How do I write more efficient code?

Obviously it is important for any programmer to make his/her code as efficient as possible. But are there any guidelines that anyone can give?

So for instance, will my code be more efficient if I do the following?

Use fewer variables
Use fewer lines

For Instance, is:

Code:

---

for (int i=0;x;i++)
{
//do stuff to x
}

---

better than:

Code:

---

for (int i=0;;i++)
{
//do stuff to x
if(x==0)
break;
}

---

Make many functions dedicated to simple tasks to avoid code repetition

So if I have:

Code:

---

if (x>y)
{
z=1;
}
else
{
z=0;
}

---

more than once is it always better to do:

Code:

---

z=compare(x,y);

//where compare() is:

int compare(int x,int y)
{
if (x>y)
{
return 1;
}
return 0;
}

---

Is it dependent upon the intelligence of your compiler (I use MSVC++)?

So for instance will:

Code:

---

s1 = s1 + s2 + s3 + s4;

---

and:

Code:

---

s1 += s2;
s1 += s3;
s1 += s4;

---

be recognised by the compiler as the same and therefore converted to the same machine code?

Can anyone gives any tips? Or point me to some info? I have tried googling but didn't get very much.

instead of...

Code:

---

for (int i=0;;i++)
{
    //do stuff to x
  if(x==0)
  break;
}

---

Why cant you use a while loop instead of this one..
[code]
while(1)
{
//do stuff to x
if (x==0)
{
break;
}
}
[code]

or simply,

Code:

---

while(x != 0)
{
        //do stuff to x
}

---

There are many other things that you can do..

Try reading the book: Efficient C++

http://win32.planet-d.net/opti/opti_c.htm

Another good book is:
Programming Pearls Second Edition by Jon Bentley

It discusses algorithms, data structures, and code optimization. It's been around for a while, but it is a classic.