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Struggling with ==

12 posts in this topic

Hello all,

Might be a basic question with a blindingly obvious answer, but I am having a slight issue.

I want to perform a conditional check on 4 variables, with the first three being equal too the last.

I thought I would go with

[source lang="cpp"]if(a == b == c == d){
// execute whatever
}[/source]
Okay, so I assumed that this would work, each variable represents a char type data. I have commented out b and c, so that there is a check to see if a == d, and this fires the execution block of code. But when I try to add b and c back in, one at a time, the conditional check fails. I have tried bracketing with c==d on the inner-most brackets. I think my brain has frazzled.

Thanks in advance for any assistance.

Stitchs.
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I think that is what I want though, I am happy if A equals B equals C equals TestValue (d). I did try Brother Bob's way before, but it seems to no avail. Or is what you guys are saying that this is incorrect and if I would like to test all the values against TestValue (d), then I should roll with logical && and go a == Test && b == Test etc.

Thanks
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What I said is not how to do it, I tried to explain why your way didn't work. You need to check them pairwise with logical and to ensure that they are all equal.
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In short, what people are saying is that you're comparing (after the first comparison) booleans and chars. Not gonna work, in most languages.
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Okay, now I get fully understand what you're all saying.

Basically, after the first comparison, which gets evaluated to 0 or 1 (depending on the condition), this may never equal the next character variable as said variable could represent a higher number/letter.
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I like pictures. I know you're understanding, but here's a little drawing of what's happening:


[code]
if (a == b == c == d) // Step 1 for the computer (let's assume a does indeed equal b here)
^^^^^^ resolves to true
if (true == c == d) // Step 2 for the computer
^^^^^^ since a == b is true, replace it with true
if (true == c == d) // Step 3 for the computer
^^^^^^^^^ Now we have to check if true == c... Wait... what? true is a boolean, c is a char, and we're trying to check if they're equal to each other? This is where everything goes wrong...
[/code]

I'm not even going to draw out the rest of what the computer does, because at this point you should see the logic is quite wrong. Edited by Cornstalks
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[quote name='Brother Bob' timestamp='1341944190' post='4957699']
It should be obvious if you rewrite it like this:
[source]
if(((a == b) == c) == d)
[/source]
[/quote]
also:
[source]
if(a == b)
{
if(1== c)
{
if(1== d)
{
...
}
}
}
[/source]
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[quote name='Krypt0n' timestamp='1342083357' post='4958316']
[quote name='Brother Bob' timestamp='1341944190' post='4957699']
It should be obvious if you rewrite it like this:
[source]
if(((a == b) == c) == d)
[/source]
[/quote]
also:
[source]
if(a == b)
{
if(1== c)
{
if(1== d)
{
...
}
}
}
[/source]
[/quote]

That's not right. If a=1, b=3, c=5, d=0 the original code would "do something", but your code would not.
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if you 'really' wanted to expand (if (a == b == c == d)) into cascaded if's you'd have to write:

[source lang="java"]
if (a == b) {
if (1 == c) {
if (1 == d) {
...
}
}else if (0 == d) {
...
}
}else {
if (0 == c) {
if (1 == d) {
...
}
}else if (0 == d) {
...
}
}
[/source]

with each ... being replaced by the body of the origin if statement :P
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