Convert phone number from alphabetic to numeric form - c

Question

I'm doing an exercise from KNKings book "C Programming: A modern approach" which involves converting a phone number in alphabetic form, entered by the user, into numeric form. When the program encounters non-alphabetic characters (digits or punctuations, for example), it should leave them unchanged. I may assume that the user only enters upper-case letters.
However, my program seems to produce garbage, to say the least.
#include <stdio.h>
#define MAX_SIZE 50
int main(void)
{
char alphabetic[MAX_SIZE], ch;
int num_elements = 0;
printf("Enter phone number: ");
int i;
for (i = 0; i < MAX_SIZE && ((ch = getchar()) != '\n'); i++){
alphabetic[i] = ch;
num_elements++;
}
for (i = 0; i <= num_elements; i++){
switch (alphabetic[i]){
case 'A': case 'B': case 'C': alphabetic[i] = '2'; break;
case 'D': case 'E': case 'F': alphabetic[i] = '3'; break;
case 'G': case 'H': case 'I': alphabetic[i] = '4'; break;
case 'J': case 'K': case 'L': alphabetic[i] = '5'; break;
case 'M': case 'N': case 'O': alphabetic[i] = '6'; break;
case 'P': case 'R': case 'S': alphabetic[i] = '7'; break;
case 'T': case 'U': case 'V': alphabetic[i] = '8'; break;
case 'W': case 'X': case 'Y': alphabetic[i] = '9'; break;
default: break;
}
}
printf("%s\n", alphabetic);
return 0;
}
In particular, I enter: COLLECT-800.
It outputs something like this: u░#■   ║k ╩
What did I do wrong?

Answer 1

You have the right idea, but there are two things missing in your program:
Most importantly, the null terminator at the end of the string. After your for loop in which you read the number, add the line:
alphabetic[i] = '\0';
If the user enters lowercase letters, they are ignored in the switch statement. To get around this, include <ctype.h> and change the switch quantity from alphabetic[i] to toupper(alphabetic[i]). Calling toupper on an already upper case letter is benign.

Answer 2

You're not putting a null-terminator anywhere so it's undefined behavior when you read the string regardless of whether you modified it afterwards. Put this line:
alphabetic[num_elements] = 0;
After your for (i = 0; i < MAX_SIZE &&... loop.
Personally, I wouldn't do the getchar loop and instead read in the string like this:
scanf("%49s", alphabetic); // reads in a string up to 50 characters
for (i = 0; alphabetic[i]; i++) { ...