C Input scanf() confusion - c

Question

I want to use scanf() to read the input string.
If the input of printf("Enter start word: "); is the symbol #, it will not execute the next printf("Enter end word: "); command, and if the input is a word, then it will execute the next command. But I don't know how to determine whether the input is a symbol or a word.
Whatever I input, it still executes printf("Enter end word: ");
#include <stdio.h>
#include <assert.h>
#include <string.h>
#include <stdbool.h>
#include <stdlib.h>
#define MAX_WORD_LENGTH 32
#define FORMAT_STRING "%31s"
#define VERY_HIGH_VALUE 999999
char **InputWords(int *n, char *start, char *end) {
printf("Enter a number: ");
scanf("%d", n); // the number of the input words
char **words = malloc(sizeof(char *) * *n);
assert(words != NULL);
for (int i = 0; i < *n; i++) {
// allocate memory for the length of each input word
words[i] = malloc(sizeof(char) * MAX_WORD_LENGTH);
assert(words[i] != NULL);
printf("Enter a word: ");
scanf(FORMAT_STRING, words[i]);
}
printf("Enter start word: ");
if (scanf("%s", start) == 1) {
printf("Enter end word: ");
scanf("%s", end);
};
printf("\n");
printf("%d, %s, %s", *n, start, end);
return words;
}
int main(void) {
int n;
char start, end; //the start/end word, which is not necessary in stage 1-4
char **words = InputWords(&n, &start, &end); //create an array to store all input words
return 0;
}
Moreover, when I add the code printf("%d", n); at the end of the function, the value of n becomes 0, although my input n is other number.
When I add printf("%d, %s, %s", *n, start, end); at the last, the output shows
Enter a number: 3
Enter a word: bad
Enter a word: ban
Enter a word: dad
Enter start word: bad
Enter end word: ban
110, an, ban
But in my input, the n = 3, start = ban and end = ban

Answer 1

There are multiple problems in the code:
you pass the addresses of single char variables to InputWords instead of arrays of sufficient length. Reading a string into single char variables causes undefined behavior because scanf() will store at least 2 bytes into the destination array.
you should always specify the maximum number of characters to store into the destination arrays for scanf("%s", ...)
to prevent reading the end word you can simply test if the word read for start starts with a #:
*end = *start = '\0';
if (scanf("%31s", start) == 1 && *start != '#') {
printf("Enter end word: ");
scanf("%31s", end);
}
Here is a modified version:
#include <stdio.h>
#include <stdlib.h>
#define MAX_WORD_LENGTH 31
#define FORMAT_STRING "%31s"
char **InputWords(int *np, char *start, char *end) {
int n = 0;
*np = 0;
*start = *end = '\0';
printf("Enter a number: ");
if (scanf("%d", &n) != 1) // the number of the input words
return NULL;
char **words = calloc(sizeof(char *), n);
assert(words != NULL);
for (int i = 0; i < n; i++) {
// allocate memory for the length of each input word
words[i] = calloc(sizeof(char), MAX_WORD_LENGTH + 1);
assert(words[i] != NULL);
printf("Enter a word: ");
if (scanf(FORMAT_STRING, words[i]) != 1) {
/* invalid input or premature end of file */
while (i --> 0) {
free(words[i]);
}
free(words);
return NULL;
}
}
printf("Enter start word: ");
if (scanf(FORMAT_STRING, start) == 1 && *start != '#') {
printf("Enter end word: ");
scanf(FORMAT_STRING, end);
}
printf("\n");
printf("%d, %s, %s\n", n, start, end);
*np = n;
return words;
}
int main() {
int n;
char start[MAX_WORD_LENGTH + 1]; // the start word
char end[MAX_WORD_LENGTH + 1]; // the end word, which is not necessary in stage 1-4
char **words = InputWords(&n, start, end); //create an array to store all input words
return 0;
}

Answer 2

Try this and tell me what doesn't work now:
#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#define FORMAT_STRING "%31s"
#define MAX_WORD_LENGTH 32
char **InputWords(int *n, char *start, char *end) {
printf("Enter a number: ");
scanf("%d", n); // the number of the input words
char **words = malloc((*n) * sizeof(*words));
assert(words);
for (int i = 0; i < *n; i++) {
// allocate memory for the length of each input word
words[i] = malloc(MAX_WORD_LENGTH);
assert(words[i]);
printf("Enter a word: ");
scanf(FORMAT_STRING, words[i]);
}
printf("Enter start word: ");
if (scanf(FORMAT_STRING, start) == 1) {
printf("Enter end word: ");
scanf(FORMAT_STRING, end);
};
printf("\n%d, %s, %s", *n, start, end);
return words;
}
int main(void){
int n;
char start[MAX_WORD_LENGTH];
char end[MAX_WORD_LENGTH];
char **words = InputWords(&n, start, end); //create an array to store all input words
// silence unused variables
(void) n;
(void) start;
(void) end;
return 0;
}