Question

Hello I am studying for a test for an intro to C programming class and yesterday I was trying to write this program to print out the even prime numbers between 2 and whatever number the user enters and I spent about 2 hours trying to write it properly and eventually I did it. I have 2 pictures I uploaded below. One of which displays the correct code and the correct output. The other shows one of my first attempts at the problem which didn’t work correctly, I went back and made it as similar to the working code as I could without directly copying and pasting everything.

unfortunately new users aren’t allowed to post pictures hopefully these links below will work.

This fails, it doesn’t print all numbers in range with natural square root:

for (i = 2; i <= x; i++)
{
   //non relevant line
   a = sqrt(i);
   aa = a * a;
   if (aa == i); 
       printf("%d ",i);
}

source: https://i.stack.imgur.com/J5wzj.jpg

While this succeeds, and prints even numbers with natural sqaure root

for (i = 2; i <= x; i++) 
{
   a = sqrt(i);
   aa = a * a;
   if (aa == i && ((i/2) *2) == i)
        printf("%d ", i);
}

source: https://i.stack.imgur.com/ufn6B.jpg

Hopefully you can see and read the screen shots I have here. I know that the ‘incorrect code’ picture does not have the (i/2)*2 == i part but I figured that it would still print just the odd and even numbers, it also has the code to calculate “sqrd” but that shouldn’t affect the output. Please correct me if I’m wrong on that last part though.

And Yes I am using Dev-C++ which I’ve read is kinda crappy of a program but I initally did this on code::blocks and it did the same thing…

Please I would very much appreciate any advice or suggestions as to what I did wrong 2 hours prior to actually getting the darn code to work for me.

Thank you,
Adam

Answer 1

your code in ‘that’ includes:

if (aa == i);
//          ^
   printf(...);

[note the ; at the end of the if condition]

Thus, if aa == i – an empty statement happens, and the print always occures, because it is out of the scope of the if statement.

To avoid this issue in the future, you might want to use explicit scoping¹ [using {, } after control flow statements] – at least during your first steps of programming the language.

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1: spartan programmers will probably hate this statement