Incremental Java
for

for ( init ; cond ; update )
  for body 

• init is a statement, usually used to initialize a looping variable (or variables)
• cond is a condition. Like the while and do while, you exit the loop when this condition evaluates to false
• update is a statement, usually used to update the looping variable (usually by incrementing it)

A single statement is now extended to mean:

• an assignment statement
• a declaration
• an empty statement
• an if-else or if statement
• a switch statement
• a while statement
• a do-while statement
• a for statement

for ( init ; cond ; update )
  for body 

1. ENTER the for loop (so far, no code is run)
2. Run init
3. Evaluate cond
4. If cond evaluates to true, run for body and go to step 3.
5. If cond evaluates to false, EXIT the for loop.

for ( int i = 0 ; i < num ; i++ )
{
   System.out.println( i ) ;
}

for ( int i = 0 ;     // LINE 1
      i < num ;    // LINE 2
      i++ )           // LINE 3
{
   System.out.println( i ) ; // LINE 4
}

• ENTER loop
• (LINE 1) Run int i = 0. This sets i to 0.
• (LINE 2) Evaluate i < num. 0 < 2 evaluates to true
• (LINE 4) Surprise! We're in the loop body. Print out 0.
• (LINE 3) Jump back to i++. i is now 1.
• (LINE 2) Evaluate i < num. 1 < 2 evaluates to true
• (LINE 4) We're in the loop body. Print out 1
• (LINE 3) Jump back to i++. i is now 2.
• (LINE 2) Evaluate i < num. 2 < 2 evaluates to false
• EXIT loop

These choices come from FORTRAN. In that language, variables starting from about i to n were considered integer variables (you didn't have to declare the types---the FORTRAN compiler assumed the type based on the first letter of the variable name).

Second, once the condition is evaluated, you jump to the loop body. The update is done afterwards.

Third, we declared i in init. This variable only has scope within the loop. After you exit the loop, i is no longer valid. It becomes valid again, once you re-enter the loop.

Fourth, the kind of loop you see above is perhaps the most common form of the for loop.

• Looping variable set to 0 in init
• Looping variable is checked against num as in i < num.
• Looping variable incremented in update using i++

Fifth, the common problem is not jumping to the loop body after the condition evaluates to true.

Practice Makes Perfect

• If you leave init blank, then there is no initialization phase. You just start with the cond.

  Here's a short example:

  int i = 0 ;
  for ( ; i < num ; i++ ) // init is blank
  {
    // CODE
  }
  

  `

• If you leave update blank, then there is no update phase. Thus, you skip, and head to cond.

  Here's a short example:

  for ( int i = 0 ; i < num ; ) // update is blank
  {
    // CODE
  }
  

• cond is a condition. We use it to determine whether to run the loop body or not. Thus, we can't skip it. If it's left blank, it must always evaluate to true or always evaluates to false? Which makes more sense? If it always evaluates to false, then you never enter the loop body. So the loop is useless. So it has to always evaluate to true.

  Normally this causes an infinite loop, which sounds just as bad (because it never leaves the loop). However, there are control flow constructs which allow us to escape the loop.