Does IF perform better than IF-ELSE?
Which one of these blocks of code performs better, and which one of them is more readable? I'd guess the gain would be negligible, particularly in the second block. I am just curious.
Block #1
string height;
string width;
if (myFlag == 1)
{
height = "60%";
width = "60%";
}
else
{
height = "80%";
width = "80%";
}
Block #2
string height = "80%";
string width = "80%";
if (myFlag == 1)
{
height = "60%";
width = "60%";
}
Updated
The results when i tested the above code were that both the blocks performed the same
Block #1
myFlag = 1: 3 Milliseconds
myFlag = 0: 3 Milliseconds
Block #2
myFlag = 1: 3 Milliseconds
myFlag = 0: 3 Milliseconds
But one important thing i noticed here(thanks to Matthew Steeples answer here) is that since the block of code that i have tested has not used the variables height and width except for assignment in the if-else and if blocks of Code Block-1 and 2 respectively, the compiler has optimized the IL code by completely removing the if and if-else blocks in question thus showing invalid results for our test here.
I have updated both the code blocks to write the values of both height and width to a file thus using them again and forcing the compiler to run our test blocks(I hope), but you can observe from the code that the actual file writing part does not effect our test results
This is the updated results, C# and IL Code
Results
Block #1
myFlag = 1: 1688 Milliseconds
myFlag = 0: 1664 Milliseconds
Block #2
myFlag = 1: 1700 Milliseconds
myFlag = 0: 1677 Milliseconds
C#.net Code
Block #1
public long WithIfAndElse(int myFlag)
{
Stopwatch myTimer = new Stopwatch();
string someString = "";
myTimer.Start();
for (int i = 0; i < 1000000; i++)
{
string height;
string width;
if (myFlag == 1)
{
height = "60%";
width = "60%";
}
else
{
height = "80%";
width = "80%";
}
someString = "Height: " + height + Environment.NewLine + "Width: " + width;
}
myTimer.Stop();
File.WriteAllText("testifelse.txt", someString);
return myTimer.ElapsedMilliseconds;
}
Block #2
public long WithOnlyIf(int myFlag)
{
Stopwatch myTimer = new Stopwatch();
string someString = "";
myTimer.Start();
for (int i = 0; i < 1000000; i++)
{
string height = "80%";
string width = "80%";
if (myFlag == 1)
{
height = "60%";
width = "60%";
}
someString = "Height: " + height + Environment.NewLine + "Width: " + width;
}
myTimer.Stop();
File.WriteAllText("testif.txt", someString);
return myTimer.ElapsedMilliseconds;
}
IL Code Generated By ildasm.exe
Block #1
.method public hidebysig instance int64 WithIfAndElse(int32 myFlag) cil managed
{
// Code size 144 (0x90)
.maxstack 3
.locals init ([0] class [System]System.Diagnostics.Stopwatch myTimer,
[1] string someString,
[2] int32 i,
[3] string height,
[4] string width,
[5] string[] CS$0$0000)
IL_0000: newobj instance void [System]System.Diagnostics.Stopwatch::.ctor()
IL_0005: stloc.0
IL_0006: ldstr ""
IL_000b: stloc.1
IL_000c: ldloc.0
IL_000d: callvirt instance void [System]System.Diagnostics.Stopwatch::Start()
IL_0012: ldc.i4.0
IL_0013: stloc.2
IL_0014: br.s IL_0070
IL_0016: ldarg.1
IL_0017: ldc.i4.1
IL_0018: bne.un.s IL_0029
IL_001a: ldstr "60%"
IL_001f: stloc.3
IL_0020: ldstr "60%"
IL_0025: stloc.s width
IL_0027: br.s IL_0036
IL_0029: ldstr "80%"
IL_002e: stloc.3
IL_002f: ldstr "80%"
IL_0034: stloc.s width
