While loop tutorial needs help

#include <iostream>
using namespace std;

int main()
{
int n1, n2; // declare two variables

cout<< "Enter two numbers: ";
cin>> n1;
cin>> n2;
i = n1; // Initialize i to n1
while( i <= n2 ){ // While i is equal or less than n2
cout<< n1 << " "; // Print out i
i = i + n2; // add i to n2
}

return 0;
}


I am just wondering what is wrong with this code. It just keeps saying that the variable i is not declared.

Here is the tutorial from the book I bought, which is recommended by this website: C++ without fear. This is the exercise.

-Write a program to print all the numbers from n1 to n2, where n1 and n2 are two numbers specified by the user.

You need to declare it then.

Code:

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int i;

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or

Code:

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int n1, n2, i;

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All variables you use must be defined. You haven't defined the variable i, so the compiler complains and rightly so.
Good luck with your exercises.

Is there any alternative to write it? According to the book, some of them, if used correctly, you do not have to declare it. How? Or do we always have to declare it with no other choice?

Besides, is it ok to have the end-user input by writing cin>> n1, n2? instead of having write cin twice( cin>> n1, and cin>> n2)?

I suspect you misread something in your book - or the book is TERRIBLY BAD (and incorrect). ALL variables MUST be declared, ALWAYS. There are different places in the code where these may be declared, but if they are not declared, the compiler will say something like "undefined symbol X" where X is the name of your variable [the exact wording depends on the compiler, but the essence is that the compiler will say "I don't know what X is, and you need to tell me"].

The comment and actual code is mismatched here:

Code:

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cout<< n1 << " "; // Print out i

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I have no idea which one is correct (because , but misleading comments are a BAD thing. No comment is better than one that is misleading).

--
Mats

Quote:

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Is there any alternative to write it? According to the book, some of them, if used correctly, you do not have to declare it. How? Or do we always have to declare it with no other choice?

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Either the book is wrong, or you are misinterpreting the book. What example does your book give to support this claim?

Quote:

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Besides, is it ok to have the end-user input by writing cin>> n1, n2? instead of having write cin twice( cin>> n1, and cin>> n2)?

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No, they mean different things. If I remember by operator precedence correctly, that expression will be evaluated as (cin >> n1), n2. This means that it will read into n1, and then just evaluate n2 (without reading anything into it).

Quote:

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Originally Posted by laserlight

No, they mean different things. If I remember by operator precedence correctly, that expression will be evaluated as (cin >> n1), n2. This means that it will read into n1, and then just evaluate n2 (without reading anything into it).

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You can however write cin >> n1 >> n2, which will read two numbers in one line of code.

--
Mats

Quote:

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Originally Posted by kenryuakuma

Is there any alternative to write it?

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None.

Quote:

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Originally Posted by kenryuakuma

According to the book, some of them, if used correctly, you do not have to declare it.

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If the book says that, then it's a bad book because it's wrong.
i has to be defined somewhere before.

Quote:

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Originally Posted by kenryuakuma

How? Or do we always have to declare it with no other choice?

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That is correct. It must be defined first.
However, when first defining it, you have one instance of it, which can be reused, so you don't necessarily need to define it before all loops, just making sure to define it before using it.

It is crystal-clear by asking you guys. Thanks so much for helping you guys. But this thing sometimes requires you to think a lot before you could solve the problems.

Quote:

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Originally Posted by kenryuakuma

It is crystal-clear by asking you guys. Thanks so much for helping you guys. But this thing sometimes requires you to think a lot before you could solve the problems.

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Yup, thinking is involved in computer programming.

--
Mats

Quote:

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Originally Posted by kenryuakuma

Is there any alternative to write it? According to the book, some of them, if used correctly, you do not have to declare it. How? Or do we always have to declare it with no other choice?

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The alternative is to use the #include directive to pull in declarations from another file. This way, you don't need to explicitly declare them, but since the file contains declarations, you are still technically declaring them.