Here is my code and i have some errors like main not valid can someone tell me whats wrong...

I have been programming in OpenGL and WinAPI I forgot the Very Basics of why this could be wrong...

Let me know plz!

#include <iostream.h>

#include <math.h>

//////////////////////////

// Constants //

// AKA //

// Global Var's //

//////////////////////////

#define PI 3.1415926535f

//////////////////////////

// Functions //

//////////////////////////

basicinit(double a)

{

cout<<a<<endl;

return 0;

};

squareinit(double a)

{

if (a < 0)

{

cout<<"Complex Number detected";

return 0;

}

cout<< sqrt(a);

return 0;

};

exponentsinit(double a, double b)

{

if (b == 0)

{

cout<<"1"<<endl;

return 0;

}

if (b == 1)

{

cout<<a<<endl;

return 0;

}

if (b < 0)

{

if (b == -1)

{

cout<<"0"<<endl;

}

for( b = b + 1; b >= 0; b++)

{

cout<<a / a<<endl;

}

for ( b = b - 1; b <= 0; b--)

{

cout<<a * a<<endl;

}

return 0;

};

//////////////////////////

// Main Function //

//////////////////////////

int main()

{

//////////////////////////

// Variablles //

//////////////////////////

const i = sqrt(-1.0f);

double x;

double y;

double z;

int choose;

//////////////////////////

// To Do //

//////////////////////////

cout<<"Choose from menu"<<endl;

cout<<"1. Basic Arithmatic\n"

<<"2. Square roots\n"

<<"3. Exponents"<<endl;

cin>>choose;

if (choose == 1)

{

cout<<"Please input expression\n"

<<"Make Sure that there is only one variable and it is on the lone side of the equal\n"

<<"EXAMPLE: 1*2+3-2"<<endl;

cin>>x;

basicinit(x, NULL, NULL);

}

if (choose == 2)

{

cout<<"INsert the number you want to squeare root\n";

cin>>x;

squareinit(x);

}

if ( choose == 3)

{

cout<<"Enter the number you want to raise exponent then write the number you want to raise it to\n"

<<"Example First: 2 Second: 2 answer is 4 because 2 squared is 4"

<<"First: ";

cin>>x;

cout<<"\nSecond: ";

cin>>y;

exponentsinit(x,y);

}

return 0;

}