for(size_t i=0;i<vec.size();i++){
if(n>vec[i]){
a=i;
break;
}
}
in this example, am I breaking the if statement or the for loop?
The break statement is used to break out of a switch or iteration statement i.e. a while, do or for loop. The C++ draft standard section 6.6.1 The break statement says:
The break statement shall occur only in an iteration-statement or a switch statement and causes termination of the smallest enclosing iteration-statement or switch statement; control passes to the statement following the terminated statement, if any.
Since the if is not an iteration statement or a switch then the break will leave the for loop.
A break statement ends only the do, for, switch, or while statement that immediately encloses it. It doesn't break out of an if statement, so your code is breaking out of the loop.
It will break from the for loop.
In loops, the break statement ends execution of the nearest enclosing
do, for, or while statement.
Source: Microsoft docs
Related
I have seen some very weird for loops when reading other people's code. I have been trying to search for a full syntax explanation for the for loop in C but it is very hard because the word "for" appears in unrelated sentences making the search almost impossible to Google effectively.
This question came to my mind after reading this thread which made me curious again.
The for here:
for(p=0;p+=(a&1)*b,a!=1;a>>=1,b<<=1);
In the middle condition there is a comma separating the two pieces of code, what does this comma do? The comma on the right side I understand as it makes both a>>=1 and b<<=1.
But within a loop exit condition, what happens? Does it exit when p==0, when a==1 or when both happen?
It would be great if anyone could help me understand this and maybe point me in the direction of a full for loop syntax description.
The comma is not exclusive of for loops; it is the comma operator.
x = (a, b);
will do first a, then b, then set x to the value of b.
The for syntax is:
for (init; condition; increment)
...
Which is somewhat (ignoring continue and break for now) equivalent to:
init;
while (condition) {
...
increment;
}
So your for loop example is (again ignoring continue and break) equivalent to
p=0;
while (p+=(a&1)*b,a!=1) {
...
a>>=1,b<<=1;
}
Which acts as if it were (again ignoring continue and break):
p=0;
while (true) {
p+=(a&1)*b;
if (a == 1) break;
...
a>>=1;
b<<=1;
}
Two extra details of the for loop which were not in the simplified conversion to a while loop above:
If the condition is omitted, it is always true (resulting in an infinite loop unless a break, goto, or something else breaks the loop).
A continue acts as if it were a goto to a label just before the increment, unlike a continue in the while loop which would skip the increment.
Also, an important detail about the comma operator: it is a sequence point, like && and || (which is why I can split it in separate statements and keep its meaning intact).
Changes in C99
The C99 standard introduces a couple of nuances not mentioned earlier in this explanation (which is very good for C89/C90).
First, all loops are blocks in their own right. Effectively,
for (...) { ... }
is itself wrapped in a pair of braces
{
for (...) { ... }
}
The standard sayeth:
ISO/IEC 9899:1999 §6.8.5 Iteration statements
¶5 An iteration statement is a block whose scope is a strict subset of the scope of its
enclosing block. The loop body is also a block whose scope is a strict subset of the scope
of the iteration statement.
This is also described in the Rationale in terms of the extra set of braces.
Secondly, the init portion in C99 can be a (single) declaration, as in
for (int i = 0; i < sizeof(something); i++) { ... }
Now the 'block wrapped around the loop' comes into its own; it explains why the variable i cannot be accessed outside the loop. You can declare more than one variable, but they must all be of the same type:
for (int i = 0, j = sizeof(something); i < j; i++, j--) { ... }
The standard sayeth:
ISO/IEC 9899:1999 §6.8.5.3 The for statement
The statement
for ( clause-1 ; expression-2 ; expression-3 ) statement
behaves as follows: The expression expression-2 is the controlling expression that is
evaluated before each execution of the loop body. The expression expression-3 is
evaluated as a void expression after each execution of the loop body. If clause-1 is a
declaration, the scope of any variables it declares is the remainder of the declaration and
the entire loop, including the other two expressions; it is reached in the order of execution
before the first evaluation of the controlling expression. If clause-1 is an expression, it is
evaluated as a void expression before the first evaluation of the controlling expression.133)
Both clause-1 and expression-3 can be omitted. An omitted expression-2 is replaced by a
nonzero constant.
