CODEWITHSUNDEEP
c++loopswhile-loopcurly-braces
user15425699
user15425699

Reputation:

How is while loop working without a block of code, shouldn't it work only for curly braces?

I know that while loops have a body part with two curly braces { }. And I know that how while loops work. But when I was reading a book on C++, I found this code:

#include <iostream>
int main()
{
    int sum = 0, value = 0;
    // read until end-of-file, calculating a running total of all values read
    while (std::cin >> value)
        sum += value;   // equivalent to sum = sum + value
    std::cout << "Sum is: " << sum << std::endl;
    return 0;
}

But there are no curly braces { } and these code is working same as:

#include <iostream>
int main()
{
    int sum = 0, value = 0;
    // read until end-of-file, calculating a running total of all values read
    while (std::cin >> value){
        sum += value;   // equivalent to sum = sum + value
    }
    std::cout << "Sum is: " << sum << std::endl;
    return 0;
}

So how can it be possible when I omitted curly braces { } in While Loops and how does it work when it omit the curly braces in while loops?

Upvotes: 1

Views: 992

Answers (9)

rawrex
rawrex

Reputation: 4064

The while's syntax is of the following form (simplified, check the link for more info):

while ( condition ) statement

Therefore, we need to supply it a statement after the condition part.

Curly braces denote a compound statement (a.k.a block, block statement). It is a group of zero or more statements that is treated by the compiler as if it were a single statement.

If you don't need a compound statement, you use an expression statements (a.k.a. one-liner). We terminate expression statements with a semicolon ;, while a block statement is terminated by closing curly brace.

In the example we can get away with an expression statement:

while (std::cin >> vInside curly braces wealue)
        sum += value;

In some cases, it is necessary to use curly braces to specify the flow of logic in program even if we use only expression statements. Also, some consider it to be a good practice to routinely use curly braces to explicitly denote boundaries every time.

Upvotes: 5

Văn Ng&#244;
Văn Ng&#244;

Reputation: 1

Beacuse while loop in your example just has only 1 statement and then it needn't have { } but still is correct. IF while loop has 2 or more statements, it has to have { } or else this loop will excute only your first statement . You can refer this while loop in C++

Upvotes: 0

Vlad from Moscow
Vlad from Moscow

Reputation: 311088

The while statement is defined the following way for example in the C++ 14 Standard

while ( condition ) statement

As you see there can be used any statement. For example a null statement like

 while ( condition );
                    ^
                    |
                   null statement

or a compound statement (that even may contain no statements) like

while ( condition ) {}

because the null statement and the compound statement are statements and there is no restriction on the kind of the statement. Also in C++ (opposite to C) declarations are also statements.

What you should take into account is that (The C++ 14 Standard, 6.5 Iteration statements)

2 The substatement in an iteration-statement implicitly defines a block scope (3.3) which is entered and exited each time through the loop. If the substatement in an iteration-statement is a single statement and not a compound-statement, it is as if it was rewritten to be a compound-statement containing the original statement. [ Example:

while (--x >= 0)
int i;

can be equivalently rewritten as

while (--x >= 0) {
int i;
}

Upvotes: 3

mugiseyebrows
mugiseyebrows

Reputation: 4743

Curly braces for if while and for are only neccesary when statement body contains more than one statement, but it's common to use it always as it protects from introducing bug by adding statement to body (see defensive programming practices).

For example there was vulnarability in SSH library recently like this:

if (condition)
   statement1;
   statement2;

statement2 intended to be executend only under condition, but it was executed unconditionally.

Probably it was only statement1 originally, but someone added statement2 and forgot to add braces. And somehow code passed review and get to production and cause some damage.

Upvotes: 2

Caleth
Caleth

Reputation: 63142

The grammar is while (condition) statement.

{ /* seqence-of-statements */ } is itself a statement.

A style guide might mandate {} for all if, for, while for the benefit of the humans who have to read the code.

Upvotes: 1

N0ll_Boy
N0ll_Boy

Reputation: 510

when you have only one statement it does not matter if you put curly braces or not. and this also applied to if, while, for, else, ...

but when you have more than one statement and if you did not put curly braces only first statement will execute during loop.

Upvotes: 2

Anas Roshdi
Anas Roshdi

Reputation: 11

Both of code is right if you just write one line of code no need at all to put curly braces.

Upvotes: -2

Raildex
Raildex

Reputation: 4765

omitting the curly braces makes the loop work on the first statement after the loop expression. so in your case, it will loop sum+=value.

This also works with if statements:

if(boolExpression)
    doSomething();
doSomethingNotDependendOnBoolExpr();

It is recommended to keep the curly braces. Or - if your codestyle omits them - keep the indendation intact so it is visible what's inside the loop and what is not.

Upvotes: -1

Michael Kruglos
Michael Kruglos

Reputation: 1286

When you omit the curly braces, only the statement right after while is executed in a loop.

My recommendation is to always use curly braces, as omitting them can create confusion, and lead to unexpected bugs, which can be difficult to notice.

Upvotes: 2

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