Is there a difference in ++i
and i++
in a for
loop? Is it simply a syntax thing?
Since you ask about the difference in a loop, i guess you mean
for(int i=0; i<10; i++)
...;
In that case, you have no difference in most languages: The loop behaves the same regardless of whether you write i++
and ++i
. In C++, you can write your own versions of the ++ operators, and you can define separate meanings for them, if the i
is of a user defined type (your own class, for example).
The reason why it doesn't matter above is because you don't use the value of i++
. Another thing is when you do
for(int i=0, a = 0; i<10; a = i++)
...;
Now, there is a difference, because as others point out, i++
means increment, but evaluate to the previous value, but ++i
means increment, but evaluate to i
(thus it would evaluate to the new value). In the above case, a
is assigned the previous value of i, while i is incremented.
As @Jon B says, there is no difference in a for loop.
But in a while
or do...while
loop, you could find some differences if you are making a comparison with the ++i
or i++
while(i++ < 10) { ... } //compare then increment
while(++i < 10) { ... } //increment then compare
a++ is known as postfix.
add 1 to a, returns the old value.
++a is known as prefix.
add 1 to a, returns the new value.
C#:
string[] items = {"a","b","c","d"};
int i = 0;
foreach (string item in items)
{
Console.WriteLine(++i);
}
Console.WriteLine("");
i = 0;
foreach (string item in items)
{
Console.WriteLine(i++);
}
Output:
1
2
3
4
0
1
2
3
foreach
and while
loops depend on which increment type you use. With for loops like below it makes no difference as you're not using the return value of i:
for (int i = 0; i < 5; i++) { Console.Write(i);}
Console.WriteLine("");
for (int i = 0; i < 5; ++i) { Console.Write(i); }
0 1 2 3 4
0 1 2 3 4
If the value as evaluated is used then the type of increment becomes significant:
int n = 0;
for (int i = 0; n < 5; n = i++) { }
As this code shows (see the dissambled MSIL in the comments), the C# 3 compiler makes no distinction between i++ and ++i in a for loop. If the value of i++ or ++i were being taken, there would definitely be a difference (this was compiled in Visutal Studio 2008 / Release Build):
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace PreOrPostIncrement
{
class Program
{
static int SomethingToIncrement;
static void Main(string[] args)
{
PreIncrement(1000);
PostIncrement(1000);
Console.WriteLine("SomethingToIncrement={0}", SomethingToIncrement);
}
static void PreIncrement(int count)
{
/*
.method private hidebysig static void PreIncrement(int32 count) cil managed
{
// Code size 25 (0x19)
.maxstack 2
.locals init ([0] int32 i)
IL_0000: ldc.i4.0
IL_0001: stloc.0
IL_0002: br.s IL_0014
IL_0004: ldsfld int32 PreOrPostIncrement.Program::SomethingToIncrement
IL_0009: ldc.i4.1
IL_000a: add
IL_000b: stsfld int32 PreOrPostIncrement.Program::SomethingToIncrement
IL_0010: ldloc.0
IL_0011: ldc.i4.1
IL_0012: add
IL_0013: stloc.0
IL_0014: ldloc.0
IL_0015: ldarg.0
IL_0016: blt.s IL_0004
IL_0018: ret
} // end of method Program::PreIncrement
*/
for (int i = 0; i < count; ++i)
{
++SomethingToIncrement;
}
}
static void PostIncrement(int count)
{
/*
.method private hidebysig static void PostIncrement(int32 count) cil managed
{
// Code size 25 (0x19)
.maxstack 2
.locals init ([0] int32 i)
IL_0000: ldc.i4.0
IL_0001: stloc.0
IL_0002: br.s IL_0014
IL_0004: ldsfld int32 PreOrPostIncrement.Program::SomethingToIncrement
IL_0009: ldc.i4.1
IL_000a: add
IL_000b: stsfld int32 PreOrPostIncrement.Program::SomethingToIncrement
IL_0010: ldloc.0
IL_0011: ldc.i4.1
IL_0012: add
IL_0013: stloc.0
IL_0014: ldloc.0
IL_0015: ldarg.0
IL_0016: blt.s IL_0004
IL_0018: ret
} // end of method Program::PostIncrement
*/
for (int i = 0; i < count; i++)
{
SomethingToIncrement++;
}
}
}
}
I dont know for the other languages but in Java ++i is a prefix increment which means: increase i by 1 and then use the new value of i in the expression in which i resides, and i++ is a postfix increment which means the following: use the current value of i in the expression and then increase it by 1. Example:
public static void main(String [] args){
int a = 3;
int b = 5;
System.out.println(++a);
System.out.println(b++);
System.out.println(b);
} and the output is:
It boggles my mind why so may people write the increment expression in for-loop as i++.
In a for-loop, when the 3rd component is a simple increment statement, as in
for (i=0; i<x; i++)
or
for (i=0; i<x; ++i)
there is no difference in the resulting executions.