Semicolon - How is messing with this exercise - c

Question

I know that I shouldn't put semicolon after a loop. But I am learning and accidentally I inserted one. And I wanted to know exactly what is happening with my error. So next time something similar happens, I know the source of the mistake.
In the following code below, in this portion of the code:
triangularNumber = 0;
for ( n = 1; n <= number; ++n )
;
I accidentally inserted a semi colon after the for loop. When I execute the entire code, it prompts the user to insert a number for calculating a TriangularNumber. But with the semicolon the result is wrong. For example, when I insert 10, the answer should be 55, but with the semicolon error it delivers to me 56. I wanted to understand why 56.
The complete code is below:
#include <stdio.h>
int main(void)
{
int n, number, triangularNumber, counter;
for(counter = 1; counter <=5; counter++)
{
printf("What Triangular Number do you want?");
scanf("%i", &number);
triangularNumber = 0;
for ( n = 1; n <= number; ++n )
**;**
triangularNumber += n;
printf("Triangular number %i is %i\n\n", number, triangularNumber);
}
return 0;
}

Answer 1

The other answers are all good, but to answer your question about why you get the specific number that you do (which would be 11 when you enter 10, not 56), it's because this:
for ( n = 1; n <= number; ++n )
;
does essentially nothing except loop until n is exactly 1 more than number, so when number is equal to 10, n will equal 11.
Then, this:
triangularNumber += n;
just sets triangularNumber to 11, since before the loop you set it to 0, so when it's sitting outside the loop as it does with the semi-colon there, triangularNumber += n; is basically equivalent to triangularNumber = n;.
Incidentally, if you'd defined n within the for loop, instead of at the beginning of main(), like so (you may need to put your compiler in C99 mode with -std=c99 or similar to do this):
for ( int n = 1; n <= number; ++n )
;
triangularNumber += n;
then you'd have spotted the error immediately because the program wouldn't compile, as by the time you got to triangularNumber += n; outside the loop, n would no longer be in scope. This is one good reason why limiting the scope of your variables to the minimum amount of code you need it can often be a good idea.

Answer 2

The second for statement has no braces, so the code runs ; number times!
You want:
for ( n = 1; n <= number; ++n )
{
**;**
triangularNumber += n;
}

Answer 3

Think of it like this:
loop {
do this;
and also this;
}
and more code outside the loop;
now, if you have a ";" with no code it will look like this:
loop {
;
}
and more code outside the loop;
the ; is a blank statement that gets run each time through the loop.
now if you remove the brackets take a look at this:
loop
;
and some code outside of the loop;
since there are no brackets the loop will only contain the very next line of code, in this example the empty line ";". then the line after that "and some code outside of the loop;" gets executed no matter what since it is outside of the loop.
basically, the code is interpeted as having brackets around one line of code if you don't have brackets. anything after the one line is outside of the brackets and is not in the loop.
i hope this clears things up!

Answer 4

The keyword for is a preface for a single statement that is run each time the loop completes. Since a semicolon ends a statement, the following loop does precisely nothing upon each iteration.
for ( n = 1; n <= number; ++n ) ;
Then execution continues to the subsequent statement (triangularNumber += n;), which is executed one time.
The same principle applies to if, while, and do keywoards as well. What gives you flexibility in all of these cases is this rule: Everything enclosed within a pair of braces is considered as one statement.
Thus, your code needs to look like one of these examples to work how you probably want it to.
for ( n = 1; n <= number; ++n ) triangularNumber += n;
-or-
for ( n = 1; n <= number; ++n )
{
triangularNumber += n;
} //everything between the braces is considered to be the one statement executed by the loop