Stuck in endless loop, not sure what I did wrong - c

Question

Program compiles, but when I try to use it, I get an endless loop. Where did i do wrong. I put comments into what I am trying to accomplish.
I have tried changing it to a for loop, but I still get issues. I think I am suppose to stick with a while loop to accomplish what I am trying to accomplish.
//Declare the arrays to hold the strings
char str[21], vowels[21], consonants[21];
int i=0;
//Declare the pointers
char *strPointer, *vowelPointer, *consonantPointer;
//Print out the prompt to the user
printf("Enter a string (20 characters maximum): ");
//Scan the user input into str
//Only allow 20 characters
scanf("%s", str);
//Set strPointer to the beginning of the user's string
strPointer = str;
//Set vowelPointer to the beginning of the vowels string
vowelPointer = vowels;
//Set consonantPointer to the beginning of tht consonant string
consonantPointer = consonants;
//Loop through the user's string until the end of the string
while(*strPointer !='\0')
{
//Check if what strPointer is pointing to is a vowel
if(strPointer[i]=='A'||strPointer[i]=='a'||strPointer[i]=='E'||strPointer[i]=='e'||strPointer[i]=='I'||strPointer[i]=='i'||strPointer[i]=='O'||strPointer[i]=='o'||strPointer[i]=='U'||strPointer[i]=='u')
{
//Move the letter from strPointer to vowelPointer
strPointer=vowelPointer
;
//Move the vowelPointer
vowelPointer=vowels
;
}
else
{
//Move the letter from strPointer to consonantPointer
strPointer=consonantPointer
;
//Move the consonantPointer
consonantPointer=consonants
;
}
//Move the strPointer
strPointer=str;
}
//Add null terminators where appropriate
strPointer[21]='\0';
str[21]='\0';
//Set the vowel and consonant pointers back to the beginning of their strings
vowelPointer[0];
consonantPointer[0];
//Print the original string and the resulting vowel and consonant strings
printf("Original string: %s\n", str);
printf("%s\n", vowelPointer);
printf("%s\n", consonantPointer);
The output is explained in my printf statements at the end. Input string, reprinted with the vowels and consonants separated and listed.

Answer 1

I would like to make some suggestions to help you out.
First, when you are scanning from user input, it is wise to use a buffer. Then, strcpy the contents of the buffer into the char array. This will help to prevent overflow. Please see the below code for more details about this topic.
Second, you can use a for loop to iterate through each of the letters. I hope my answer will help you understand what I mean.
#include <stdio.h>
#include <ctype.h>
#include <string.h>
int main(void)
{
//Declare the arrays to hold the strings
char c, str[21], vowels[21], consonants[21], buffer[21];
int i = 0, j = 0, h = 0;
//Declare the pointers
char *strPointer, *vowelPointer, *consonantPointer;
//Print out the prompt to the user
printf("Enter a string (20 characters maximum): ");
//Scan the user input into str
scanf("%s", buffer);
// Copy the buffer into the str
strcpy(str, buffer);
// go letter by letter checking if it is a vowel or consonant
for (int i = 0; str[i]; i++)
{
// make the character uppercase
c = toupper(str[i]);
// if the letter is a vowel add the letter to the vowel array,
// then increase the position
if (c == 'A' || c == 'E' || c == 'I' || c == 'O' || c == 'U')
vowels[j++] = str[i];
else
// add the letter to the consonants array, then increase the position
consonants[h++] = str[i];
}
// properly terminate the strings
vowels[j] = '\0';
consonants[h] = '\0';
//Print the original string and the resulting vowel and consonant strings
printf("Original string: %s\n", str);
printf("%s\n", vowels);
printf("%s\n", consonants);
return 0;
}