IL_0036: ldc.i4.5
IL_0037: newarr [mscorlib]System.String
IL_003c: stloc.s CS$0$0000
IL_003e: ldloc.s CS$0$0000
IL_0040: ldc.i4.0
IL_0041: ldstr "Height: "
IL_0046: stelem.ref
IL_0047: ldloc.s CS$0$0000
IL_0049: ldc.i4.1
IL_004a: ldloc.3
IL_004b: stelem.ref
IL_004c: ldloc.s CS$0$0000
IL_004e: ldc.i4.2
IL_004f: call string [mscorlib]System.Environment::get_NewLine()
IL_0054: stelem.ref
IL_0055: ldloc.s CS$0$0000
IL_0057: ldc.i4.3
IL_0058: ldstr "Width: "
IL_005d: stelem.ref
IL_005e: ldloc.s CS$0$0000
IL_0060: ldc.i4.4
IL_0061: ldloc.s width
IL_0063: stelem.ref
IL_0064: ldloc.s CS$0$0000
IL_0066: call string [mscorlib]System.String::Concat(string[])
IL_006b: stloc.1
IL_006c: ldloc.2
IL_006d: ldc.i4.1
IL_006e: add
IL_006f: stloc.2
IL_0070: ldloc.2
IL_0071: ldc.i4 0xf4240
IL_0076: blt.s IL_0016
IL_0078: ldloc.0
IL_0079: callvirt instance void [System]System.Diagnostics.Stopwatch::Stop()
IL_007e: ldstr "testifelse.txt"
IL_0083: ldloc.1
IL_0084: call void [mscorlib]System.IO.File::WriteAllText(string,
string)
IL_0089: ldloc.0
IL_008a: callvirt instance int64 [System]System.Diagnostics.Stopwatch::get_ElapsedMilliseconds()
IL_008f: ret
} // end of method frmResearch::WithIfAndElse
Block #2
.method public hidebysig instance int64 WithOnlyIf(int32 myFlag) cil managed
{
// Code size 142 (0x8e)
.maxstack 3
.locals init ([0] class [System]System.Diagnostics.Stopwatch myTimer,
[1] string someString,
[2] int32 i,
[3] string height,
[4] string width,
[5] string[] CS$0$0000)
IL_0000: newobj instance void [System]System.Diagnostics.Stopwatch::.ctor()
IL_0005: stloc.0
IL_0006: ldstr ""
IL_000b: stloc.1
IL_000c: ldloc.0
IL_000d: callvirt instance void [System]System.Diagnostics.Stopwatch::Start()
IL_0012: ldc.i4.0
IL_0013: stloc.2
IL_0014: br.s IL_006e
IL_0016: ldstr "80%"
IL_001b: stloc.3
IL_001c: ldstr "80%"
IL_0021: stloc.s width
IL_0023: ldarg.1
IL_0024: ldc.i4.1
IL_0025: bne.un.s IL_0034
IL_0027: ldstr "60%"
IL_002c: stloc.3
IL_002d: ldstr "60%"
IL_0032: stloc.s width
IL_0034: ldc.i4.5
IL_0035: newarr [mscorlib]System.String
IL_003a: stloc.s CS$0$0000
IL_003c: ldloc.s CS$0$0000
IL_003e: ldc.i4.0
IL_003f: ldstr "Height: "
IL_0044: stelem.ref
IL_0045: ldloc.s CS$0$0000
IL_0047: ldc.i4.1
IL_0048: ldloc.3
IL_0049: stelem.ref
IL_004a: ldloc.s CS$0$0000
IL_004c: ldc.i4.2
IL_004d: call string [mscorlib]System.Environment::get_NewLine()
IL_0052: stelem.ref
IL_0053: ldloc.s CS$0$0000
IL_0055: ldc.i4.3
IL_0056: ldstr "Width: "
IL_005b: stelem.ref
IL_005c: ldloc.s CS$0$0000
IL_005e: ldc.i4.4
IL_005f: ldloc.s width
IL_0061: stelem.ref
IL_0062: ldloc.s CS$0$0000
IL_0064: call string [mscorlib]System.String::Concat(string[])
IL_0069: stloc.1
IL_006a: ldloc.2
IL_006b: ldc.i4.1
IL_006c: add
IL_006d: stloc.2
IL_006e: ldloc.2
IL_006f: ldc.i4 0xf4240
IL_0074: blt.s IL_0016
IL_0076: ldloc.0
IL_0077: callvirt instance void [System]System.Diagnostics.Stopwatch::Stop()
IL_007c: ldstr "testif.txt"
IL_0081: ldloc.1
IL_0082: call void [mscorlib]System.IO.File::WriteAllText(string,
string)
IL_0087: ldloc.0
IL_0088: callvirt instance int64 [System]System.Diagnostics.Stopwatch::get_ElapsedMilliseconds()
IL_008d: ret
} // end of method frmResearch::WithOnlyIf
So we can say that the IF-Else block(Block #1) runs faster than the if block(Block #2) as pointed by many in this forum.