133) Thus, clause-1 specifies initialization for the loop, possibly declaring one or more variables for use in
the loop; the controlling expression, expression-2, specifies an evaluation made before each iteration,
such that execution of the loop continues until the expression compares equal to 0; and expression-3
specifies an operation (such as incrementing) that is performed after each iteration.
The comma simply separates two expressions and is valid anywhere in C where a normal expression is allowed. These are executed in order from left to right. The value of the rightmost expression is the value of the overall expression.
for loops consist of three parts, any of which may also be empty; one (the first) is executed at the beginning, and one (the third) at the end of each iteration. These parts usually initialize and increment a counter, respectively; but they may do anything.
The second part is a test that is executed at the beginning of each execution. If the test yields false, the loop is aborted. That's all there is to it.
The C style for loop consists of three expressions:
for (initializer; condition; counter) statement_or_statement_block;
The initializer runs once, when the loop starts.
The condition is checked before each iteration. The loop runs as long it evaluates to true.
The counter runs once after each iteration.
Each of these parts can be an expression valid in the language you write the loop in. That means they can be used more creatively. Anything you want to do beforehand can go into the initializer, anything you want to do in between can go into the condition or the counter, up to the point where the loop has no body anymore.
To achieve that, the comma operator comes in very handy. It allows you to chain expressions together to form a single new expression. Most of the time it is used that way in a for loop, the other implications of the comma operator (e.g. value assignment considerations) play a minor role.
Even though you can do clever things by using syntax creatively - I would stay clear of it until I find a really good reason to do so. Playing code golf with for loops makes code harder to read and understand (and maintain).
The wikipedia has a nice article on the for loop as well.
Everything is optional in a for loop. We can initialize more than one variable, we can check for more than one condition, we can iterate more than one variable using the comma operator.
The following for loop will take you into an infinite loop. Be careful by checking the condition.
for(;;)
Konrad mentioned the key point that I'd like to repeat: The value of the rightmost expression is the value of the overall expression.
A Gnu compiler stated this warning when I put two tests in the "condition" section of the for loop
warning: left-hand operand of comma expression has no effect
What I really intended for the "condition" was two tests with an "&&" between. Per Konrad's statement, only the test on to the right of the comma would affect the condition.
the for loop is execution for particular time for(;;)
the syntex for for loop
for(;;)
OR
for (initializer; condition; counter)
e.g (rmv=1;rmv<=15;rmv++)
execution to 15 times in for block
1.first initializ the value because start the value
(e.g)rmv=1 or rmv=2
2.second statement is test the condition is true or false ,the condition true no.of time execution the for loop and the condition is false terminate for block,
e.g i=5;i<=10 the condition is true
i=10;i<10 the condition is false terminate for block,
3.third is increment or decrement
(e.g)rmv++ or ++rmv
I read Why I don't need brackets for loop and if statement, but I don't have enough reputation points to reply with a follow-up question.
I know it's bad practice, but I have been challenged to minimise the lines of code I use.
Can you do this in any version of C++?
a_loop()
if ( condition ) statement else statement
i.e. does the if/else block count as one "statement"?
Similarly, does if/else if.../else count as one "statement"? Though doing so would become totally unreadable.
The post I mentioned above, only says things like:
a_loop()
if(condition_1) statement_a; // is allowed.
You can use ternary operator instead of if...else
while(true) return condition_1 ? a : b;
while seems redundant here if the value of its argument is always true so you can simply write
return condition_1 ? a : b;
Yes, syntactically you can do that.
if/else block is a selection-statement, which is a kind of statement.
N3337 6.4 Selection statements says:
selection-statement:
if ( condition ) statement
if ( condition ) statement else statement
switch ( condition ) statement
In fortran 90, does an if stop statement require a closing endif?
example:
if(foo.eq.1) stop
!do some stuff
Is do some stuff part of the loop or does stop imply endif as the program is ended?
