Why the loops has a problem (pset2 Substitution) - c

Question

I have been working CS50X pset2 substitution. I think it almost has done. When I input single char such as A or B D and so on... the ciphertext will get the right result. (e.g A will get J, "B" will get T and D will get E and so on...
However, if I input ABC, the ciphertext will only show J and the other cannot show. What did I do wrong?
#include <cs50.h>
#include <stdio.h>
#include <ctype.h>
#include <string.h>
#include <stdlib.h>
int ciphertext = 0;
//Key
//JTREKYAVOGDXPSNCUIZLFBMWHQ
int main(int argc, string argv[])
{
//Check that program was run with one command-line argument
if (argc == 2)
{
string key = argv[1];
//check the key does it validate
for (int i = 0, n = strlen(key); i < n; i++)
{
string plaintext = get_string("plaintext: ");
printf("ciphertext: \n");
int u = 64;
for (int k = 0, p = strlen(plaintext); k < p; k++)
{
if (isupper(plaintext[k]) != 0)
{
for (int j = 0; j < 26; j++)
{
u = u + 1;
//printf("u is %c\n", u);
if (u == plaintext[k])
{
ciphertext = key[j];
printf("Key is %c\n", key[j]);
printf("Plaintext is %c\n", plaintext[k]);
printf("ciphertext is %c\n", ciphertext);
//break;
}
}
}
else
{
printf("%c", plaintext[k]);
}
}
return 0;
}
}
}

Answer 1

Your problems are logic errors that appear to result from changes you attempted to make to the code that got out of hand. You have unnecessary loops and unused variable in your code. You need to change the scope of the u declaration so it is reinitialized on each iteration. You simply need to remove the for (int i = 0, n = strlen(key); i < n; i++) loop altogether, neither i or n is used. The same for ciphertext and l (ell).
You need to move int u = 64; immediately after if (isupper(plaintext[k])) so it is reset on each iteration.
I suspect all the printf statements were for debugging and you really only want the key output. Putting it altogether, you can rearrange your code to:
#include <cs50.h>
#include <stdio.h>
#include <ctype.h>
#include <string.h>
#include <stdlib.h>
int main (int argc, string argv[]) {
if (argc == 2) { /* validate key given */
string key = argv[1];
string plaintext = get_string("plaintext: ");
printf("ciphertext: ");
for (int k = 0, p = strlen(plaintext); k < p; k++)
{
if (isupper(plaintext[k]))
{
int u = 64;
for (int j = 0; j < 26; j++)
{
u = u + 1;
if (u == plaintext[k]) {
printf("%c", key[j]);
break;
}
}
}
else
printf("%c", plaintext[k]);
}
putchar ('\n');
}
}
Example Use/Output
$ ./bin/cs50_cypher2 JTREKYAVOGDXPSNCUIZLFBMWHQ
plaintext: ABD
ciphertext: JTE
Simplify Your Logic
If you think about what the code above is actually doing, it can be reduced to:
#include <cs50.h>
#include <stdio.h>
#include <ctype.h>
#include <string.h>
int main (int argc, string argv[]) {
if (argc < 2) { /* validate key given */
fputs ("usage: ./program key\n", stderr);
return 1;
}
string key = argv[1];
string plaintext = get_string("plaintext: ");
printf("ciphertext: ");
for (int k = 0, p = strlen(plaintext); k < p; k++)
{
if (isupper(plaintext[k]))
putchar (key[plaintext[k] - 'A']);
else
putchar (plaintext[k]);
}
putchar ('\n');
}
Or if you use a ternary operator, your for loop condenses to simply:
for (int k = 0, p = strlen(plaintext); k < p; k++)
putchar (isupper(plaintext[k]) ? key[plaintext[k] - 'A'] : plaintext[k]);
And since there is no need to call strlen(plaintext), you can eliminate string.h entirely and just loop for (int k = 0; plaintext[k]; k++) since plaintext is a nul-terminated string, your entire program can reduce to:
#include <cs50.h>
#include <stdio.h>
#include <ctype.h>
int main (int argc, string argv[]) {
if (argc < 2) { /* validate key given */
fputs ("usage: ./program key\n", stderr);
return 1;
}
string key = argv[1];
string plaintext = get_string("plaintext : ");
printf("ciphertext: ");
for (int k = 0; plaintext[k]; k++)
putchar (isupper(plaintext[k]) ? key[plaintext[k] - 'A'] : plaintext[k]);
putchar ('\n');
}
Handling Lowercase Letters
You know your key entered as the first argument is all uppercase. So if you want to output a lowercase key, you must call tolower() on the key after you have applied the offsets. Since for an uppercase letter, you just want to know the offset within the 26-character key to use, you simply find out how many letters your current letter is from 'A' and get that offset in key[], e.g.
key[plaintext[k] - 'A']
For lowercase input, you need to find out the offset in key for the difference in lowercase letters, and then apply tolower() to the key, e.g.
tolower(key[plaintext[k] - 'a'])
See ASCII Table & Description
Putting that altogether, your for loop can be written as:
for (int k = 0; plaintext[k]; k++) {
if (isupper(plaintext[k]))
putchar (key[plaintext[k] - 'A']);
else if (islower(plaintext[k]))
putchar (tolower(key[plaintext[k] - 'a']));
else
putchar (plaintext[k]);
}
Example Use/Output
$ ./bin/cs50_cypher4 JTREKYAVOGDXPSNCUIZLFBMWHQ
plaintext : AbZ 50
ciphertext: JtQ 50
Look things over and let me know if you have further questions.