while loop in C# with multiple conditions

Question

This is my code:

while( Func(x) != ERR_D)
{
   if(result == ERR_A) 
         throw...; 
   if(result == ERR_B)
         throw...;

  mydata.x = x;
 }

The problem is that I want to use result = Func(x) in the while condition as the result will be checked inside the while loop. The while loop should call Func(x) untill it returns ERR_D. I can't use

do{ 
    result = Func(x);
   if(result == ERR_A) 
         throw ...; 
   if(result == ERR_B)
         throw ...;
    mydata.x = x;
   }while(result != ERR_D);

in my project as it first calls Func(x) which is what I don't want. But I have tried while(result = Func(x) != ERR_D), it doesn't work. Any ideas to solve this?

Well, x never changes. Maybe that has something to do with it? Hard to say, since "doesn't work" could mean almost anything and we have no idea what's supposed to be happening. — Ant P
– Ant P, Commented Sep 24, 2013 at 8:59
var result = Func(x); while (result != ERR_D) { doStuff(); result = Func(x); }? — Corak
– Corak, Commented Sep 24, 2013 at 8:59
Does it really make sense to throw exceptions in a while-loop? the loop will terminate as soon as one is thrown... — Stig-Rune Skansgård
– Stig-Rune Skansgård, Commented Sep 24, 2013 at 9:08

Gene · Accepted Answer · 2013-09-24 09:11:51Z

8

You just need to add some parentheses:

while((result = Func(x)) != ERR_D) { /* ... */ }

The != operator has a higher priority than the assignment, so you need to force the compiler to perform the assignment first (which evaluates to the assigned value in C#), before comparing the values on both sides of the != operator with each other. That's a pattern you see quite often, for example to read a file:

string line;

while ((line = streamReader.ReadLine()) != null) { /* ... */ }

edited Sep 24, 2013 at 9:11

answered Sep 24, 2013 at 9:02

Gene

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Comments

Darren · Accepted Answer · 2013-09-24 08:59:21Z

6

Try declaring result outside the loop and then assign it Funcs return value on each iteration.

For instance:

var result = Func(x);

while(result != ERR_D)
{
   if(result == ERR_A) 
         throw...; 
   if(result == ERR_B)
         throw...;

  mydata.x = x;
  result = Func(x);
 }

answered Sep 24, 2013 at 8:59

Darren

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Comments

King King · Accepted Answer · 2013-09-24 09:08:56Z

2

Try this:

while((result=Func(x)) != ERR_D){
 if(result == ERR_A) 
      throw...; 
 if(result == ERR_B)
      throw...;
 mydata.x = x;
}

NOTE: the assignment is done first in the parenthesis (result=Func(x)), this assignment is in fact done by the overload of the operator = and this operator returns the reference to the left side operand, that is result. After that, the result will be compared against the ERR_D via the operator !=.

edited Sep 24, 2013 at 9:08

answered Sep 24, 2013 at 8:58

King King

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Comments

Raidri · Accepted Answer · 2013-09-24 09:00:10Z

1

Try

while((result = Func(x)) != ERR_D)

answered Sep 24, 2013 at 9:00

Raidri

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Comments

Matthew Watson · Accepted Answer · 2013-09-24 09:13:27Z

0

You could express this using a while (true)...:

while (true)
{
    var result = Func(x);

    if (result == ERR_D)
        break;

    if (result == ERR_A) 
        throw ...; 

    if (result == ERR_B)
        throw ...;

    mydata.x = x;            
}

This is a loop-with-one-exit (if you ignore the throws ;)) so it is a structured loop.

You could also use a for loop, although it looks a little funky (pun not intended!):

for (var result = Func(x); result != ERR_D; result = Func(x))
{
    if (result == ERR_A) 
        throw ...; 

    if (result == ERR_B)
        throw ...;

    mydata.x = x;     
}

edited Sep 24, 2013 at 9:13

answered Sep 24, 2013 at 9:08

Matthew Watson

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