There are mainly two places (apart from the arithmetic if) where the if keyword can be met.
Firstly it is the logical if statement
if (condition) statement_if_true
If the condition is true, the statement_if_true is executed. Anything that follows is not part of the if statement. There is no then and no end if here.
Secondly there is the if conditional construct
if (condition) then
body with statements
end if
The body can contain any number of statements or constructs and must be followed by end if. The then keyword is obligatory for the construct and the body begins on a new line after the then.
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for (...)
for (...)
{
break;
break; // 1
}
Will the code at (1) execute? There could also be there continue or anything else. I know I could just check in my debugger but I wanna know what C++ standard says about it as my compiler might just be a special case.
As per the specifications
6.6.1 The break statement [stmt.break]
The break statement shall occur only in an iteration-statement or a switch statement and causes termination
of the smallest enclosing iteration-statement or switch statement; control passes to the statement
following the terminated statement, if any.
Hence 1 should not even reach . Some Java compiler might even flag that code unreachable.
As per standard
Within nested statements, the break statement ends the do, for, switch, or while statement that immediately encloses it.
So in your case it will always break at first and never come to second break statement.
The code at(1) will not be executed, break means jump out of the loop, so after the first break, the code will jump out of the inner loop.
Within that particular loop, no, no code after break will execute as break will drop out of that loop and continue execution from after the end of the inner for loop. Although any code outside of the inner loop that appears after the break (within the outer loop), will be executed.
No, each time the first break statement is hit the inner loop will exit and the second break statement will never be reached.
You can see here.
And I am sure the next break never exce . No matter what compiler you use.
No. The code execution jumps out of the inner first loop the moment a break is encountered.
Even if the code is this way
break;
continue;
break;
the same thing happens. Continue is just a way to tell the compiler to iterate the loop further skipping any code in between.
I was asked this question in an interview.
I replied that it was a conditional construct because
It executes once, unlike a loop which has the capability to execute multiple times.
There is no loop control mechanisms, there is only conditional switching based on different cases.
So is my answer right or wrong, is there a better answer?
Also he asked me the reason why break; statements work with switch-case since, break; only works with loops.
This question I could not answer.
In C++
switch is selection-statement
n3376 6.4/1 and 6.4.2 is about switch
selection-statement:
...
switch ( condition ) statement
break is jump-statement
n3376 6.6.1/1
The break statement shall occur only in an iteration-statement or a switch statement and causes termination
of the smallest enclosing iteration-statement or switch statement; control passes to the statement following
the terminated statement, if any.
C answer
There is no formal term called "conditional construct". The C standard uses the term selection statement. The C language has three different selection statements: if, if-else and switch (C11 6.8.4). Loops sort under the category of iteration statements (6.8.5).
The break statement is a jump statement, just like goto. It has some restrictions of where it is allowed to appear:
C11 6.8.6.3
A break statement shall appear only in or as a switch body or loop
body.
So to answer the interview questions:
Is switch case a loop or a conditional construct?
If you by conditional construct mean a selection statement, then yes, switch is a conditional construct.
why break; statements work with switch-case since, break; only works with loops
No, the question is incorrect, it does not only work with loops. It works with switch and loops. This is because the C language is defined in that way (6.8.6.3).
A switch case is a way of wrapping a block of instructions and saying execute (part of) it, beginning here and ending here. The matching case marks the beginning and the following break marks the end.
The block could be a few instructions:
{
instruction_A;
instruction_B;
instruction_C;
instruction_D;
}
The case statements say where to dynamically start based upon the switch value:
switch(value)
{
case one:
instruction_A;
instruction_B;
case two:
instruction_C;
case three:
instruction_D;
}
In case one, all the instructions will be called, as there is no break. Case two will execute C and D, if there are no exceptions (c;.
The break statements say where to stop, and mean it's possible to drop through a number of case statements:
switch(value)
{
case one:
instruction_A;
instruction_B;
case two:
instruction_C;
break;
case three:
instruction_D;
}
Case one will now execute A, B, and C, but not D.