About arguments by default in C++

Question

I am asking help to understand: - In this program:

#include <iostream> int main() { int j = 0; int k = 1; int m = -1; int n = 1; int p = ((j * 300) - (4 * n) - (7 * m)); if (int l = ((j * 300) - (4 * n) - (7 * m)) > 0) { std::cout << "l is egual: " << l - 1 << std::endl; } else { std::cout << "l is negative: " << l << std::endl; } if (p > 0) { std::cout << "p is egual: " << p - 1 << std::endl; } else { std::cout << "p is negative: " << p << std::endl; } return 0; }

I got the output: gcc circonfEn.cc -lstdc++ -o circonfEn ./circonfEn
l is egual: 0 p is egual: 2

Why the instruction in the if do not work?

It's thé boolean operation that gets into l. You get int l = 2>0 then l=1 — PilouPili
– PilouPili, Commented Sep 16, 2018 at 8:33

HolyBlackCat · Accepted Answer · 2018-09-16 08:36:38Z

Your code essentially means (modulo the scope of l):

int l = ((j * 300) - (4 * n) - (7 * m)) > 0; if (l != 0) { std::cout << "l is egual: " << l - 1 << std::endl; } else { std::cout << "l is negative: " << l << std::endl; }

Since > returns a bool, the value of l is always 0 or 1.

The code should be rewritten as:

int l = (j * 300) - (4 * n) - (7 * m); if (l > 0) { std::cout << "l is egual: " << l - 1 << std::endl; } else { std::cout << "l is negative: " << l << std::endl; }

Or:

if (int l = (j * 300) - (4 * n) - (7 * m); l > 0) { std::cout << "l is egual: " << l - 1 << std::endl; } else { std::cout << "l is negative: " << l << std::endl; }

Anybody who tries the last solution, make sure your compiler is in C++17 mode.

seunggabi · Accepted Answer · 2018-09-16 08:34:53Z

2

int l = ((j * 300) - (4 * n) - (7 * m)) > 0 // true; l = 1; // because true == 1; 1 - 1 = 0; // so, l is egual: 0;

Do you understand?

edited Sep 16, 2018 at 8:34

answered Sep 16, 2018 at 8:33

seunggabi

1,83216 silver badges13 bronze badges

1 Comment

ripeisripe Over a year ago

Yes I understand. Thank you.

StoryTeller - Unslander Monica · Accepted Answer · 2018-09-16 08:35:21Z

Defining variables in the condition of an if statement is a little known feature. I'm afraid you are not using it as prescribed. You see, when you write:

if (int l = <whatever>)

Then it's the same as if you've written

int l = <whatever>; if(l)

l itself is evaluated for truthiness. You can't specify it needs to be evaluated as l > 0. So when you write:

int l = ((j * 300) - (4 * n) - (7 * m)) > 0

l will be either 0 or 1. Because you initialize it with ((j * 300) - (4 * n) - (7 * m)) > 0

Collectives™ on Stack Overflow

About arguments by default in C++

3 Answers 3

1 Comment

1 Comment

Comments

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

1 Comment

1 Comment

Comments

Related