C++ 结构体初始化向量
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Vector of structs initialization
提问by SRN
I want know how I can add values to my vector of structs using the push_backmethod
我想知道如何使用该push_back方法将值添加到我的结构向量中
struct subject
{
string name;
int marks;
int credits;
};
vector<subject> sub;
So now how can I add elements to it?
那么现在我该如何添加元素呢？
I have function that initializes string name(subject name to it)
我有初始化字符串名称的函数（主题名称）
void setName(string s1, string s2, ...... string s6)
{
// how can i set name too sub[0].name= "english", sub[1].name = "math" etc
sub[0].name = s1 // gives segmentation fault; so how do I use push_back method?
sub.name.push_back(s1);
sub.name.push_back(s2);
sub.name.push_back(s3);
sub.name.push_back(s4);
sub.name.push_back(s6);
}
Function call
函数调用
setName("english", "math", "physics" ... "economics");
回答by John Humphreys - w00te
Create vector, push_back element, then modify it as so:
创建向量，push_back 元素，然后像这样修改它：
struct subject {
string name;
int marks;
int credits;
};
int main() {
vector<subject> sub;
//Push back new subject created with default constructor.
sub.push_back(subject());
//Vector now has 1 element @ index 0, so modify it.
sub[0].name = "english";
//Add a new element if you want another:
sub.push_back(subject());
//Modify its name and marks.
sub[1].name = "math";
sub[1].marks = 90;
}
You cant access a vector with [#] until an element exists in the vector at that index. This example populates the [#] and then modifies it afterward.
在该索引处的向量中存在元素之前，您无法使用 [#] 访问向量。此示例填充 [#]，然后对其进行修改。
回答by Sebastian Mach
If you want to use the new current standard, you can do so:
如果你想使用新的现行标准，你可以这样做：
sub.emplace_back ("Math", 70, 0);
or
或者
sub.push_back ({"Math", 70, 0});
These don't require default construction of subject.
这些不需要subject.
回答by Rikard S?derstr?m
You may also which to use aggregate initialization from a braced initialization list for situations like these.
对于此类情况，您还可以使用花括号初始化列表中的聚合初始化。
#include <vector>
using namespace std;
struct subject {
string name;
int marks;
int credits;
};
int main() {
vector<subject> sub {
{"english", 10, 0},
{"math" , 20, 5}
};
}
Sometimes however, the members of a struct may not be so simple, so you must give the compiler a hand in deducing its types.
然而，有时结构体的成员可能并不那么简单，因此您必须帮助编译器推断其类型。
So extending on the above.
所以在上面延伸。
#include <vector>
using namespace std;
struct assessment {
int points;
int total;
float percentage;
};
struct subject {
string name;
int marks;
int credits;
vector<assessment> assessments;
};
int main() {
vector<subject> sub {
{"english", 10, 0, {
assessment{1,3,0.33f},
assessment{2,3,0.66f},
assessment{3,3,1.00f}
}},
{"math" , 20, 5, {
assessment{2,4,0.50f}
}}
};
}
Without the assessmentin the braced initializer the compiler will fail when attempting to deduce the type.
如果没有花assessment括号初始化器，编译器在尝试推断类型时将失败。
The above has been compiled and tested with gcc in c++17. It should however work from c++11 and onward. In c++20 we may see the designator syntax, my hope is that it will allow for for the following
上面已经在c++17中用gcc编译和测试过。然而，它应该从 c++11 开始工作。在 c++20 中，我们可能会看到指示符语法，我希望它允许以下内容
{"english", 10, 0, .assessments{
{1,3,0.33f},
{2,3,0.66f},
{3,3,1.00f}
}},
source: http://en.cppreference.com/w/cpp/language/aggregate_initialization
来源：http: //en.cppreference.com/w/cpp/language/aggregate_initialization
回答by Alok Save
You cannot access elements of an empty vector by subscript.
Always check that the vector is not empty & the index is valid while using the []operator on std::vector.[]does not add elements if none exists, but it causes an Undefined Behaviorif the index is invalid.
您不能通过下标访问空向量的元素。
使用[]on 运算符时，请始终检查向量是否为空且索引是否有效std::vector。[]如果不存在元素，则不添加元素，但如果索引无效，则会导致未定义行为。
You should create a temporary object of your structure, fill it up and then add it to the vector, using vector::push_back()
您应该创建结构的临时对象，填充它，然后将其添加到向量中，使用 vector::push_back()
subject subObj;
subObj.name = s1;
sub.push_back(subObj);
回答by Onk_r
After looking on the accepted answer I realized that if know size of required vector then we have to use a loop to initialize every element
查看接受的答案后，我意识到如果知道所需向量的大小，那么我们必须使用循环来初始化每个元素
But I found new to do this using default_structure_element like following...
但我发现新的使用 default_structure_element 来做到这一点，如下所示......
#include <bits/stdc++.h>
typedef long long ll;
using namespace std;
typedef struct subject {
string name;
int marks;
int credits;
}subject;
int main(){
subject default_subject;
default_subject.name="NONE";
default_subject.marks = 0;
default_subject.credits = 0;
vector <subject> sub(10,default_subject); // default_subject to initialize
//to check is it initialised
for(ll i=0;i<sub.size();i++) {
cout << sub[i].name << " " << sub[i].marks << " " << sub[i].credits << endl;
}
}
Then I think its good to way to initialize a vector of the struct, isn't it?
然后我认为初始化结构向量的方法很好，不是吗？
