Mathematical difficulties

Question

New guy here, and feeling like a bit of a dumbass, to be honest.

Basically, I am at university and one of my modules is Introduction to Programming (learning C). I really enjoy learning the programming and really want to take that further; however, I am really struggling with the math/logic. For example, each week we get these small tests which I have been doing OK on - unless they involve (what seems to me complicated but to many of you quite easy, I am sure) mathematical lines of code.

Here is one of the questions:

Which of the following is output when the following code is run:

#include<stdio.h>

void main()

{

    int a = 300, b = 100, c = 100;

    if(a >= 400)

        b = 300;

    c = 200;

    printf("%d, %d, %d\n", a, b, c);

}

So I basically answered 300,100,100 (in the multiple choice answers), but the answer was 300,100,200. And (if I am being totally honest with you) if the option to choose 300,300,200 was there in the multiple choice I would have chosen that. I cant seem to understand why - in the result - only the value of c was changed?

Honestly, where this part of programming is concerned I am really struggling. I am determined to keep working hard on it though.

Any tips, or advice you guys have will be greatly appreciated.

Kind regards.

Answer 1

If you look at the code, b and c have values assigned since their initialization.

But b is changed if the condition a >= 400 is true i.e. b is set to 200 if 300 >= 400 is true. Is that true? No. So, b is left unchanged.

200 is assigned to c regardless anything else in your code.

In summary, since the declarations of a (=300), b(=100), and c(=100):

a isn't changed at all.

b is changed on a condition. But since that condition is false, it's not changed either.

c is always changed (200 is assigned).

So, you can now work out why the answer is so?

Answer 2

Execution proceeds line by line, and control branches as required.

Initially, the line with the initializations runs:

int a = 300, b = 100, c = 100;

At this point, the values are a = 300, b = 100 and c = 100.

if(a >= 400)
    b = 300;

The if statement works as follows:

if(condition)
    then-statement;

If the condition is true, then the then-statement is executed. Otherwise, we skip it. In your example, we are testing to see if a >= 400, since a is 300 so far, 300 >= 400 is not true, so we skip the statement b = 300.

At this point, the values are still a = 300, b = 100 and c = 100 (since we did not change anything).

Finally, you have the line:

c = 200;

This statement is not guarded by an if, so it will always be executed. So, we update the value of c and set it to 200.

At this point, the values are a = 300, b = 100 and c = 200.

So, when we get to the print statement, these are precisely the values that are printed out.