3.5 for Loops
[This section corresponds to the other half of K&R Sec. 3.5]
Our second loop, which we've seen at least one example of already, is the for loop. The first one we saw was:
for (i = 0; i < 10; i = i + 1)
printf("i is %d\n", i);
More generally, the syntax of a for loop is
for( expr<sub>1</sub> ; expr<sub>2</sub> ; expr<sub>3</sub> )
statement
(Here we see that the for loop has three control expressions. As always, the statement can be a brace-enclosed block.)
Many loops are set up to cause some variable to step through a range of values, or, more generally, to set up an initial condition and then modify some value to perform each succeeding loop as long as some condition is true. The three expressions in a for loop encapsulate these conditions: expr<sub>1</sub> sets up the initial condition, expr<sub>2</sub> tests whether another trip through the loop should be taken, and expr<sub>3</sub> increments or updates things after each trip through the loop and prior to the next one. In our first example, we had i = 0 as expr<sub>1</sub>, i < 10 as expr<sub>2</sub>, i = i + 1 as expr<sub>3</sub>, and the call to printf as statement, the body of the loop. So the loop began by setting i to 0, proceeded as long as i was less than 10, printed out i's value during each trip through the loop, and added 1 to i between each trip through the loop.
When the compiler sees a for loop, first, expr<sub>1</sub> is evaluated. Then, expr<sub>2</sub> is evaluated, and if it is true, the body of the loop (statement) is executed. Then, expr<sub>3</sub> is evaluated to go to the next step, and expr<sub>2</sub> is evaluated again, to see if there is a next step. During the execution of a for loop, the sequence is:
expr<sub>1</sub>
expr<sub>2</sub>
statement
expr<sub>3</sub>
expr<sub>2</sub>
statement
expr<sub>3</sub>
...
expr<sub>2</sub>
statement
expr<sub>3</sub>
expr<sub>2</sub>
The first thing executed is expr<sub>1</sub>. expr<sub>3</sub> is evaluated after every trip through the loop. The last thing executed is always expr<sub>2</sub>, because when expr<sub>2</sub> evaluates false, the loop exits.
All three expressions of a for loop are optional. If you leave out expr<sub>1</sub>, there simply is no initialization step, and the variable(s) used with the loop had better have been initialized already. If you leave out expr<sub>2</sub>, there is no test, and the default for the for loop is that another trip through the loop should be taken (such that unless you break out of it some other way, the loop runs forever). If you leave out expr<sub>3</sub>, there is no increment step.
The semicolons separate the three controlling expressions of a for loop. (These semicolons, by the way, have nothing to do with statement terminators.) If you leave out one or more of the expressions, the semicolons remain. Therefore, one way of writing a deliberately infinite loop in C is
for(;;)
...
It's useful to compare C's for loop to the equivalent loops in other computer languages you might know. The C loop
for(i = x; i <= y; i = i + z)
is roughly equivalent to:
for I = X to Y step Z (BASIC)
do 10 i=x,y,z (FORTRAN)
for i := x to y (Pascal)
In C (unlike FORTRAN), if the test condition is false before the first trip through the loop, the loop won't be traversed at all. In C (unlike Pascal), a loop control variable (in this case, i) is guaranteed to retain its final value after the loop completes, and it is also legal to modify the control variable within the loop, if you really want to. (When the loop terminates due to the test condition turning false, the value of the control variable after the loop will be the first value for which the condition failed, not the last value for which it succeeded.)
It's also worth noting that a for loop can be used in more general ways than the simple, iterative examples we've seen so far. The ``control variable'' of a for loop does not have to be an integer, and it does not have to be incremented by an additive increment. It could be ``incremented'' by a multiplicative factor (1, 2, 4, 8, ...) if that was what you needed, or it could be a floating-point variable, or it could be another type of variable which we haven't met yet which would step, not over numeric values, but over the elements of an array or other data structure. Strictly speaking, a for loop doesn't have to have a ``control variable'' at all; the three expressions can be anything, although the loop will make the most sense if they are related and together form the expected initialize, test, increment sequence.
The powers-of-two example of the previous section does fit this pattern, so we could rewrite it like this:
int x;
for(x = 2; x < 1000; x = x * 2)
printf("%d\n", x);
There is no earth-shaking or fundamental difference between the while and for loops. In fact, given the general for loop
for(expr<sub>1</sub>; expr<sub>2</sub>; expr<sub>3</sub>)
statement
you could usually rewrite it as a while loop, moving the initialize and increment expressions to statements before and within the loop:
expr<sub>1</sub> ;
while(expr<sub>2</sub>)
{
statement
expr<sub>3</sub> ;
}
Similarly, given the general while loop
while(expr)
statement
you could rewrite it as a for loop:
for(; expr; )
statement
Another contrast between the for and while loops is that although the test expression (expr<sub>2</sub>) is optional in a for loop, it is required in a while loop. If you leave out the controlling expression of a while loop, the compiler will complain about a syntax error. (To write a deliberately infinite while loop, you have to supply an expression which is always nonzero. The most obvious one would simply be while(1) .)
If it's possible to rewrite a for loop as a while loop and vice versa, why do they both exist? Which one should you choose? In general, when you choose a for loop, its three expressions should all manipulate the same variable or data structure, using the initialize, test, increment pattern. If they don't manipulate the same variable or don't follow that pattern, wedging them into a for loop buys nothing and a while loop would probably be clearer. (The reason that one loop or the other can be clearer is simply that, when you see a for loop, you expect to see an idiomatic initialize/test/increment of a single variable, and if the for loop you're looking at doesn't end up matching that pattern, you've been momentarily misled.)
