Loops: while and for

while: general purpose loop

while (condition is true)
    do something

  • Most general looping construct

  • Serves all uses

  • With a couple of extra variables everything is doable

  • … in many cases complicated though

⟶ for, do - while

From while to for (1)

Iteration over sets of elements using while

Iteration using while

i = 0;
sum = 0;
while (i < 100) {
    sum += i;
    ++i;
}

Generally

init-expression
while (cond-expression) {
    body-statement
    next-expression
}

From while to for (2)

The following constructs are equivalent:

init-expression
while (cond-expression) {
    body-statement
    next-expression
}

for (init-expression; cond-expression; next-expression)
    body-statement

Plus:

  • init-expression, cond-expression and next-expression are optional

for, a Little Closer

for (i = 0, sum = 0; i < 100; ++i)
    sum += i;

init-expression

i = 0, sum = 0

cond-expression

i < 100

next-expression

++i

Comma Operator

Comma operator:

  • The expression expr-1, expr-2 has the value expr-2

  • The operator “,” is left-associative

  • Precedence: lowest precedenceof all operators (see operator table)

value = expr-1, expr-2; /* expr-2 */
value = expr-1, expr-2, expr-3; /* expr-3 */
value = 1, 2, 3; /* 3 */

for: Infamous Idioms

C is infamous for excessive compactness …

As above, only more compact

for (i = 0, sum = 0; i < 100; sum += i++);

Infinite loop

for (;;) {
    do_something();
    sleep(5);
}

A crash, in microcontroller terminology

for (;;);