+= and -=

I've come across += and -= and have no clue as to what they do, and I cant find any tutorials either. Can anyone explain?

EDIT: I've also noticed some code..

Code:

---

if (i)
{ etc etc etc }

---

How can 'i' be a condition?

• L += R;
  
  Equivalent to:
  
  
  Code:

  ---

  L = L + R;

  ---

• L -= R;
  
  Equivalent to:
  
  
  Code:

  ---

  L = L - R;

  ---

Quote:

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Originally Posted by Sn0mAN

I've come across += and -= and have no clue as to what they do, and I cant find any tutorials either. Can anyone explain?

EDIT: I've also noticed some code..

Code:

---

if (i)
{ etc etc etc }

---

How can 'i' be a condition?

---

In C, everything non-zero is true (ie 0 is false). So if i is not 0, etc etc etc will be executed.

The operator != itself returns 1 or 0, so when you use

Code:

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if(x != 0)

---

it's like you're using

Code:

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if( (x != 0) != 0)

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or

Code:

---

if ( (x != 0) != 0) != 0)

---

ad infinitum. So you can just leave out the first != 0 and go

Code:

---

if(x)

---

Every comparison operator (indeed, every operator) returns a zero or non-zero value. When you use if(x != 0), you're just using the zero or non-zero value value of x != 0. You can also use the zero or non-zero value of a variable. You could go

Code:

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int x = 1, y = x != 0;
if(y)

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or even

Code:

---

if(x)

---

instead of

Code:

---

if(x != 0)

---

. . . okay, so that wasn't a very good explanation. But I hope you know what I mean. :)

Quote:

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or
Code:

if ( (x != 0) != 0) != 0)

ad infinitum. So you can just leave out the first != 0 and go
Code:

if(x)

---

lol... Funny way of putting it

I know, it's definitely a novel idea. :D

With += and -= (or any of the compound assignment operators), the left-hand side is only evaluated once, instead of twice with the expanded form, so if its evaluation has a side effect, they're not equivalent (although this is uncommon).
Example:

http://books.google.com/books?id=svQ...nidOjt7qKR7yl8

http://c-faq.com/bool/bool2.html