Alteration of variable value in C - c

Question

[UPDATE]: The answer by Keine Lust, to explicitly declare the size of array solves the problem.
I am trying to write my own decimal to binary converter in C (posted below), which constantly is giving wrong outputs due to alteration of value of variable named a. Why is the alteration happening? And the compiler I'm using is gcc.
#include <stdio.h>
#include <string.h>
#include <math.h>
int main(void) {
int j = 0; // This variable is used to counter the iterations of while-loop
int a; // This variable holds user-input
char b[] = {}; // This is character array to hold the value of bits after operation
printf("Enter a number to convert into binary\n>>>");
scanf("%d", &a);
while(a!=0){ // Loop until the value of a is 0
printf("a before modulus: %d \n", a); // printing a for debugging
int bit = a % 2; // represents binary digit
printf("a after modulus:%d \n", a); // print a for debugging
switch(bit){ // switch-case to identify the bit
case 1:
b[j] = '1';
break;
case 0:
b[j] = '0';
break;
}
printf("a after switch-case:%d \n", a); // the value of a is altered after switch-case, check output.
a = (int) floor((float)(a/2));
j += 1;
}
printf("\n");
return 0;
}
OUTPUT: (input: 2)
a before modulus:2
a after modulus:2
a after switch-case:48
a before modulus: 24
a after modulus:24
a after switch-case:12312
a before modulus: 6156
a after modulus:6156
..... continues
COMPLETED CODE THAT WORKS: [UPDATE]
#include <stdio.h>
#include <string.h>
#include <math.h>
int main(void) {
const int MAX_SIZE = sizeof(char) * 10000; // Will hold 10000 characters
int j = 0; // This variable is used to counter the iterations of while-loop
int a,temp; // This variable holds user-input
char b[MAX_SIZE]; // This is character array to hold the value of bits after operation will hol
printf("Enter a number to convert into binary\n>>>");
scanf("%d", &a);
temp = a;
int bits = log(a)/log(2);
bits += 1;
while(a!=0){ // Loop until the value of a is 0
int bit = a % 2; // represents binary digit
switch(bit){ // switch-case to identify the bit
case 1:
b[j] = '1';
break;
case 0:
b[j] = '0';
break;
}
a = a/2;
j += 1;
}
printf("The %d in binary is: ",temp);
for (int c=(bits-1);c>=0;c--) {
printf("%c", b[c]);
}
printf("\n");
return 0;
}