Vector of structs initialization

Question

I want know how I can add values to my vector of structs using the push_back method

struct subject { string name; int marks; int credits; }; vector<subject> sub;

So now how can I add elements to it?

I have function that initializes string name(subject name to it)

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

setName("english", "math", "physics" ... "economics");

other means of vector initialization: stackoverflow.com/questions/4155845/vector-of-structures — jpalecek
– jpalecek, Commented Nov 9, 2011 at 16:09

Natan Streppel · Accepted Answer · 2013-11-16 14:37:34Z

Create vector, push_back element, then modify it as so:

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.

subject.resize(2); may be a slightly quicker and clearer solution... or even vector<subject> sub(2);.
Yeah, I agree. :) I didn't think he'd quite understand overallocation for efficiency and how a vector grows in size though - this looks more like an intro to c++ problem.
@w00te I added a couple of quotation marks that you probably forgot to add. Hope that's ok.
@nims All good! :) I'm surprised that error hung around for 3 years on a question with 25,000 views, haha.

Sebastian Mach · Accepted Answer · 2022-03-30 13:15:03Z

71

If you want to use the new current standard, you can do so:

sub.emplace_back ("Math", 70, 0); // requires a fitting constructor, though

or

sub.push_back ({"Math", 70, 0}); // works without constructor

.

edited Mar 30, 2022 at 13:15

answered Nov 9, 2011 at 18:53

Sebastian Mach

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2 Comments

Jean-Simon Brochu Over a year ago

The first part of your answer is not accurate; you cannot emplace_back a struct without a matching ctor rextester.com/VQWCT51042

Sebastian Mach Over a year ago

@Jean-SimonBrochu: Sorry for the slightly delayed reply. Yes, you are right, to be even more accurate, it was specifically incomplete :) I added a clarifier.

Rikard Söderström · Accepted Answer · 2018-04-29 10:00:36Z

You may also which to use aggregate initialization from a braced initialization list for situations like these.

#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.

So extending on the above.

#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 assessment in the braced initializer the compiler will fail when attempting to deduce the type.

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

 {"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

Alok Save · Accepted Answer · 2011-11-09 15:41:14Z

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 Behavior if the index is invalid.

You should create a temporary object of your structure, fill it up and then add it to the vector, using vector::push_back()

subject subObj; subObj.name = s1; sub.push_back(subObj);

James · Accepted Answer · 2019-06-21 06:13:12Z

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

But I found new to do this using default_structure_element like following...

#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?

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