A common question is, how do I break out of two nested loops at once? For example, how can I examine pairs of characters in a string, stopping when I find an equal pair? The classic way to do this is to write two nested loops that iterate over the indexes of the string:
- C++ Break Statement
- How To Break A For Loop In C++ Word
- How To Break A For Loop In C Sharp
- How To Exit A For Loop In C++
- The break statement is used inside loops and switch case. C – break statement. It is used to come out of the loop instantly. When a break statement is encountered inside a loop, the control directly comes out of loop and the loop gets terminated. It is used with if statement, whenever used inside loop.
- When a break statement is encountered inside a loop, the loop is immediately terminated and the program control resumes at the next statement following the loop. It can be used to terminate a case in the switch statement (covered in the next chapter).
s='a string to examine'
foriinrange(len(s)):
forjinrange(i+1,len(s)):
ifs[i]s[j]:
answer=(i,j)
break# How to break twice???
A better example is two functions that have the same behaviour, so they could be compiled to the same asm if the compiler 'saw through' this odd idiom. I moved the ret += arri above the if in your example.As it turns out gcc 6.1 uses cmov for the if, which increases the length of the loop-counter dependency chain from 1 cycle to 3 cycles (Intel pre-Broadwell) or 2 cycles (Broadwell and later).
Here we are using two loops to generate the two indexes that we want to examine. When we find the condition we’re looking for, we want to end both loops.
There are a few common answers to this. But I don’t like them much:
- Put the loops into a function, and return from the function to break the loops. This is unsatisfying because the loops might not be a natural place to refactor into a new function, and maybe you need access to other locals during the loops.
- Raise an exception and catch it outside the double loop. This is using exceptions as a form of goto. There’s no exceptional condition here, you’re just taking advantage of exceptions’ action at a distance.
- Use boolean variables to note that the loop is done, and check the variable in the outer loop to execute a second break. This is a low-tech solution, and may be right for some cases, but is mostly just extra noise and bookkeeping.
C++ Break Statement
My preferred answer, and one that I covered in my PyCon 2013 talk, Loop Like A Native, is to make the double loop into a single loop, and then just use a simple break.
This requires putting a little more work into the loops, but is a good exercise in abstracting your iteration. This is something Python is very good at, but it is easy to use Python as if it were a less capable language, and not take advantage of the loop abstractions available.
Let’s consider the problem again. Is this really two loops? Before you write any code, listen to the English description again:
How can I examine pairs of characters in a string, stopping when I find an equal pair?
I don’t hear two loops in that description. There’s a single loop, over pairs. So let’s write it that way:
defunique_pairs(n):
''Produce pairs of indexes in range(n)''
foriinrange(n):
forjinrange(i+1,n):
yieldi,j
s='a string to examine'
fori,jinunique_pairs(len(s)):
ifs[i]s[j]:
answer=(i,j)
break
Here we’ve written a generator to produce the pairs of indexes we need. Now our loop is a single loop over pairs, rather than a double loop over indexes. The double loop is still there, but abstraced away inside the unique_pairs generator.
This makes our code nicely match our English. And notice we no longer have to write len(s) twice, another sign that the original code wanted refactoring. The unique_pairs generator can be reused if we find other places we want to iterate like this, though remember that multiple uses is not a requirement for writing a function.
I know this technique seems exotic. But it really is the best solution. If you still feel tied to the double loops, think more about how you imagine the structure of your program. The very fact that you are trying to break out of both loops at once means that in some sense they are one thing, not two. Hide the two-ness inside one generator, and you can structure your code the way you really think about it.
How To Break A For Loop In C++ Word
Python has powerful tools for abstraction, including generators and other techniques for abstracting iteration. My Loop Like A Native talk has more detail (and one egregious joke) if you want to hear more about it.
- C++ Basics
- C++ Object Oriented
- C++ Advanced
- C++ Useful Resources
- Selected Reading
There may be a situation, when you need to execute a block of code several number of times. In general, statements are executed sequentially: The first statement in a function is executed first, followed by the second, and so on.
Programming languages provide various control structures that allow for more complicated execution paths.
A loop statement allows us to execute a statement or group of statements multiple times and following is the general from of a loop statement in most of the programming languages −
C++ programming language provides the following type of loops to handle looping requirements.
How To Break A For Loop In C Sharp
Sr.No | Loop Type & Description |
---|---|
1 | while loop Repeats a statement or group of statements while a given condition is true. It tests the condition before executing the loop body. |
2 | for loop Execute a sequence of statements multiple times and abbreviates the code that manages the loop variable. |
3 | do...while loop Like a ‘while’ statement, except that it tests the condition at the end of the loop body. |
4 | nested loops You can use one or more loop inside any another ‘while’, ‘for’ or ‘do..while’ loop. |
Loop Control Statements
Loop control statements change execution from its normal sequence. When execution leaves a scope, all automatic objects that were created in that scope are destroyed.
C++ supports the following control statements.
Sr.No | Control Statement & Description |
---|---|
1 | break statement Terminates the loop or switch statement and transfers execution to the statement immediately following the loop or switch. |
2 | continue statement Causes the loop to skip the remainder of its body and immediately retest its condition prior to reiterating. |
3 | goto statement Transfers control to the labeled statement. Though it is not advised to use goto statement in your program. |
The Infinite Loop
How To Exit A For Loop In C++
A loop becomes infinite loop if a condition never becomes false. The for loop is traditionally used for this purpose. Since none of the three expressions that form the ‘for’ loop are required, you can make an endless loop by leaving the conditional expression empty.
When the conditional expression is absent, it is assumed to be true. You may have an initialization and increment expression, but C++ programmers more commonly use the ‘for (;;)’ construct to signify an infinite loop.
NOTE − You can terminate an infinite loop by pressing Ctrl + C keys.