Answer 2

You invoke an infinite loop by setting:
strPointer=str;
at the end of your while (*strPointer !='\0') loop. This resets the address held by strPointer to the beginning of str. strPointer is never incremented, so your loop loops with strPointer pointing to the first character in str over-and-over-and-over again...
Further, you initialize vowelPointer=vowels; which sets the address of vowelPointer to point to the beginning element of the uninitialized vowels array. The same occurs with consonantPointer=consonants;
Since both vowelPointer and consonantPointer point to arrays declared with automatic storage duration, you invoke Undefined Behavior when you attempt to access the uninitialized values with:
printf("%s\n", vowelPointer);
printf("%s\n", consonantPointer);
(but luckily you never get there because of your endlesss loop over while (*strPointer !='\0'))
Additionally, validate EVERY input and affirmatively protect your array bounds by using the field-width modifier with scanf (or better, use fgets()). For example:
//Only allow 20 characters
if (scanf( "%20s", str) != 1) {
fputs ("error: (user canceled input)\n", stderr);
return 1;
}
(note: when reading with "%s" the trailing '\n' is not consumed and will remain in your input buffer)
To fix your pointer problems, you can do something similar to the following:
//Declare the arrays to hold the strings
char str[21], vowels[21], consonants[21];
size_t vidx = 0, cidx = 0; /* indexes for vowels/consonants */
...
//Loop through the user's string until the end of the string
while (*strPointer)
{
//Check if what strPointer is pointing to is a vowel
if (strPointer[i]=='A'|| strPointer[i]=='a'||
strPointer[i]=='E'|| strPointer[i]=='e'||
strPointer[i]=='I'|| strPointer[i]=='i'||
strPointer[i]=='O'|| strPointer[i]=='o'||
strPointer[i]=='U'|| strPointer[i]=='u')
{
//Copy the letter from strPointer to vowelPointer
if (vidx < 20) {
vowelPointer[vidx] = *strPointer;
vidx++;
}
/* or using pointer arithmetic */
// if (vowelPointer - vowels < 20) {
// *vowelPointer = *strPointer;
// vowelPointer++;
// }
}
else {
//Copy the letter from strPointer to consonantPointer
if (cidx < 20) {
consonantPointer[cidx] = *strPointer;
cidx++;
}
/* same alternative available for consonantPointer */
}
//Move the strPointer
strPointer++;
}
//Add null terminators where appropriate
vowelPointer[vidx] = 0;
consonantPointer[cidx] = 0;
//Reset the ponters
vowelPointer = vowels;
consonantPointer = consonants;
//Print the original string and the resulting vowel and consonant strings
printf("Original string: %s\n", str);
printf("%s\n", vowelPointer);
printf("%s\n", consonantPointer);
If you are still stuck, you can put it altogether in a short, slightly more concise example as:
#include <stdio.h>
#include <ctype.h>
#define MAXC 1024 /* don't skimp on buffer size */
int main (void) {
char str[MAXC], cons[MAXC], vowels[MAXC], /* arrays */
*strptr = str, *consptr = cons, *vowelptr = vowels; /* pointers */
fputs ("Enter a string (1022 characters maximum): ", stdout);
if (!fgets (str, MAXC, stdin)) { /* validate EVERY read */
fputs ("error: (user canceled input)\n", stderr);
return 1;
}
while (*strptr) { /* loop over each character */
char lc = tolower (*strptr); /* convert to lowercase to compare */
if (lc == 'a' || lc == 'e' || lc == 'i' || lc == 'o' || lc == 'u')
*vowelptr++ = *strptr; /* copy vowel to array */
else if (!isspace (*strptr)) /* otherwise if not whitespace */
*consptr++ = *strptr; /* copy to consonant array */
strptr++; /* advance string pointer */
}
*vowelptr = *consptr = 0; /* nul-terminate arrays */
printf ("str : %scons : %s\nvowels: %s\n", str, cons, vowels);
}
(question: do you know why there is no '\n' needed after "str : %s" in the printf above?)
Note above that the character is converted to lowercase before comparison for a vowel to cut in-half the number of conditionals needed to check for a vowel. Don't skimp on buffer size. Generally you have 1M of stack space (4M on Linux). Use a buffer of at least 256-chars, or a simple 1K buffer as used above. Also note if you wanted to print using the pointer, you would simply reset them to point to the original arrays immediately after exiting the loop, e.g.
strptr = str;
consptr = cons;
vowelptr = vowels;
Regardless, you can print using the arrays or the pointers after resetting them as both will both point to the same address. See C11 Standard - 6.3.2.1 Other Operands - Lvalues, arrays, and function designators(p3)
Example Use/Output
$ ./bin/sepvowels
Enter a string (1022 char max): A quick brown fox jumps over the lazy dog
str : A quick brown fox jumps over the lazy dog
cons : qckbrwnfxjmpsvrthlzydg
vowels: Auioouoeeao