auto keyword with for-loop produced Odd result

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i have got weird result when i executed this for-loop with auto keyword. could someone explain why i got weird result in this for-loop?

C++

#include <iostream>

int main()
{
	for (auto i = 0u; i < 3u; ++i) // Okay
	{
		std::cout << i << ' ';
	}

	for (auto i = 2u; i >= 0u; --i) // ?????? Awful
	{
		std::cout << i << ' ';
	}
}

Posted 3-Jan-16 2:16am

Member 10870731

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Richard MacCutchan · Accepted Answer · 2016-01-03T02:36:00

Solution 1

You are declaring your values as unsigned so when i goes from 0 to -1 it is treated as the unsigned value 0xFFFFFFFF, and the loop goes on forever. If you change the second loop statement to

C++

for (auto i = 2u; i > 0u; --i)
// or
for (auto i = 2u; i != 0xFFFFFFFFu; --i)
// or better 
for (auto i = 2; i >= 0; --i) // use signed numbers

it will work. It is better to use signed numbers as much as possible to avoid issues such as this.

Posted 3-Jan-16 2:36am

Richard MacCutchan

Comments

Andreas Gieriet 3-Jan-16 9:32am

My 5!
Cheers
Andi

Richard MacCutchan 3-Jan-16 9:43am

My first 5 of 2016, thank you.

Sergey Alexandrovich Kryukov 3-Jan-16 12:39pm

5ed.
—SA