How to structure If/Else?

[gozron]

Search on this forum is confusing. I searched for "else" or "if" (with and without quotes) with all topics selected and returned no results.

Anyway, here's my dilemma. (spell checked and doesn't look right) :P

I'm trying to go through a string of bits flipping every other one. Like so:

Code: Select all

t1=""
for i=1 to 7 step 2
   if mid(t,i,1)="0" then t1=t1+"1"
   if mid(t,i,1)="1" then t1=t1+"0"
   t1=t1+mid(t,i+1,1)
next i

Of course you see the problem. If the first condition is true then both lines get executed accomplishing nothing. I went through help looking for if/else formats trying every combination I can think of and keep getting "Else without If" errors. Can someone point me in the right direction? It's been several years and starting to enjoy this stuff again.

Regards

[Kot]

Since you can have only two values - 1 or 0, the second line is useless. Can't you do it like this:

if mid(t,i,1)="0" then t1=t1+"1" else t1=t1+"0"

[elsairon]

Of course you see the problem. If the first condition is true then both lines get executed accomplishing nothing. I went through help looking for if/else formats trying every combination I can think of and keep getting "Else without If" errors. Can someone point me in the right direction? It's been several years and starting to enjoy this stuff again.

Regards

You could add an elseif to the code, or use a select case, although that might be a bit much for only 2 choices.

Code: Select all

t1=""
for i=1 to 7 step 2
   if mid(t,i,1)="0" then
        t1=t1+"1"
   elseif mid(t,i,1)="1" then 
       t1=t1+"0"
   end if
   t1=t1+mid(t,i+1,1)
next i

[Richard]

Why make a decision when you can compute the inverse.

Code: Select all

Dim As String t, t1
t = "1111000" ' input test
t1 = t
For i as integer = 0 To 6 Step 2
    t1[i] = 1 Xor t1[i]
Next i
print t
print t1
Sleep

Here are three different ways of computing the inverse.

Code: Select all

Dim As String t = "0000000011111111"    ' input test string
Dim As String t1 = t, t2 = t, t3 = t
For i As Integer = 0 To Len(t1)-1 Step 2
    t1[i] = Asc("0")  +  Asc("1")  -  t1[i] ' subtract, the other remains
    t2[i] = Asc("0")  *  Asc("1")  /  t2[i] ' one cancels, the other remains
    t3[i] = Asc("0") Xor Asc("1") Xor t3[i] ' flip only the bits that differ
Next i
Print t
Print t1
Print t2
Print t3
Sleep

[Theunis Jansen]

On a pure QB4.5 basis -

I would suggest the following because it is more structured then one line coding and is easier to read. I tested it with -lang fblite and there are no problems.
If you use Fbide then there are some examples on the structure of IF then Else ElseIf. I prefer to use Select Case when more than one decision is involved. You can code precisely what you want in a more readable manner.
If we process t as indicated t1 will become t1="00000000"

Code: Select all

DEFSTR t
t="10101010"           REM or whatever it is you want to change
t1=t                       REM make t1 the same as t
for i=1 to 7 step 2   REM process t but change t1
   if mid(t,i,1)="0" then
        mid(t1,i,1)="1"
     else
       mid(t1,i,1) = "0" 
        Rem since we have checked for 0 we don't have to check for 1
   end if
next i
    Rem the i isn't really necessary. it slows down the loop in QB4.5

[bfuller]

@gozron

I see you are new on the forum, with only a couple of posts. Have you found the WIKI yet?

http://www.freebasic.net/wiki/wikka.php?wakka=DocToc for table of contents and

http://www.freebasic.net/wiki/wikka.php ... eyPgIfthen for IF....THEN

sorry if I'm being simplistic but you did post in the "beginners" forum and the WIKI is a huge help to me when I'm just trying to get something to run.