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Depends
thatrajaNobody remains a virgin, Life screws everyone 
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Do you use them? I feel sorry for you... 
If your actions inspire others to dream more, learn more, do more and become more, you are a leader." - John Quincy Adams
You must accept one of two basic premises: Either we are alone in the universe, or we are not alone in the universe. And either way, the implications are staggering” - Wernher von Braun
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Use as many as the algorithm requires.
If you need to examine every single point in a three dimensional space, then trying to do it without nesting three deep is messy, inefficient, and wastefull.
for (int x = 0; x < 100; x++)
{
for (int y = 0; y < 100; y++)
{
for (int z = 0; z < 100; z++)
{
...
}
}
}Worse, moving the nesting to methods to "make it look tidier" may well hide the amount of processing that is going on since it is no longer obvious that the single instruction in the middle of the centre loop is being executed 100 * 100 * 100 times, not just 100 times...
Having a blanket rule "This is messy!" is wrong, and counter productive.
Ideological Purity is no substitute for being able to stick your thumb down a pipe to stop the water
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From that article:
Here's a common example:
double sum = 0;for (int i = 0; i < array.length; i++) { sum += array[i];}
What's going on here? I've been programming for years, and I'm comfortable speed-reading this idiom; it's obviously a summation of a set of values in an array. But to actually read this block of code, I need to process about 30 tokens spread out over four lines.
And here are programmers writing pragma, karma and dogma at the beginning of their programs and they have the gall to complain about a for loop? 
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I never go further than 26 levels, because then I've run out of variables to control the loops with.
I could of course start using aa, ab, ac...but that gets confusing.
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I like to use Z1, Z2, Z3, etc... Then I can have hundreds and thousands of nested loops
If your actions inspire others to dream more, learn more, do more and become more, you are a leader." - John Quincy Adams
You must accept one of two basic premises: Either we are alone in the universe, or we are not alone in the universe. And either way, the implications are staggering” - Wernher von Braun
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Problem with that is the indentation gets too large and you keep having to scroll to the right.
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Buy a wider monitor.
Ideological Purity is no substitute for being able to stick your thumb down a pipe to stop the water
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Mmmmmm....Hundreds and Thousands[^]
Ideological Purity is no substitute for being able to stick your thumb down a pipe to stop the water
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mmmmmmmmmmmm, doughnut 
If your actions inspire others to dream more, learn more, do more and become more, you are a leader." - John Quincy Adams
You must accept one of two basic premises: Either we are alone in the universe, or we are not alone in the universe. And either way, the implications are staggering” - Wernher von Braun
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Wrong of you to use a thru z as control variables.
Use a1, a2, ......... and it will not be confusing at all! 
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Add, as comments to the end of the loops, the containing logic and you can have as many nested as required...
for(int i=0; i<100; i++) {
for(int j=0; j<100; j++) {
for(int k=0; k<100; k++) {
}
}
}
“That which can be asserted without evidence, can be dismissed without evidence.”
― Christopher Hitchens
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As many as required, but no more.
Limiting the number of nested loops is like prescribing a length for variable names:
"Variable names must be at least six characters and no more than 31 characters in length, must begin with an upper case alphabetic character, may not include an underscore, and must consist of one or more complete English words, signified through use of upper case characters at the beginning of each word".
Picking names will be like playing Scrabble...
Software Zen: delete this;
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Sounds like a valid password policy!
'As programmers go, I'm fairly social. Which still means I'm a borderline sociopath by normal standards.' Jeff Atwood
'I'm French! Why do you think I've got this outrrrrageous accent?' Monty Python and the Holy Grail
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Is this from a wikipedia study:
Shameel wrote: Most people would agree that three levels is acceptable
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How about looping through a n-dimensional table where n>3?
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I would say that a rule is ok and if that rule gets broken, a good justification as to why it was broken needs to be given.
One can always refactor code to eliminate loops/complexity.
I feel that it is especially important if the code needs to be tested to make such a rule. (else the testing cost more money than you're making on the project)
Bad Example:
String name[100][100] = fill_string();
void loop_level_1()
{
for(int i=0; i<100; i++)
{
loop_level_2(i);
}
}
void loop_level_2(int i)
{
for(int j=0; j<100; j++)
{
loop_level_3(i, j);
}
}
void loop_level_3(int i, int j)
{
for(int k=0; k<100; k++)
{
print(k + ": " + name[i][j]);
}
}
This way, each function can be tested in isolation.
"Program testing can be used to show the presence of bugs, but never to show their absence."
<< please vote!! >>
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The question is why you think nested loops are broken in the first place?
The only thing you achieve is separate the context in which the need for nested loops arose, and therefore remove the ability to spot possible optimizations easily.
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Not with you regarding the question you are asking... I was referring to the braking of the rule, not the loop.
"Program testing can be used to show the presence of bugs, but never to show their absence."
<< please vote!! >>
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Refer to:
http://en.wikipedia.org/wiki/Cyclomatic_complexity[^] more specifically the section on "Implications for Software Testing", for a better understanding from where I'm coming from.
"Program testing can be used to show the presence of bugs, but never to show their absence."
<< please vote!! >>
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Interesting article. However, according to these definitions a triple nested loop only has a complexity of 4, and the inventer suggested to break up code when exceeding a maximum complexity of 10(!).
(1)->(2)->(3)->(4)->(5)->(6)->(7)->(8)
^ ^ ^ | | |
| | | | | |
| | (9)<--- | |
| (0)<------------- |
(a)<-----------------------
M = E − N + 2P = 13 - 11 + 2*1 = 4
Also, extracting the inner loop into a separate function does not even help, as it increases P:
(1)->(2)->(3)---(call)-->(4)->(5)->(6)
^ ^ | |
| | | |
| (7)<------------- |
(8)<-----------------------
M1 = E1 − N1 + 2P1 = 9 - 8 + 2*1 = 3
(1)->(2)->(3)->(4)
^ |
| |
(5)<---
M2 = E2 − N2 + 2P2 = 5 - 5 + 2*1 = 2
and
M = E - N + 2P = 14 - 13 +2*2 = 5
So, by extracting a loop into another function, you are actually increasing the complexity, instead of reducing it!
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"One common testing strategy, espoused for example by the NIST Structured Testing methodology, is to use the cyclomatic complexity of a module to determine the number of white-box tests that are required to obtain sufficient coverage of the module."
The word module that they are referring to is basically a function.
So the complexity generally gets measured on the function and not the whole program. Therefor braking a nested if into function calls will actually reduce/spread the complexity of each function making them simpler and easier to test.
So instead of having one function with example a complexity of 25, you'll have 3 functions with a complexity of 10 each.
"Program testing can be used to show the presence of bugs, but never to show their absence."
<< please vote!! >>
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That's what I thought at first, too. But then, what is P?
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"For a single program (or subroutine or method), P is always equal to 1." Thus each function without having any loops/if-statements ect. starts with a complexity of 1.
"Program testing can be used to show the presence of bugs, but never to show their absence."
<< please vote!! >>
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