Java control if else statement

Question

I'm using the below free text to learn intro to java and I am having trouble understanding the difference between the code segments:

http://math.hws.edu/eck/cs124/downloads/javanotes7-linked.pdf

Example 1

    int x;
    x = -1;
    if (x < 0)
        x = 1;
    else
        x = 2;

Example 2

    int x;
    x = -1;
    if (x < 0)
        x = 1;
    if (x >= 0)
        x = 2;

In Example 1, x is 1; In Example 2, x is 2.

On the right, if -1 is not > or = to 0 then shouldn't the output be 1? Could someone please explain why the output would instead be 2?

on the left; program executes else block only if condition is false. so no need to explain right one its just executing lines in order. — Ömer Erden
– Ömer Erden, Commented Aug 12, 2016 at 18:50
@ShivaShinde: What on earth is wrong with basic questions? Everyone starts with the basics. — Mark Peters
– Mark Peters, Commented Aug 12, 2016 at 18:52
Can someone send a mail to the author of that... thing. Is on the second page. — Braiam
– Braiam, Commented Aug 12, 2016 at 19:03
@JarrodRoberson or maybe DO use it. Because that code is there to explain why the code is bad. "The author is literally using it to address misconception or misunderstanding of new programmers about how if statements operate by saying that the two are not equivalent even if they can look the same to a new programmer. The author's point being (coincidentally) illustrated by OP being confused at what the difference would be. — VLAZ
– VLAZ, Commented Aug 12, 2016 at 19:14

Mark Peters · Accepted Answer · 2016-08-12 18:52:02Z

5

Once the second if statement is evaluated, x the first has already taken effect--so the value of x is now 1.

if (x < 0)
   x = 1;


if (x >= 0) //x is 1 because you've already evaluated the above!
   x = 2;

The else keyword creates mutually exclusive branches: only one can be executed. So if you had this, the behavior would be as you expected:

if (x < 0)
   x = 1;
else if (x >= 0)
   x = 2;

edited Aug 12, 2016 at 18:52

answered Aug 12, 2016 at 18:47

Mark Peters

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1 Comment

Rabbit Guy Over a year ago

This answer is correct, @TheJuniorProgrammer... the second example contains two if statements.

Bill the Lizard · Accepted Answer · 2016-08-12 18:54:44Z

2

In the Example 1, only one of the statements x = 1; or x = 2; will be executed because it uses an if...else statement.

In the Example 2, both statements will be executed because it uses two separate if statements, and because both conditions are true at the time they are evaluated.

edited Aug 12, 2016 at 18:54

user3458

answered Aug 12, 2016 at 18:48

Bill the Lizard

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Comments

Mingle Li · Accepted Answer · 2016-08-12 19:45:05Z

0

In the second example; first you go through the first if condition:

x = -1;

if (x < 0) x = 1;

x is less than 0, so x becomes 1.

When you go to the second if condition:

if (x >= 0) x = 2;

x is still 1, and 1 is greater than 0, so x becomes 2.

edited Aug 12, 2016 at 19:45

answered Aug 12, 2016 at 18:50

Mingle Li

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