Cannot understand for loop with two variables

Question

When I use two variables in a for loop with different conditions two conditions like I have used below i<3,j<2 the for loop is always executing till the second condition fails.

#include<iostream>
#include<conio.h>
using namespace std ;
int main()
{
int i,j ;
for(i=0,j=0;i<3,j<2;i++,j++)
{
    cout<<"hello" ;
}
getch() ;
return 0 ;
}

In that code, hello is printed 2 times. Why?

If I use i<3,j<10, "Hello" is printed 10 times. I can't understand why the first condition is being neglected. Is it compiler dependent or something else?

Every thing works normal if I replace with conditions like || (OR) or &&(AND).An other thing is that I cannot initialize i and j in the for loop itself, it is showing me an error, but works fine when I declare variables in C style or one variable outside the for loop, why is it so?

Compiler I have used is Orwell Dev C++.
Thanks in advance.

You, like many other confused people, ran into the comma operator. — chris, Oct 05 '13 at 05:44
`i<3,j<2` is an interesting way to write a condition... (compilable, but ...) — Alexei Levenkov, Oct 05 '13 at 05:47
go through this: it mght help http://stackoverflow.com/questions/1232176/how-to-put-two-increment-statements-in-a-c-for-loop?answertab=votes#tab-top — Rat-a-tat-a-tat Ratatouille, Oct 05 '13 at 05:51
Not so much "interesting" as it is just attempting to be confusing. The first part gets discarded, so you could say, "i > 0, j < 2" and it would do the same thing. — Zac Howland, Oct 05 '13 at 05:52
`g++-4.8` `7:14: warning: left operand of comma operator has no effect` — Alex, Oct 05 '13 at 06:22

score 7 · Accepted Answer · answered Oct 05 '13 at 05:47

7

for(i=0,j=0;i<3,j<2;i++,j++)

is equivalent to

for(i=0,j=0;j<2;i++,j++)

The comma expression takes on the value of the last expression.

Whichever condition is first, will be disregarded, and the second one will be used only.

answered Oct 05 '13 at 05:47

P0W

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But why is it that i cant declare i and j in the loop itself.The second part of the question in the post – Arun Oct 05 '13 at 05:56
@Arun may be you should re-visit `for` loop, from a good book – P0W Oct 05 '13 at 05:59
@Arun You don't show us the code that generated the error nor the specific error you got, so we could only guess. – David Schwartz Oct 05 '13 at 05:59

CygnusX1 · Answer 2 · 2013-10-05T13:47:42.763

The for loop consists of:

for(START_STATEMENT; CONDITION_EXPRESSION, LOOP_EXPRESSION) BODY_BLOCK

Where:

START_STATEMENT is any single statement, which may include variable declaration. If you want to declare 2 variables, you can write int i=0, j=0, but not int i=0; int j=0 because the latter are actually 2 statements. Also node, that variable declaration is a part of statement, but cannot be a part of (sub) expression. That is why int i=0, int j=0 would also be incorrect.
CONDITION_EXPRESSION is any single expression that evaluates to a boolean value. In your case you are using a coma operator which has the following semantics: A, B will do:
- evaluate A (it will evaluate, not just ignore)
- ditch the result of A
- evaluate B
- return B as the result
In your case: i<3,j<2 you are comparing i<3, you are just ignoring the result of this comparison.

Comma expressions are useful when the instructions have some side effects, beyond just returning a value. Common cases are: variable increment/decrement or assignment operator.
LOOP_EXPRESSION is any single expression that does not have to evaluate to anything. Here you are using the comma expression again, ignoring the result of the left-hand-side. In this case however, you are not using the result anyway, and just using the ++ side effect - which is to increment the values of your variables.
BODY_BLOCK is either a single statement or a block, encapsulated with curly braces.

The above for can be compared to:

{
    START_STATEMENT;
    while(EXPRESSION) {
        BODY_BLOCK;
        LOOP_EXPRESSION;
    }
}

Not really needed. It was there in previous (incorrect) version which I quickly corrected, but left it there. — CygnusX1, Oct 05 '13 at 13:47

score -1 · Answer 3 · answered Oct 05 '13 at 05:51

-1

The c complier always used second condition.

therefore j<2 is used.

use this for loop

      for(i=0,j=0;j<10;i++,j++)

answered Oct 05 '13 at 05:51

Dev

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Cannot understand for loop with two variables

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