Test Results
10,000,000 iterations of Block 1
myFlag = 0: 23.8ns per iteration
myFlag = 1: 23.8ns per iteration
10,000,000 iterations of Block 2
myFlag = 0: 23.8ns per iteration
myFlag = 1: 46.8ns per iteration
Block 2 is 96% slower than Block 1. Makes sense, since Block 2 does twice the work in the pessimistic case.
i prefer either case, depending on the situation. If
myFlagis rarely ever 1, then it want it to stand out as the edge case that we have to handle. If both are equally likely, i want theif-elsesyntax. But that's preference, not fact.
Decades ago, the intel 80286 dual pipeline would stall if a conditional jump was taken, rather than falling through to the next instruction. By the time of the Pentium that went away; the CPU pre-fetches both branch paths. But in the back of my mind i still have a twinge of fear whenever i write code that has the most common outcome in the else clause. Every time i have to remind myself that it doesn't matter anymore.
Int32 reps = 10000000;
private void Block1(int myFlag)
{
String width;
String height;
Stopwatch sw = new Stopwatch();
sw.Start();
for (int i = 0; i < reps; i++)
{
if (myFlag == 1)
{
width = String.Format("{0:g}%", 60);
height = String.Format("{0:g}%", 60);
}
else
{
width = String.Format("{0:g}%", 80);
height = String.Format("{0:g}%", 80);
}
}
sw.Stop();
Double time = (Double)sw.Elapsed.Ticks / Stopwatch.Frequency * 1000000000.0 / reps;
MessageBox.Show(time.ToString() + " ns");
}
private void Block2(int myFlag)
{
String width;
String height;
Stopwatch sw = new Stopwatch();
sw.Start();
for (int i = 0; i < reps; i++)
{
width = String.Format("{0:g}%", 80);
height = String.Format("{0:g}%", 80);
if (myFlag == 1)
{
width = String.Format("{0:g}%", 60);
height = String.Format("{0:g}%", 60);
}
}
sw.Stop();
Double time = (Double)sw.Elapsed.Ticks / Stopwatch.Frequency * 1000000000.0 / reps;
MessageBox.Show(time.ToString() + " ns");
}
-
String.FormatmakesIF96% slower -
GetPercentageString(0.60)makesIF96% slower
const
reps = 10000000;
procedure Block1(myflag: Integer);
var
width, height: string;
i: Integer;
t1, t2: Int64;
time: Extended;
freq: Int64;
begin
QueryPerformanceCounter(t1);
for i := 1 to reps do
begin
if myFlag = 1 then
begin
width := '60%';
height := '60%';
end
else
begin
width := '80%';
height := '80%';
end;
end;
QueryPerformanceCounter(t2);
QueryPerformanceFrequency(freq);
time := (t2-t1) / freq * 1000000000 / reps;
ShowMessage(FloatToStr(time)+ 'ns');
end;
procedure Block2(myflag: Integer);
var
width, height: string;
i: Integer;
t1, t2: Int64;
time: Extended;
freq: Int64;
begin
QueryPerformanceCounter(t1);
for i := 1 to reps do
begin
width := '80%';
height := '80%';
if myFlag = 1 then
begin
width := '60%';
height := '60%';
end;
end;
QueryPerformanceCounter(t2);
QueryPerformanceFrequency(freq);
time := (t2-t1) / freq * 1000000000 / reps;
ShowMessage(FloatToStr(time)+ 'ns');
end;
Doing twice the amount of work takes roughly twice the amount of time.
Answer: IF does not perform better than IF-ELSE.
The performance gain here is negligible to the tune I'd call this micro-micro-micro-optimization. Go for readability here unless you plan to do this a couple of million times.
Edit: (re: question in comments)
In my opinion the first one is the more readable. It explicitly shows in a ready format what the strings should be for each case. The second one omits a case, so a reviewer would have to look through other areas of the code to determine the default value. To put it into perspective, imagine 50 lines of code between the original declaration/initialization and this particular code block. If it becomes unclear in that case then that would decide it for me.