C++ Syntax: for

Description

The for loop corresponds roughly to the FORTRAN do loop. The syntax is:-

for ( init-expr; test-expr; increment-expr) statement

any or all of the expressions insides the brackets may be empty. The sequence is as follows:-

init-expr, if it exists, is executed. Typically this initialises one or more counters, and may also declare them as well.
test-expr is evaluated. If it evaluates to true (nonzero) or is missing then statement, which is often a compound statement is executed. Otherwise the for loop exits.
increment-expr is executed if it exists. Typically this increments one or more counters.
Control returns to step 2 and the test repeated.

For example to sum the contents the 10 element array my_ints:-

Int_t sum = 0;
for (Int_t index=0; index<10; index++) sum += my_ints[index];

Usage Notes

When init-expr is used to declare, as well as initialise a variable, it is defined to have the same scope as the for statement itself, that is to say the innermost compound statement that holds the for loop. Two important consequences:-

The variable can be tested once the loop has completed. As the user describes exactly how the loop has to work, the value should be what is expected even if the loop does not end prematurely (see blow). Contrast this with FORTRAN that only guarantees the value if breaking out of the loop.
If using two or more similar for loops in the same compound statement, only the first should declare the loop variable, the others should just reinitialise it:-
```
{
  ...

  for (Int_t index=0; ...

  ...

  for ( index=0; ...

}
```

To break out of a loop use the break statement and to skip directly to the next cycle of a loop use the continue statement.

Go Back to the The C++ Crib Top Page

If you have any comments about this page please send them to Nick West