question about for loop..........

Question

3.00/5 (3 votes)

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C++

#include <iostream.h>
void main()
{

for(int n=1;n<4;n++);
cout<<"abc";

my question is that when i use semicolon at the end of loop then for loop execute body of loop only once. how for loop works in this case

Posted 21-Aug-13 19:12pm

ALIWAZ

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5 solutions

Solution 4

for the sake of understanding the for loop construct usually have following format

C#

for(<statement>;<condition>;<statement>;)
  <body statement>;
</statement></condition></statement>

Lets take a simple example, the below for loop will execute the body 4 times;

C#

for (int i=1;i < 5;i++)
  cout << i;

but semicolon at the end of for loop instruct that the said for loop does not have any body so will not execute the cout instruction as part of for loop

C#

for (int i=1;i < 5;i++)
  cout << "Test";

For loop parameter are optional for example this is valid for loop, the only difference is that it does not have declaration and increment statements;

C#

For (;i < 5;)
  cout << i++;

similarly this is also valid but it will be an infinite for loop

C#

for (;;)
  cout << i;

Posted 21-Aug-13 19:53pm

_Asif_

Updated 22-Aug-13 20:14pm

v2

Comments

Stefan_Lang 22-Aug-13 7:22am

<quote>
for (int i=1;i < 5;i++);
cout << i;

Actually this will not compile as the scope of the variable i ends at the end of the first line. If you got an up-to-date compiler, it will issue an error "undefined symbol: i".

_Asif_ 23-Aug-13 2:12am

My bad, corrected!

Solution 3

In this case abc is print only once because you use ; at the end of loop because cout<<"abc"; not body of loop actually .

Posted 21-Aug-13 19:48pm

Antariksh Verma

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Sergey Alexandrovich Kryukov · Accepted Answer · 2013-08-21T19:34:00

Solution 1

Way too simple.

Semicolon means "empty statement". If you use it, the it becomes the body of the loop; so the loop repeats 4 times but does nothing. The second line goes out of the loop, so it is only executed once.

If you remove this semicolon, the body of the loop statement is your second line; it will be executed 4 times.

—SA

Posted 21-Aug-13 19:34pm

Sergey Alexandrovich Kryukov

Comments

ridoy 22-Aug-13 1:45am

exactly what i will say,my 5!

Sergey Alexandrovich Kryukov 22-Aug-13 2:07am

Thank you.
—SA

Thanks7872 · Accepted Answer · 2013-08-21T19:37:00

Refer this link

Effect of Semicolon after 'for' loop[^]

It states that:

Quote:
Semicolon is a legitimate statement called null statement * that means "do nothing". Since the for loop executes a single operation (which could be a block enclosed in {}) semicolon is treated as the body of the loop, resulting in the behavior that you observed.

The following code

C#
for (i=0;i<5;i++);
 {
     printf("hello\n");
 }
is interpreted as follows:

C#
Repeat five times for (i=0;i<5;i++)
... do nothing (semicolon)
Open a new scope for local variables {
... Print "hello"
Close the scope }

If we look at your code

C#

for(int n=1;n<4;n++);//loop four times,do nothing.
cout<<"abc";//Executed once

Regards..:)

SaqibRasheed · Accepted Answer · 2013-08-23T03:03:00

for(int n=1;n<4;n++); means that there is no other statement inside the for loop. So, the loop will be execute 4 times but will print nothing or will do no processing. After the loop execution is finished, the statements following the for loop will be executed; that's why you get the "abc" printed only once. The reason is very simple that cout<<"abc"; is not part of for loop because for loop has finished without doing any processing or printing.