Incorrect logic in C++

Question

I have the following code in C++:

(where arr.data[i-1] = 20; arr.date[i] = 30; payload = 50;)

if (i > 0 && arr.data[i-1] < payload < arr.data[i]) {
    cout << arr.data[i-1] << " < " << payload << " < " << arr.data[i] << "\n";
}

and I am getting the statement returned as

20 < 50 < 30

What am I doing incorrect?

Use a compiler with warnings enabled! See [Coliru example here](http://coliru.stacked-crooked.com/a/2f02d1c30f9b7b3f). — Csq, Sep 22 '14 at 00:23

score 7 · Accepted Answer · answered Sep 22 '14 at 00:16

7

You can't chain comparison operators like this:

if (i > 0 && arr.data[i-1] < payload < arr.data[i]) {

Instead, it should look like this:

if (i > 0 && arr.data[i-1] < payload && payload < arr.data[i]) {

Note that you do this in a few places in your code.

answered Sep 22 '14 at 00:16

Bill Lynch

Thank you! I was coming from other languages where you can string the comparisons together. – biw Sep 22 '14 at 00:19
2

@Ben: "a – deviantfan Sep 22 '14 at 00:20
3

There's only one language I've heard of that allows chaining comparisons like that. Most won't accept that – phuclv Sep 22 '14 at 02:10

score 1 · Answer 2 · answered Sep 22 '14 at 00:50

1

Relational operators in C++ are left associative, so this is how the implementation reads it:

(i > 0 && arr.data[i-1] < payload) = TRUE = 1 (Boolean logic)

so, (1 < arr.data[i]) = TRUE

answered Sep 22 '14 at 00:50

Manol

2 Answers2