Scanning an octal and hexadecimal number in %d format specifier with scanf

I was experimenting with the printing octal and hexadecimal number in C. The following is printing as expected
```
int a = 012;
printf("%d",a); //printing as 10. The decimal representation of octal 12.
```
However, when I am scanning 012 using %d 0 is ignored and it is printing 12. What is the reason?
```
int b;
scanf("%d",&b);
printf("%d", b); //printing 12.
```
When I am scanning 0x12 using %d the output is giving as 0 as follows
```
int b;
scanf("%d",&b);
printf("%d", b); //printing 0
```

What is the reason for this? Why they are not giving any runtime errors?

Thanks in advance!!!

Solution

Explanation

int a = 012;    // a is assigned decimal value 10 (012 is octal = 10 in decimal)
printf("%d",a); // printing a in decimal as 10

In the case of first code snippet, you are assigning a=10 (which is represented as octal 012) and printing it in decimal format.

int b;
scanf("%d",&b);  // read an integer in decimal format
printf("%d", b); //printing 12 because the input was 12 after ignoring the 0.

When you're using scanf with %d, it expects an integer in decimal format. So it ignores the first 0 and assigns b = 12. And when you print it, you see 12.

int b;
scanf("%d",&b);
printf("%d", b); //printing 0

In the above case, you are asking scanf to read an integer in decimal format and when you enter 0x12, it stops scanning at character x after reading 0 since x cannot be a part of a valid integer.

Solution

So here is the way to go.

"%d" - to input decimal value
"%o" - to input integer value in an octal format
"%x" - to input integer value in hexadecimal format
"%i" - to input integer in any format (decimal/octal/hexadecimal)