Questions tagged [range-based-loop]
66 questions
454
votes
8 answers
C++ Loop through Map
I want to iterate through each element in the map without knowing any of its string-int values or keys.
What I have so far:
void output(map table)
{
map::iterator it;
for (it = table.begin(); it…
NoName
- 9,824
- 5
- 32
- 52
26
votes
8 answers
Range-based for loop with special case for the first item
I find myself often with code that looks like this:
bool isFirst = true;
for(const auto &item: items)
{
if(!isFirst)
{
// Do something
}
// Normal processing
isFirst = false;
}
It seems that there ought to be a better…
bpeikes
- 3,495
- 9
- 42
- 80
14
votes
8 answers
How can I skip elements in a range-based for loop based on 'index'?
Is there a way to access the iterator (I suppose there's no loop index?) in a C++11 range-based for loop?
Often we need to do something special with the first element of a container and iterate over the remaining elements. So I'm looking for…
Jay
- 6,572
- 3
- 37
- 65
9
votes
4 answers
Iterating over odd (even) elements only in a range-based loop
Suppose we have a plain array (or other container which supports range-based loops):
const int N = 8;
int arr[N] = {0, 1, 2, 3, 4, 5, 6, 7};
Using indexes or iterators, we can loop over odd elements and increment the index by two:
for (int i = 0; i…
Dmytro Dadyka
- 2,208
- 5
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- 31
7
votes
1 answer
How to pass in a brace-enclosed initializer list to a function?
I want to write a function that can be used with an argument that otherwise could directly occur in a range-based loop:
template
void sayIt(const Iterable& stuff) {
for (const auto& e : stuff) {
cout << e << endl;
…
shinjin
- 2,858
- 5
- 29
- 44
6
votes
1 answer
What is member interpretation in Range-based for loop (since C++11)?
I read this documentation for a range-based for loop:
The member interpretation is used if the range type has a member named begin and a member named end. This is done regardless of whether the member is a type, data member, function, or…
Ethanabc
- 311
- 2
- 7
5
votes
1 answer
Why doesn't iterate over a container accessed directly through std::optional::value() work?
I'm trying to iterate over a std::vector contained in a struct T that I access through a std::optional. Contrary to my expectations, the behavior is different if I first store the std::optional into a copy versus if I iterate it…
w128
- 4,680
- 7
- 42
- 65
5
votes
1 answer
Is there a technical reason why range-based for loop doesn't detect whether it's looping on an rvalue?
The reason for the question is that I've seen code like this:
auto fun(std::vector&& v) {
std::vector w;
for (auto&& e : v /* not an rvalue, but keep reading */) {
w.push_back(std::move(e));
}
// do stuff with…
Enlico
- 23,259
- 6
- 48
- 102
4
votes
1 answer
how to write forward iterator using private std::vector base class
I need a vector class that exposes a small subset of the std::vector API. Everything works except range-based for. Here my attempt at implementing a forward iterator, which however does not compile.
#include
#include
template…
Joachim W
- 7,290
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- 59
4
votes
6 answers
Is it possible to implement a DefaultIfNull function in C++?
Disclaimer: This is rather more out of curiosity than for a lack of other solutions!
Is it possible to implement a function in C++ that:
gets passed a pointer of type T
either returns a reference-like-thing to the object pointed to by T
or, if the…
Martin Ba
- 37,187
- 33
- 183
- 337
3
votes
1 answer
What does the auto "t{1U}" do in the following range-based for loop from std::make_heap example?
I was browsing the standard algorithm library and came across an example which used a range based for loop in a way that I had not seen before: https://en.cppreference.com/w/cpp/algorithm/is_heap
In the example given, they use a range based for loop…
HarryP2023
- 298
- 1
- 13
3
votes
1 answer
Deleting map elements in a range-based loop
I would like to drop a number of elements from a map based on some condition:
#include
#include
#include
int main() {
std::unordered_map numbers = {{1,2}, {2,1}, {3,2}, {4,5}};
auto even =…
Stein
- 3,179
- 5
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- 51
3
votes
1 answer
Don't print space after last value with iterator
I'm having a problem with a beginner concept
in competitive programming extra space in print may cause wrong answer judgment
I want to iterate through a container like map or set but last value should not have a space
#include
#include…
Jee
- 35
- 5
3
votes
1 answer
Why can't I use this 'void' type in a range based for loop?
for(auto i: {{1,2,3}, {4,5,6}, {7,8,9}}){
/* loop body */
}
I know I have other ways to get my work done. But I was just wondering why we cannot use such type of list in this loop.
It is giving me this error:
cannot use type 'void' as a range
sparsh goyal
- 89
- 1
- 7
3
votes
3 answers
Is std::filesystem::directory_iterator really an iterator?
Something doesn't make to sense. According to what I've read you use std::filesystem like this:
#include
#include
#include
int main()
{
auto iterator = std::filesystem::directory_iterator("c:/somefolder");
…
Zebrafish
- 11,682
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