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There are other combinations of (upper-case) letters whose sum of ascii values is 2015.

This declaration string ciphertext[text_length]; creates an array of strings. From the context it should be a char array, not a string array. Changing that will likely lead to cascading compile error …

Suggest set a breakpoint in get_letters at if (isalpha(text[i]) == true) and see what happens next. If it is not what you expect, review the man page for the char "is" functions, expecially the functi …

The compiler doesn't differentiate between int and char. char is essentially a subset of integers. I will venture a guess that 'A' does not in fact work (single-quotes makes it a char), whereas "A" do …

One problem is: after a letter is substituted, and text[i] is changed, the j loop continues and might substitute again, depending on the key. After substitution program should break out of the j loop. …

One "problem" is that leeted in the replace function is declared as a Variable Length Array (VLA) The key takeaway for me: ...lifetime of a VLA ends when the declaration goes out of scope Change th …

The segfault is coming because there does not exist argv[1] at this line b = isdigit(argv[1][i]); Wait what???? The function definition declares a string array with 1 element. The index of that elemen …

From the spec: Suppose that a machine sells bottles of Coca-Cola (Coke) for 50 cents and only accepts coins in these denominations: 25 cents, 10 cents, and 5 cents. 30 is an invalid entry. See the f …

This doc shows the correct syntax for the typecast operator in C. Syntax ( type-name ) expression Typecasting is demonstrated in the lecture at around 1:03:00.

Which number is both less than 0 and greater than 9? No number would ever produce "true". To test a value between 0 and 9, the conditional is: if num >= 0 && num <= 9

Program doesn't know if all the characters in key are digits until after it has processed all of them. The "Success" message should come after the for loop is finished. And remember this hint from the …

The program is trying to use argv[1] here int l = strlen(argv[1]); before it knows whether argv[1] exists (if (argc != 2)).

if/else is binary. true/false. An If cannot have two else clauses. It needs an else if to add additional conditions, as described here. [The page is for C++, but the lesson is valid in C]

Looks like l is trending high. It counts every character in the text except the terminating null byte (which never makes it into the loop anyway!). Check out man isalpha to get back on track. This ma …

To see the whole picture, add text to the printf. From man scanf re the %s directive: s Matches a sequence of non-white-space characters; the next pointer must be a pointer to character array that is …