerk])
return false;
keychk[lowerk]++;
}
return true;
}
string encipher (string key, string s, string cipher)
{
int i = 0;
for (; s[i]; i++) {
if (isalpha (s[i])) {
int pos = tolower (s[i]) - 'a';
cipher[i] = islower(s[i]) ? tolower(key[pos]) : toupper(key[pos]);
}
else
cipher[i] = s[i];
}
cipher[i] = 0;
return cipher;
}
int main (int argc, string argv[])
{
if (argc < 2) {
fputs ("Usage: ./substitution key\n", stderr);
return 1;
}
size_t len = 0;
string key = argv[1];
if (!validkey (key)) {
fputs ("Key must contain 26 characters.\n", stderr);
return 1;
}
string plain = get_string ("plaintext: "), cipher;
len = strlen (plain);
cipher = malloc (len + 1);
printf ("ciphertext: %s\n", encipher (key, plain, cipher));
free (cipher);
return 0;
}
(note: main() updated to allocate separately for cipher allocating for the number of characters in plain (+1 for the nul-terminating character) and changed output prompts, e.g. "plaintext " and "ciphertext: " as per the problem, and added #include <stdlib.h> which is not included by default with the online ide.cs50.io CS50 IDE)
Give that a try and let me know if you have problems or need additional help.