C++ primer 5th edition, "for" loop problem.

[Glyk]

Hi, im new to C++ just started learning with C++ primer 5th edition and encountered a problem, in exercise 1.13 i am asked to write a range with a "for" loop, i have managed to get the range but it starts from " 0 " instead of the value i specified in "cin". here is my code check it out, thanks in advance.

#include "stdafx.h"
#include <iostream>

int main()
{
	using namespace std;
cout << " enter two numbers " << endl;	
int v1 = 0, v2 = 0;								
cin >> v1 >> v2;
for( v1 = 0; v1 <= v2; ++v1)

	cout << v1 << endl;
	
return 0;
}

[anup30]

for( v1 = 0; v1 <= v2; ++v1)

v1 is initialized by cin before for loop, so need not re-initialize in for()

-> for( ; v1 <= v2; ++v1)

[firedraco]

On line 10, you are assigned 0 to v1 in your for loop, which overwrites the previous value you have read from cin.

The solution is to simply not do anything in the initialization section of the for loop.

[oliverkenny]

I don't want to complicate things but just for future reference, it's common practice to put the using namespace std; line after declaring the headers (the headers are declared on the top two lines of the file).

[jlb]

I don't want to complicate things but just for future reference, it's common practice to put the using namespace std;

It may seem like common practice, however IMO keeping this statement as local as possible, if used at all, is the better practice.

[Glyk]

Thanks alot for all your help, i wanted to ask about the correct line "for( ; v1 <= v2; ++v1),"
what does the " ; " before the condition means? isn't it supposed to be at an end of a statement? and why if i leave only the condition and the "++v1" inside the () all i am getting is infinitely running numbers?

[firedraco]

The ";" is an empty statement; it means "do nothing". Remember how a for loop is structured: the first statement is the initialization section.

why if i leave only the condition and the "++v1" inside the () all i am getting is infinitely running numbers?

What do you mean by this? Could you give an example of when this occurs/the code that is doing that?

[Glyk]

for example:

#include "stdafx.h"
#include <iostream>

int main()
{
	using namespace std;
cout << " enter two numbers " << endl;	
		int v1 = 0, v2 = 0 ;																																													

cin >> v1 >> v2;
for(v1 <= v2; ++v1;)

	cout << v1 << endl;
	
return 0;
}

when compiled the program infinitely runs numbers in a climbing order ,like it's not meeting the condition of stopping on " v2 " but when i add the empty statement it does meet the condition and everything is fine. btw did i miss an explanation on empty statements in the book?.

[nickeeromo]

I think what is happening is that the for statement is structured like:

for ( initialization; tillCondition; finalExecutedStatement)

without the first ";" if there is no actual initialization, it sees

v1 <= v2; AS the initialization statement, which just is the same as saying true is true. Only 2 semicolons are necessary in a for's parenthetical set. The incrementor is not necesssary.

THEN, simply the next statement is evaluated as the tillCondition statement, but it physically changes the value of v1 when it executes. I THINK that basically if this statement evaluates to true (or a number greater than 0), the loop will execute again. So what you have is:

(true; ++v1; /*noIncrementor*/)

SO essentially nothing is happening at initialization, then v1 is added onto / evaluated as true, then the loop runs, thus running infinitely incrementing v1 for every iteration.

Hope I was helpful, and as always, anyone let me know if I am wrong on any point. Thanks.

[anup30]

when initializer is empty like this
for( ; v1 <= v2; ++v1)

use while loop

while(v1<=v2) {
//...
++v1;
}

[Glyk]

Now i understand, that's great thanks for the help guys.

btw the "while" loop is not a problem but my exercise specifically asks me to write it in "for".

[dhayden]

You could also do it with a separate counter variable. I mention this because sometimes you don't want to change the variables holding the limits (v1 and v2 in your case):

for(int i = v1; i <= v2; ++i) {
	cout << i << endl;
}

Some other comments:
- always use { and }, even when there is just one statement. Otherwise you will find that you actually needed two statements one day, add the second one (without adding braces), and spend 6 hours trying to figure out why the code doesn't work.

- It's a matter of taste, but I prefer "for" loops over "while" loops even when there is no initialization. With "for" loops, your code basically says

here is everything about a loop
     here is what I'm doing at each iteration 

with a while loop it's more like

here is a loop
    here is what I'm doing during each iteration and buried in this code is the thing that changes during each iteration.