Counter in C Language - c

Question

I'm mainly an electronics hardware guy, but I've learned the basics of D from a friend, so I decided I'd pick up a more standard language like C. So, any advice would be great.
Basically, I'm using the integer 'phew' as a counter, to reverse all numbers. Super basic, but I'm having trouble finding the way to do this in C. My code:
#include <stdio.h>
int main()
{
int input;
int phew;
printf("Binary Number: ");
scanf("%d", &input);
while(phew < sizeof(input))
{
if(input[phew] == 0)
printf("1");
else
printf("0");
phew++;
}
return 0;
}
And the compiler error was:
helloworld.c: In function ‘main’:
helloworld.c:11:11: error: subscripted value is neither array nor pointer nor vector
if(input[phew] == 0)
^

Answer 1

First of all, in the below statement,
while(phew < sizeof(input))
invokes undefined behavior as phew is an automatic local variable and not initialized. You need to initialize phew like int phew = 0;
In this scenario, sizeof(input) is valid, but does not make any sense.
That said, you can only use the [] operator on an array type. In your case, input is an int, so you are not allowed to write input[n].
to elaboate, quoting the C11 standard, chapter §6.5.2.1
Syntax
postfix-expression [ expression ]
and the description
One of the expressions shall have type pointer to complete object type, the other
expression shall have integer type, and the result has type type.
So, clearly, in your case input is not a "pointer-to-type", hence the error.
To solve the issues,
You can change the type of input as char input[32] = {0};
change the scan statement as `scanf("%29s", input);
add while(phew < strlen(input))
to make sense. You'll be needing string.h header file for that. Check details about strlen() here.
You have to change it like
while(phew < strlen(input) )
{
if(input[phew] == '0') //ASCII 48, decimal
printf("1");
else
printf("0");
phew++;
}

Answer 2

You cannot use [] to get bits of an integer in C; that operator only works on pointer values and an int is not a pointer. You may be familiar with Verilog, where bit vectors and integers are the same thing; but C doesn't have a syntax for accessing bits of an object and [] is only for accessing members of an array.
One correct way to do this is using a bit mask. For example:
for (size_t i = 0; i < (sizeof input)*CHAR_BIT; i++) {
unsigned mask = 1 << i;
if (input & mask) {
putchar('1');
} else {
putchar('0');
}
}
putchar('\n');
What this code does on each iteration of the loop is create an unsigned integer mask by shifting the number 1 into the ith place of the integer. Then mask is bitwise-ANDed with input to give a value that is 0 if that bit is 0 in input (and otherwise some nonzero value equivalent to the place value of the bit). Don't forget to print out a newline at the end or the output may not be visible.
I'm using unsigned arithmetic here because << can have implementation-defined or undefined behavior on signed numbers in C. input can be a regular (signed) int because it will be promoted to unsigned by the & operator.
Also notice that I had to multiply sizeof input by CHAR_BIT (which is defined in <limits.h>) to get the number of bits; sizeof always measures sizes in bytes.
Also see this C FAQ (20.7) about bit manipulations.