"Then Else" together with no code between?

[moorea21]

Hi, I found a bit of syntax that I don't understand.

For I = LBound(TextArr) To J + 5

If Left(TextArr(I), 2) = "SB" Then Else TextArr(I) = ""

Next I

I've not seen 'Then' and 'Else' together without any code between, I don't know how that works.

Does it mean that the line 'NextI' should execute if the first 2 chars of TextArr(I) are "SB"? That might explain the lack of code after "Then".
In which case the Else statement means "If the first 2 chars are not "SB", set the array element's contents to null".

Have I got this right?

[jmcilhinney]

That is stupid code. I see people do that sort of thing quite a bit because they apparently don't understand logic. They basically do this:

If someExpression = True Then
    'Do nothing
Else
    'Do something
End If

when they should do this:

If someExpression = False Then
    'Do something
End If

You simply negate the condition and get rid of the empty block. In your case, that would be:

If Left(TextArr(I), 2) <> "SB" Then
    TextArr(I) = ""
End If

[moorea21]

Yes, I thought that too. I wondered if maybe there was some code after the "Then", and it got commented out at some stage and eventually deleted. Not logical otherwise, as far as I can see, as you said.

[DataMiser]

In which case the Else statement means "If the first 2 chars are not "SB", set the array element's contents to null".

Don't confuse an empty string with null as they are 2 different things. The code is setting it to an empty string in cases where the If tests false

[moorea21]

Thanks DataMiser, I'll look into that. So it's not deleting an array element, or making it's value null, but it is setting its value as an empty string. I got that now.

[Logophobic]

It isn't "stupid code." It is an optimization for speed.

If A = B Then Else C

will execute faster than

If A <> B Then C

That said, speed of execution usually isn't an issue. Habitual use of the former is poor style as the latter is more intuitive.

[jmcilhinney]

It isn't "stupid code." It is an optimization for speed.

If A = B Then Else C

will execute faster than

If A <> B Then C

That said, speed of execution usually isn't an issue. Habitual use of the former is poor style as the latter is more intuitive.

It is stupid code because, if you need that level of performance optimisation, what are you using VB for in the first place?

[Bonnie West]

It is an optimization for speed.

Whatever optimization was gained by using the = operator over the <> (if there is indeed one) will be negated by the fact that (in VB6, at least) the Else block is always slower than the Then block.

[DataMiser]

Personally I prefer to use Not with = rather than the <> as it is more readable at least for me, not sure if it would be faster than <> or not

If Not X=Y Then

[techgnome]

DM - exactly... if you're truly going for speed optimization a Not will out perform <>
It's one of the things I learned from doing ASM a number of years ago... even with a full if...then...else block... if a majority of your time drops into the else... rewrite it to use the inverted logic ... the THEN clause should always be the primary code path ... if you're looking for speed. (goes with Bonnie's comment). And that's probably true for just about any compiled language, it's not unique to VB.


-tg

[dw85745]

78:     i = 1
004026E2   mov         dword ptr [i],1
79:     If Not i = 0 Then
004026E9   cmp         dword ptr [i],0
004026ED   je          frmMain::cmdCrash_Click+29Ah (004026fa)
80:       i = i + 1
004026EF   mov         eax,dword ptr [i]
004026F2   add         eax,1
004026F5   jo          $L93+1Fh (00402775)
004026F7   mov         dword ptr [i],eax
81:     End If
82:
83:     j = 1
004026FA   mov         dword ptr [j],1
84:     If j <> 0 Then
00402701   cmp         dword ptr [j],0
00402705   je          frmMain::cmdCrash_Click+2B2h (00402712)
85:       j = j + 1
00402707   mov         eax,dword ptr [j]
0040270A   add         eax,1
0040270D   jo          $L93+1Fh (00402775)
0040270F   mov         dword ptr [j],eax
86:     End If

techgnome: Was curious whether there was a difference. Breaking out of a compiled VB6 program to the VC++ compiler, it looks to me that both the
use of "NOT" and "<>" gives the same ASM. Whether this is optimized further???