Halt while loop if the condition is not met - c

Question

I'm trying to add a while loop inside the program. After it executes it's supposed to ask you if you want to continue and type in "y" if you do and "n" if you don't. If they do it's supposed to start over again, if not then it stops.
The thing is I wanted to make a message pop up if someone types in something other then "y" or "n" to require them to do that. The problem is the while loop will go on whatever they write in.
char cnd[100];
while(cnd[0] != 'n') {
printf("Program executing\n");
printf("Would you like to launch the program again? If not then type in 'n', if you do then type in 'y': ");
scanf("%s", &cnd);
while(cnd[0] != 'n' || cnd[0] != 'y') {
printf("You have to type in either 'y' or 'n':");
scanf("%s", &cnd);
}
}
return 0;
I tried to even test this by printing the user input before and after the statement. It does seem to be correct, so I don't know why the while loop won't work.

Answer 1

Let's translate your loop condition to plain English:
cnd[0] != 'n' || cnd[0] != 'y'
Basically, you're saying:
If the first character in cnd is something other than n, or the first character in cnd is something other than y, enter the loop.
This is unfalsifiable since cnd[0] will always be either not n or not y - it can't be both at the same time.
You should instead be asking:
If the first character in cnd is something other than n, and the first character in cnd is something other than y, enter the loop.

Answer 2

your while loop checks if it isn't y or isn't n one of those is always true try this instead
while(cnd[0] != 'n' && cnd[0] != 'y') {

Answer 3

The condition cnd[0] != 'n' || cnd[0] != 'y' is always true. You meant to use && (logical and).
You also need to drop & from &cnd (in both scanf calls) as that'd result in format specifier mismatch.
Also in the very first iteration cnd is uninitialized. You're better off using a do ..while loop or start cnd with "y" such as: char cnd[100] = "y";
Note the problems with scanf too.

Answer 4

read what you wrote (as code, not as english)
while(cnd[0] != 'n' || cnd[0] != 'y') {
if cnd[0] = 'n' then it doesnt = 'y' and vice versa.
you mean
while(cnd[0] != 'n' && cnd[0] != 'y') {
ie give the error is its not n and its not y

Answer 5

First and foremost: Why do you read a string when you want a single character? Use getchar(). Then check if there is other garbage after that character in the stream and act accordingly: Error message, clearing stdin:
#include <stdbool.h>
#include <ctype.h>
#include <stdio.h>
void clear(FILE *stream)
{
int ch;
// eat everything until a newline or EOF is encountered:
while ((ch = fgetc(stream)) != EOF && ch != '\n');
}
bool only_whitespace(FILE *stream)
{
int ch;
// eat all whitespace until a newline or EOF is encountered:
while ((ch = fgetc(stream)) != EOF && isspace((char unsigned)ch) && ch != '\n');
// ^^^^^^^^^^^^^^ don't pass signed values
// to the functions from <ctype.h>
if (ch == '\n') { // when the last character read was a newline
ungetc(ch, stream); // put it back into the stream
return true; // and tell the caller that all we encountered
} // was whitespace
return false;
}
int main(void)
{
int again;
do {
puts("Program executing\n");
while (printf("Would you like to launch the program again? If not then type in 'n', if you do then type in 'y': "),
(again = getchar()) != EOF && again != 'y' && again != 'n' || !only_whitespace(stdin))
{
fputs("Input error!\n\n", stderr);
if (again != '\n') // when the character read was not a newline
clear(stdin); // there could be other garbage left in the stream
}
clear(stdin);
} while (again == 'y');
// return 0; // Since C99 (i think) the effect of execution reching the end of main() without
// encountering a return-statement is the same as return 0; (same in C++)
}