Type Specifiers. HELP!

Paul_help_

Hi friends.

I am currently trying to understand the meaning of 'Type Specifiers'.

I am reading a book and I have been learning the following code:

printf( "rounded to a whole number is: %d", 7.9 );

I understand what it does. The second argument replaces the type specifier: %d and it is converted to an integer: 7.

The only thing that doesn't make ANY sense to me is the definition they give to %d in the book i am reading.

They say that the type specifier %d treats the argument as an integer and formats it as a signed decimal number.

Now I have no idea what the difference between 'treating' and 'formatting' is.

And what on earth is the difference between a 'decimal' and a 'signed decimal'????

My understanding of a decimal i a float???

The output is '7' which is clearly an integer!! how the hell can it be both???

Please help.

Paul.

sneakyimp

I'm not sure what exactly your book says, but would recommend reviewing the PHP documentation online for pretty helpful usage details:
http://us2.php.net/manual/en/function.sprintf.php

to 'treat the argument as an integer' could mean a variety of things. try suppling a string like "foo" as your argument and see what comes out. Try supplying an integer and then a float and see if there's any difference. Keep in mind that every number ends up being stored in a memory location somewhere (or a series of memory locations, depending on how much data it contains). By "treating" I think the documentation means to suggest that the function will look at the memory location of the supplied 2nd parameter and interpret its contents as an integer. I'm not sure how to ferret out what exactly the difference is between these two vars, but I expect the memory location that stores them might have different bits and bytes in it. I have no idea how PHP will "treat" your second parameter, I just want to show you that numbers have different binary encodings depending on how you "treat" them:

$v1 = 7;
echo gettype($v1) . PHP_EOL; // "integer"
$bytes1 = pack('i', $v1); // pack as signed integer (machine dependent size and byte order)
echo bin2hex($bytes1) . PHP_EOL; // "07000000" on my machine, your results may vary

$v2 = 7.00000;
echo gettype($v2) . PHP_EOL; // "double"
$bytes2 = pack('d', $v2); // pack as double (machine dependent size and representation)
echo bin2hex($bytes2) . PHP_EOL; "0000000000001c40" on my machine, your results may vary

You may want to check out the docs on [man]pack[/man] to get an idea of how data types may vary in their binary representation. There is also a mention in there of signed decimals and unsigned decimals.

As for 'formatting' of these numbers, we can output them as binary, octal, decimal, hexadecimal, or possibly other ways. The number 15 (a decimal number) is represented in hexadecimal as F

NogDog

I'm just guessing they used the term "decimal" in this case in the sense of it being a base 10 integer, versus, say, a base 16 (hexadecimal) integer?

Paul_help_

Thanks guys.

I think the main problem I am having is confusion between %d and %f

It seems like when I use %d it returns an integer. But in the description it says that the type specifier %d treats the argument as an integer and formats it as a signed decimal number.

I have always believed that a decimal and a float are the same thing. So why do people call a 'base 10' number a decimal even though its not really a decimal (its an integer)? I imagine this can confuse the hell out of people??

Weedpacket

Paul help! wrote:
So why do people call a 'base 10' number a decimal even though its not really a decimal (its an integer)? I imagine this can confuse the hell out of people??

Funny, I've always known "base 10" as "decimal" (from the Latin decem, meaning ten).

Whether a number is an integer or not is independent of what base is used to write it; the former is a property of the number, the latter is cosmetic (i.e., how it's presented).

As NogDog says - just as "base 16" is known as "hexadecimal" (see the "%x" format specifier) - or for that matter "base -2" is "negabinary". It doesn't usually mean simply that the number as written contains a "." (which doesn't necessarily make it a floating-point number either - it could be fixed-point). Most people call that a "decimal point", but then they don't have to deal with the hexadecimal number "14A.BB" (it's more formally called the "radix point").

I guess what it boils down to is that, like anything in mathematics, things only mean what they're defined to mean.

NogDog

And if you're programming in Ruby, it doesn't even matter how things are defined, as you can override the definitions at any time. 😉