Using Auto in a for loop

**Scatman** · 10-11-2020

Hi all,

i was doing the following:

Code:

for (int i = 0; i < numbers.capacity(); i++) {
        numbers.push_back(rand() % 30);
}

so i got the warning:

Code:

warning C4018: '<': signed/unsigned mismatch

i get the same warning if i use "auto" instead of "int" :

Code:

for (auto i = 0; i < numbers.capacity(); i++) {
        numbers.push_back(rand() % 30);
}

i get only rid of the warning when i use "unsigned int".

My question:
itsn't that where "auto" is supposed to help? so that i don't need to bother aboutthe data type ?
what i am i doing wrong? is there some best practice where not to use "auto" ?

i used a lot with range based for loops, it works fine

Thanks in advance

**laserlight** · 10-11-2020

The type of 0 is int, so using auto here still causes i to be of type int. If you really wanted to, you could write:

Code:

for (decltype(numbers.capacity()) i = 0; i < numbers.capacity(); i++)

But that seems unnecessarily repetitive to me. Maybe:

Code:

auto capacity = numbers.capacity();
for (decltype(capacity) i = 0; i < capacity; ++i)

EDIT:
I might test this later to be sure, but the thought came to mind that you might be able to use generate_n with back_inserter to accomplish this:

Code:

std::generate_n(std::back_inserter(numbers),
                numbers.capacity(),
                []() { return rand() % 30; });

**Scatman** · 10-11-2020

The type of 0 is int, so using auto here still causes i to be of type int

alright alright, now i got it, and thanks for the alternatives.

generate_n with back_inserter are very new to me

Thread: Using Auto in a for loop

Thread Tools

Search Thread

Display

Using Auto in a for loop

Similar Threads

do while loop to auto execute when there is a tie in a game ? completely stuck

Auto-response

auto log in program ....

auto?

Auto continue

Tags for this Thread