Java question

[duragezic]

What is the difference between these two statements?

I know that i++ increments by one whatever i is, but I don't understand what ++i means. Because I'm looking at loops that I thought simply went from 0 to some number "for ( int i=0; i<100; i++ )", but they are written as: "for ( int i=0; i<100; ++i )". So they are not the same operation?

 

[MrChad]

Nope.

for (int i=0; i < 100; i++){
....System.out.println(i + " ");
}

will output: 0 1 2 3 4 5 6 ... 99

for (int i=0; i < 100; ++i){
....System.out.println(i + " ");
}

will output: 1 2 3 4 5 6 7 ... 100

 

[SleepWalkerX]

If you have the ++ before the variable then it'll add one to the variable first (in this for loop it'll add one before it starts). If you have the ++ after then it'll add one afterwards.

 

[Templeton]

Originally posted by: MrChad
Nope.

for (int i=0; i < 100; i++){
....System.out.println(i + " ");
}

will output: 0 1 2 3 4 5 6 ... 99

for (int i=0; i < 100; ++i){
....System.out.println(i + " ");
}

will output: 1 2 3 4 5 6 7 ... 100

No, they will output the same thing. the loop itirator isn't evaluated until after the first pass through the loop.

The difference is in expressions:

int i = 5;
int j = ++i;
//i is now 6, j is 6

int i = 5;
int j = i++;
//i is now 6, j is 5 since i incremented after assignment

 

[kamper]

Originally posted by: Templeton
No, they will output the same thing. the loop itirator isn't evaluated until after the first pass through the loop.

Confirmed.

The difference is in expressions:

int i = 5;
int j = ++i;
//i is now 6, j is 6

int i = 5;
int j = i++;
//i is now 6, j is 5 since i incremented after assignment

Good example. In more depth: both fragments have both an action (increment) and a value. With pre-increment, the action takes place before the evaluation. With post-increment, the evaluation takes place before the action.

 

[MrChad]

Originally posted by: Templeton

Originally posted by: MrChad
Nope.

for (int i=0; i < 100; i++){
....System.out.println(i + " ");
}

will output: 0 1 2 3 4 5 6 ... 99

for (int i=0; i < 100; ++i){
....System.out.println(i + " ");
}

will output: 1 2 3 4 5 6 7 ... 100

No, they will output the same thing. the loop itirator isn't evaluated until after the first pass through the loop.

The difference is in expressions:

int i = 5;
int j = ++i;
//i is now 6, j is 6

int i = 5;
int j = i++;
//i is now 6, j is 5 since i incremented after assignment

😱

I stand corrected.

 

[Markbnj]

Templeton is correct. The difference is that the prefix operator (++i) applies the operator before the value is "taken" for use in the operation. The postfix operator (i++) applies the operator after the value is taken. Can lead to some subtle bugs, but in practice postfix is the right way to go 90% of the time.

 

[kamper]

Originally posted by: Markbnj
Can lead to some subtle bugs, but in practice postfix is the right way to go 90% of the time.

IMHO, in practice, the right way to go is to never use either in combination with another statement. The extra line of code is well worth the clarity.

 

[Markbnj]

IMHO, in practice, the right way to go is to never use either in combination with another statement. The extra line of code is well worth the clarity.

:thumbsup: Agree 100%. There are relatively few good uses for either form.

 

[kamper]

The only place I've really seen them inlined into other code is from my math-oriented comp-sci profs who still write in fortran for fun and seem to think that extremely densely packed code with a multitude of single-lettered variable names is somehow more elegant than code that humans can read 😛