How to read data from a Vector

Question

How can I use the following vector to read true/false from using a while or for loop. With this implemtation of the loop I get an error for the oprator !=

no operator "!=" matches these operands

vector<bool> Verification;

Verification.push_back(true);
Verification.push_back(false);
Verification.push_back(true);
Verification.push_back(false);
Verification.push_back(true);

for (int it = Verification.begin(); it != Verification.end(); it++) {   
                    if (it==true) cout<<"true";
                    else if (it == false) cout<<"false";
}

*it, an iterator itself is not true or false it points to an element of the container that may be true or false, also your iterator shouldnt be an int — Borgleader
– Borgleader, Commented Dec 2, 2021 at 14:26
int it = Verification.begin() should be a compile error too. — Caleth
– Caleth, Commented Dec 2, 2021 at 14:26
I don't think the error is the operator. You are declaring it as int but trying to put an iterator in it. Change int to vector<bool>::iterator. Although, personally I prefer this style- for(size_t i = 0; i < vector_name.size(); i++) — Abdur Rakib
– Abdur Rakib, Commented Dec 2, 2021 at 14:27
Aside: *it==true is the same as *it, is the same as (*it == true) == true. Just use the bool value directly — Caleth
– Caleth, Commented Dec 2, 2021 at 14:28
Read the introduction to iterators in your favourite C++ book. — molbdnilo
– molbdnilo, Commented Dec 2, 2021 at 14:36

Caleth · Accepted Answer · 2021-12-02 14:30:35Z

3

You are declaring it as the wrong type. The result of Verification.begin() is a std::vector<bool>::iterator. But you don't need to specify that.

Use a range-for loop instead

for (bool b : Verification) 
{
    std::cout << std::boolalpha << b;
}

answered Dec 2, 2021 at 14:30

Caleth

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2 Comments

Omar Over a year ago

Thank you for the answer works great, side question what does bool b : Verification do

Federico Over a year ago

@Omar click on that blue link range-for and everything will be clear

Slava · Accepted Answer · 2021-12-02 14:44:25Z

There are various ways to iterate over an std::vector

Using iterator

Long example:

for( std::vector<bool>::iterator it = v.begin(); it != v.end(); ++it ) std::cout << *it;

or the same but shorter:

for( auto it = v.begin(); it != v.end(); ++it ) std::cout << *it;

Using index

Here:

for( unsigned int i = 0; i != v.size(); ++i ) std::cout << v[i];

Range loop

Here:

for( bool b : v ) std::cout << b;

(there are some more but we will omit them for clarity)

Looks like you mixed 1 and 2 hense you have compilation errors. Choose one.

Richard · Accepted Answer · 2021-12-02 14:37:20Z

The problem is that Verification.begin() gives you an iterator while it is an int.

To solve this you could modify your for loop to:

for (std::vector<bool>::iterator it = Verification.begin(); it != Verification.end(); it++) {   
                    if (*it==true) cout<<"true";
                    else if (*it == false) cout<<"false";
}

Note *it means we're dereferencing the iterator it and then comparing the result.

Also you don't need the else if because you can just use else.

Alternative solution

You can also use a range-base for loop as shown below:


for (bool element : Verification) 
{
    std::cout << std::boolalpha << element;
}

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