variable always output 0, C++

Question

#include<iostream>

int main (){

double a, prsnt;

std::cout << "Imput one of the three numbers \n";
std::cin >> a;

if (a = 2 || 12){
    prsnt == 15.55;
}else if(a = 3 || 11){
    prsnt == 29.03;
}else if(a = 4 || 10){
    prsnt == 40.67;
}else if(a = 5 || 9){
    prsnt == 50.57;
}else if(a = 6 || 8){
    prsnt == 59.23;
}else if(a = 7){
    prsnt == 66.51;
}else{
    std::cout << "ERROR, INVALID INPUT";
}

std::cout << "\n" << prsnt; 
}

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prsnt is always 0 no matter what the "a" variable is. the code allows me to input for a, then gives me a number, but it is always 0