break out of if statement

Question

Can you break out of an if statement or is it going to cause crashes  I m starting to acquaint myself with C  but this seems controversial  The first image is from a book on C   Head First C   and the snippet shows code written by Harvard s CS classes staff  What is actually going on and has it something to do with C standards      breaks don t break if statements       On January 15  1990  AT amp T s long-distance telephone system crashed  and 60 000 people lost their phone service   The cause  A developer working on the C code used in the exchanges tried to use a break to break out of an if statement   But breaks don t break out of ifs  Instead  the program skipped an entire section of code and introduced a bug that interrupted 70 million phone calls over nine hours    for  size   0  size  lt  HAY MAX  size             wait for hay until EOF     printf   nhaystack  d       size       int straw   GetInt        if  straw    INT MAX          break          add hay to stack     haystack size    straw    printf   n

User · Answer

I think the question is a little bit fuzzy - for example, it can be interpreted as a question about best practices in programming loops with if inside. So, I'll try to answer this question with this particular interpretation.

If you have if inside a loop, then in most cases you'd like to know how the loop has ended - was it "broken" by the if or was it ended "naturally"? So, your sample code can be modified in this way:

bool intMaxFound = false;
for (size = 0; size < HAY_MAX; size++)
{
  // wait for hay until EOF
  printf("\nhaystack[%d] = ", size);
  int straw = GetInt();
  if (straw == INT_MAX)
     {intMaxFound = true; break;}

  // add hay to stack
  haystack[size] = straw;
}
if (intMaxFound)
{
  // ... broken
}
else
{
  // ... ended naturally
}

The problem with this code is that the if statement is buried inside the loop body, and it takes some effort to locate it and understand what it does. A more clear (even without the break statement) variant will be:

bool intMaxFound = false;
for (size = 0; size < HAY_MAX && !intMaxFound; size++)
{
  // wait for hay until EOF
  printf("\nhaystack[%d] = ", size);
  int straw = GetInt();
  if (straw == INT_MAX)
     {intMaxFound = true; continue;}

  // add hay to stack
  haystack[size] = straw;
}
if (intMaxFound)
{
  // ... broken
}
else
{
  // ... ended naturally
}

In this case you can clearly see (just looking at the loop "header") that this loop can end prematurely. If the loop body is a multi-page text, written by somebody else, then you'd thank its author for saving your time.

UPDATE:

Thanks to SO - it has just suggested the already answered question about crash of the AT&T phone network in 1990. It's about a risky decision of C creators to use a single reserved word break to exit from both loops and switch.

Anyway this interpretation doesn't follow from the sample code in the original question, so I'm leaving my answer as it is.

User · Answer

This is actually the conventional use of the break statement  If the break statement wasn t nested in an if block the for loop could only ever execute one time   MSDN lists this as their example for the break statement

User · Answer

break interacts solely with the closest enclosing loop or switch  whether it be a for  while or do    while type  It is frequently referred to as a goto in disguise  as all loops in C can in fact be transformed into a set of conditional gotos   for  A  B  C  D     translates to A  goto test  loop  D  iter  C  test  if  B  goto loop  end   while  B  D              Simply doesn t have A or C do   D    while  B       Omits initial goto test continue                 goto iter  break                    goto end    The difference is  continue and break interact with virtual labels automatically placed by the compiler  This is similar to what return does as you know it will always jump ahead in the program flow  Switches are slightly more complicated  generating arrays of labels and computed gotos  but the way break works with them is similar    The programming error the notice refers to is misunderstanding break as interacting with an enclosing block rather than an enclosing loop  Consider   for  A  B  C       D     if  E           F         if  G  break       Incorrectly assumed to break if E   breaks for          H          I    J    Someone thought  given such a piece of code  that G would cause a jump to I  but it jumps to J  The intended function would use if   G  H  instead

User · Answer

As already mentioned that  break-statement works only with switches and loops  Here is another way to achieve what is being asked  I am reproducing  https   stackoverflow com a 257421 1188057 as nobody else mentioned it  It s just a trick involving the do-while loop   do        do something   if  error        break           do something else   if  error        break           etc     while  0     Though I would prefer the use of goto-statement

[c] "break;" out of "if" statement?

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