Converting for loop to do while loop in C++

Question

I need to convert this for loop to a do while:

for (int i = 0; i <= input; i = i + 2)
{
    cout << name << " is number " << input << endl;
    total = total + i;
}
cout << endl;
cout << total << endl;

This is what I have so far:

do
{
    cout << name << " is number " << input << endl;
    i += 2;
    total += i;
} while (i <= input);
cout << endl;
cout << total << endl;

It doesn't give the same total value as the for loop. What am I doing wrong?

Did you initialize i correctly to zero before the loop? Are you sure that input can never by negative? — Patrick
– Patrick, Commented Apr 3, 2012 at 16:16
Yeah I already have all of the variables listed with a value of 0, except for the string name, so I know it can't be that. — quasipsychotic
– quasipsychotic, Commented Apr 3, 2012 at 16:16

user700284 · Accepted Answer · 2012-04-03 16:17:00Z

4

You have to add i to the total before incrementing it by 2

So the do..while loop should be like this:

do
{
    cout << name << " is number " << input << endl;
    total += i;
    i += 2;
} while (i <= input);
cout << endl;
cout << total << endl;

answered Apr 3, 2012 at 16:17

user700284

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2 Comments

user700284 Over a year ago

Glad to be of help.Please mark the answer as accepted if you think it has solved your query.

Seagull Over a year ago

It won't do the same if the i > input before entering the loop.

Seagull · Accepted Answer · 2012-04-03 16:41:45Z

The main difference between for loop and do-while loop is the fact that:

For loop executes 0 or more times
But the do-while loop executes 1 or more times

Example:

int input = 100;

//Will never execute as i is bigger than 5
for (int i = input; i<5; ++i)
    cout << i;

//Will execute only one time as i < 5 is checked only
//after first execution
int i = input;
do
{
    cout << i;
} while(i < 5);

The way to correctly do you task is:

int i = 0;
//if used to prevent first execution
if (i <= input)
{
    do
    {
        cout << name << " is number " << input << endl;
        total = total + i;
        i = i + 2;
    } while(i <= input);
}

But for is better to rewrite for loop like

for(BEFORE_STATEMENT; FINISH_STATEMENT; ITERATE_STATEMENT)
{
    LOOP_CODE
}

as while loop, which will work the same

BEFORE_STATEMENT
while(FINISH_STATEMENT)
{
    LOOP_CODE
    ITERATE_STATEMENT
}

talnicolas · Accepted Answer · 2012-04-03 16:16:29Z

1

You just need to change

i += 2;
total += i;

to

total += i;
i += 2;

In your for loop:

total = total + i;

i is equal to 0 at the first iteration. The way you were doing it in the do - while loop, i was set to 2 before the total addition.

answered Apr 3, 2012 at 16:16

talnicolas

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1 Comment

talnicolas Over a year ago

Damn looks like responding first isn't a criteria for upvotes anymore.

cquadrini · Accepted Answer · 2012-04-03 16:22:19Z

0

If you haven't done so in a previous portion of the code, you need to initialize i in the do...while.

Also, in the do...while, change the order to have total incremented before i is incremented.

answered Apr 3, 2012 at 16:22

cquadrini

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Comments

user934691 · Accepted Answer · 2012-04-03 16:22:54Z

0

Your code is incorrect. Corect is

do
{
    cout << name << " is number " << input << endl;
    total += i;//swap these lines
    i += 2;//
} while (i <= input);
cout << endl;
cout << total << endl;

answered Apr 3, 2012 at 16:22

user934691

Comments

Rps · Accepted Answer · 2012-04-03 16:16:42Z

-1

a do while will be exectued at least one time, no mather what the value is i is. Also you should initialise your i.

answered Apr 3, 2012 at 16:16

Rps

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