Using c code to make a caesar cipher? - c

Question

Hello good people of the stack overflow.
Explanation
I am trying to to the following.
My program needs to take a stream of letters as the input and then for the output rotate 13 paces forward.
For example
A becomes N
B becomes O
C becomes P
D becomes Q
and so on
For this program I need to use the ascii table. So for example lower case a=97 once my program is done it becomes n=110
I wrote a little formula for this
c=(c+13-97)% 26+97 where c is my letter. as you can see if c=97 then c will end up being 110.
So here is my program
As it is seen I used an if statment to determine if I have a capital or lower case letter.
/* 97 is lower case a in ascii and 122 is lower case z*/
# include <stdio.h>
int main() {
char c;
printf("please enter a character:");
while (c!=-1) {
c=getchar();
putchar (c);
if ( c>=97 && c<=122) {
c=(c+13-97)% 26+97 ;
printf("%c \n " ,c);
}
else
c=(c+13-65) % 26 +65 ;
printf("%c \n " ,c);
}
return 0;
}
My problem is with the output for example if I plug in a
instead of n I get
an
n
1

Answer 1

Your code is absolutely fine with some minor changes in it.
/* 97 is lower case a in ascii and 122 is lower case z*/
# include <stdio.h>
int main()
{
char c;
printf("please enter a character:");
while (c!=-1)
{
c=getchar();
//putchar (c); // Avoid Printing entered character
if ( c>=97 && c<=122)
{
c=((c+13-97)% 26)+97 ;
printf("%c \n " ,c);
}
else
{ // Braces missing
c=((c+13-65) % 26) +65 ;
printf("%c \n " ,c);
}
return 0;
}
}
Output :
please enter a character : a
n

Answer 2

One other issue that you will run into is the need to flush the \n remaining in the input buffer before reading the next character. Basically, what occurs is after a character is entered and the user presses Enter, c is read from the input buffer, but '\n' remains. So the next time the loop is entered \n is already present in the input buffer and is taken as c for that iteration.
To prevent this from occurring, the easiest way to flush the input buffer is to declare a throw-away int say int flush and following each read of a character, add the following:
do { flush=getchar(); } while (flush != '\n');
This will extract all remaining characters from the input buffer preparing it for the next read. (Note: fflush does not do this for input streams) An implementation with this incorporated is:
#include <stdio.h>
int main () {
int c = 0; /* Always initialize variables */
int flush = 0;
int new = 0;
printf ("\nPlease enter a character (ctrl+d to exit):\n\n");
while (printf (" char: ") && (c = getchar()) != -1) {
do { flush=getchar(); } while (flush != '\n'); /* flush input buffer */
if (c >= 'a' && c <= 'z')
{
new = (c + 13 - 97) % 26 + 97;
printf ("\t lower-case '%c' becomes: '%c'\n\n", c, new);
}
else if (c >= 'A' && c <= 'Z')
{
new = (c + 13 - 65) % 26 + 65;
printf ("\t upper-case '%c' becomes: '%c'\n\n", c, new);
}
else
{
printf ("\n invalid character, try again\n\n");
}
}
printf ("\n\nexiting.\n\n");
return 0;
}
output:
Please enter a character (ctrl+d to exit):
char: a
lower-case 'a' becomes: 'n'
char: b
lower-case 'b' becomes: 'o'
char: n
lower-case 'n' becomes: 'a'
char: o
lower-case 'o' becomes: 'b'
char: A
upper-case 'A' becomes: 'N'
char: B
upper-case 'B' becomes: 'O'
char: N
upper-case 'N' becomes: 'A'
char: O
upper-case 'O' becomes: 'B'
char:
exiting.
Good luck with your project. Drop a comment if you run into additional problems.
Multiple Character Version Using getline
Regarding your question about line input. Here is a version using getline to read multiple characters from stdin:
#include <stdio.h>
int main () {
int new = 0;
ssize_t nread = 0; /* number of chars read by getline */
char *line = NULL; /* string holding chars - getline allocates when NULL */
size_t n = 0; /* limit number of bytes to (ignored when 0) */
char *p = NULL; /* point to use to iterate over each char in line */
int index = 0; /* simple index for formatted output. */
printf ("\nPlease enter characters to translate (ctrl+d to exit):\n");
while (printf ("\n input: ") && (nread = getline (&line, &n, stdin) != -1)) {
index = 0; /* reset index */
p = line; /* assign pointer to line */
printf ("\n"); /* just because it looks nice */
while (*p != '\n') /* getline consumes the '\n' */
{
if (*p >= 'a' && *p <= 'z')
{
new = (*p + 13 - 97) % 26 + 97;
printf ("\t char[%2d] : %c => %c\n", index, *p, new);
}
else if (*p >= 'A' && *p <= 'Z')
{
new = (*p + 13 - 65) % 26 + 65;
printf ("\t char[%2d] : %c => %c\n", index, *p, new);
}
else
{
printf ("\n char[%2d] : %c => invalid character\n", index, *p);
}
p++;
index++;
}
}
printf ("\n\nexiting.\n\n");
return 0;
}
output:
Please enter characters to translate (ctrl+d to exit):
input: aAbBcCmMnNoO
char[ 0] : a => n
char[ 1] : A => N
char[ 2] : b => o
char[ 3] : B => O
char[ 4] : c => p
char[ 5] : C => P
char[ 6] : m => z
char[ 7] : M => Z
char[ 8] : n => a
char[ 9] : N => A
char[10] : o => b
char[11] : O => B
input:
exiting.

Answer 3

The while loop should be
while ((c=getchar()) != EOF)
Your code is checking the value of c before c is initialized. You should have gotten a warning about that.
You should not be using hard-coded ASCII values in your code. Lower case a is 'a'. Lower case z is 'z'. So, for example, you can write
if ( c >= 'a' && c <= 'z' )
Note that stdin buffers characters until you press the return key. When you do press the return key, getchar will give you the character that you typed followed by a newline '\n' character. Your code only handles lower case and upper case characters. You need to modify the code to handle non-alpha characters correctly, e.g. spaces, punctuation, newlines.