Why the need to initialize before using the variable?

[karuga]

I was recently using the Fibonacci series(iteratively not recursively) and it left me with a question.

int a=0;
int b=1;
int c=1; *
System.out.print ln(a);
System.out.print ln(b);

while(c>=89)
{
c=a+b;


System.out.print ln(c+",");
a=b;
b=c;
}
* the code could not run before initializing the variable c first to something like 1 or 0.
What is the explanation behind this?It is not clear why i need to do this initialization first.

[bitmason]

the main reason is that the instance of the variable "c" later in the code would have been undefined - the compiler would not have been able to allocate storage for it, since c could be anything - an int, a char, float, or even an instance of a class.

sorry, missed the point - initializing a variable is important at establishing an initial value. when compiled, a variable that is declared but not defined would have an "arbitrary" value. declaration only allocates storage for an object, and does not apply a value.

[AlbertP]

Just use:

Code: Select all

int c;

This code initializes the variable without giving it any value.

[eiver]

In fact it is possible to compile "int c;" depending on the compiler. The computer will know it is an int and not a float, but its value will be proably some random garbage from memory. Try creating an array and print its contents without initializing any values and you will get a nice memory dump I was sometimes even able to pick up some memory structures from my previous runs of the program. This all depends on the language and compiler. Typically higher level languages will automatically initialize variables to their default values.

[AlbertP]

Oh yes, I am used to VB and C# .NET which are quite different from C.

But if you assign your own value to the variable, it doesn't really matter that there is some memory garbage in it: that will become overwritten.

[cliste]

The reason being that the variable c would otherwise be used before it is defined.

ie the program when run will do the following:
1- create variable 'a', assign the value 0 to it
2- create variable 'b', assign the value 1 to it
3- create variable 'c', assign the value 1 to it
4- print the value of 'a' (namely 0)
5- print the value of 'b' (namely 1)
6- check if c > 98
7- set c = a+b
...

On step six it would break down as it cannot compare an empty variable to a number. The problem being that you ar actually using the variable as part of the while loop before it is initialised.

(On a side note you *may* be able to get away without assigning a value to the variable at step 3 if you use a do{ ... }while() loop, but I may be wrong on that)

[AlbertP]

With a do { ... } while() instead of while() { ... } it works 100% OK.