C++ brainteasers - Have fun and learn. You may get a job too

[Andy Nguyen]

To quote Mr. Connor on his experience interviewing C++ programmers for FE jobs

In recent times, I've handed over a laptop and asked a candidate to write some code for a standard algorithm like quicksort or even just a hash table. In fact, I've found the best predictor of programming skill is actually the expression on their face when I do.

So I thought the best way to learn is play. So it may be fun and beneficial to post programming problems I encouter on various forum, brainteaser books. I do not know the answers before hand. If you see some nontrivial programming questions, please post here. You may very well see them during your interviews.

1) Write one line of code to swap the contents two variables without using a temp variable
2) Write a program to print 1-100 and backward without using loops.
3) Write a C++ program to print out "Hello world" without using any ';'

 

[alain]

Re: C++ puzzles - Have fun and learn. You may get a job too

To quote Mr. Connor on his experience interviewing C++ programmers for FE jobs

In recent times, I've handed over a laptop and asked a candidate to write some code for a standard algorithm like quicksort or even just a hash table. In fact, I've found the best predictor of programming skill is actually the expression on their face when I do.

So I thought the best way to learn is play. So it may be fun and beneficial to post programming problems I encouter on various forum, brainteaser books. I do not know the answers before hand. If you see some nontrivial programming questions, please post here. You may very well see them during your interviews.

1) Write one line of code to swap the contents two variables without using a temp variable
2) Write a program to print 1-100 and backward . . . without using loops.

These are not difficult but a person with no programming knowledge might struggle. These are my answers. I haven't tried them but they should work.
1)

#include <algorithm>
...
   std::swap(a, b);
...

2) This is a little more conversome but it's also straight forward:

#include <iostream>
...
void printForward(int i, int max)
{
    if (i > max) {
        return;
    } else {
        std::cout << i << std::endl;
        printForward(i + 1, max);
    }
}

void printBackward(int i, int min)
{
    if (i < min) {
        return;
    } else {
        std::cout << i << std::endl;
        printForward(i - 1, max);
    }
}

int main()
{
    printForward(1, 100);
    printBackward(100, 1);
    return 0;
}

 

[Andy Nguyen]

Here is C++ style code to print 0 - 100 and then 100-0 using one function without loops

#include <iostream>  
void count(int n, int max)
{
std::cout <<n<<" ";
if(n < max) count(n+1, max);
std::cout << n<<" ";
}
int main()
{
int n = 100;
count(0, n);
}

C:\Fall 2006\9815>swap.exe
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56
 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 8
3 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 100 99 98 97 96 95 94 93 9
2 91 90 89 88 87 86 85 84 83 82 81 80 79 78 77 76 75 74 73 72 71 70 69 68 67 66
65 64 63 62 61 60 59 58 57 56 55 54 53 52 51 50 49 48 47 46 45 44 43 42 41 40 39
 38 37 36 35 34 33 32 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 1
2 11 10 9 8 7 6 5 4 3 2 1 0

 

[DominiConnor]

Here's one that gets a few people:

1: Write a function to compute factorial using recursion.
2: Do the same for Fibonacci numbers.

What's basically wrong with the Fibonacci code ?
Almost everyone makes the same mistake.

 

[alain]

Here's one that gets a few people:

1: Write a function to compute factorial using recursion.
2: Do the same for Fibonacci numbers.

What's basically wrong with the Fibonacci code ?
Almost everyone makes the same mistake.

double factorial(double n)
{
    if (n == 0.) return 1.
    return n * factorial(n-1);
}
// I think this should work - haven't tested it
double fibonacci_aux(double n, double f0, double f1)
{
     if (n == 0) return f1;
     return fibonacci_aux(n-1., f1, f0 + f1);
}
double fibonacci(double n)
{
    if (n == 0) return 1.;
    return fibonacci_aux(n, 0, 1);
}

 

[RussianMike]

Not a c++ programmer but doesnt fibonacci using recursion take too long? I mean if you want the 1000th fib number it can take some time. 4 mins?

 

[Andy Nguyen]

1) Write one line of code to swap the contents two variables without using a temp variable

Here is my one line swapping

a-=b=(a+=b)-b; //one line swapping using +/-

Same idea but will not work if one of the numbers is 0 since it involves multiplication and division

a/=b=(a*=b)/b;//one line swapping using */:

here is my test code and output

#include <iostream>
int main(){
    double a,b;
    std::cout <<"Input a,b separated by space : ";
    std::cin >> a >>b;
    a-=b=(a+=b)-b; //one line swapping +/-
    std::cout <<"a= "<< a <<", b= "<< b<<std::endl;
}

Input a,b separated by space : 4.5 -6
a= -6, b= 4.5

 

[alain]

Not a c++ programmer but doesnt fibonacci using recursion take too long? I mean if you want the 1000th fib number it can take some time. 4 mins?

That's not totally true. A good compiler will unwind the tail recurssion but that's another discussion.

 

[RussianMike]

lol. when I was taking c++ class to learn it, the teacher was trying to use recursion to find 1000th fib number and it was taking a long time. He was using visual C++.

 

[jay.berg]

1) Write one line of code to swap the contents two variables without using a temp variable

void swap(int &a, int &b) {
         a^=(b^=(a^=b));
}

 

[DavidKDeutsch]

I know this thread is a bit old, but hereis what I came up with for the "Hello, world" with no semicolon:

void main()
{
if(printf("Hello World!\n")){}
}

 

[vbaranov]

Nice one Dave! That took me some time to figure out.

 

[Andriy]

Factorial calculated during compilation:

#include <iostream>

template<unsigned n>
struct factorial
{
    enum { value = n*factorial<n-1>::value };
};

template<>
struct factorial<0>
{
    enum { value = 1 };
};

int _tmain(int argc, _TCHAR* argv[])
{
    std::cout << factorial<10>::value << std::endl;
    return 0;
}

 

[Daniel Duffy]

// Fibonacci.cpp
//
// Fibonacci numbers using templates.
//
// 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 
// 987, 1597, 2584, 4181, 6765, 10946, 17711, 28657, 46368, 75025, 
// 121393, ...[1] 
//
// DJD
#include<iostream>
usingnamespace std;
template <unsigned N>
struct Fibonacci
{
enum { value = Fibonacci<N-1>::value + Fibonacci<N-2>::value};
};
template <>
struct Fibonacci<0>
{
enum { value = 0 };
};
template <>
struct Fibonacci<1>
{
enum { value = 1 };
};
int main()
{
cout << Fibonacci<21>::value << endl;
return 0;
}

 

[SeanPro]

Very cheeky, but works user-side!

//Swap two numbers without temp
#include<iostream.h>
#include<conio.h>
void main()
{
clrscr();
int a,b;
cout<<"a="<<endl;
cin>>a;
cout<<"b="<<endl;
cin>>b;
cout<<"a="<<b<<endl;
cout<<"b="<<a<<endl;
getch();
}

This is another one. I'm trying to do this without using any built-in functions or anything. This one requires a bit of work from the user.

//Swap numbers without temp variable
#include<iostream.h>
#include<conio.h>
void main()
{
clrscr();
int a,b, i;
cout<<"a="<<endl;
cin>>a;
cout<<"b="<<endl;
cin>>b;

if (a<b)
{
for(i=a;i<b;i++)
{
++a;
}
cout<<"--------->a="<<a<<"<---------"<<endl;
}
else if(a>b)
{
for(i=a;i>b;i--)
{
--a;
}
cout<<"--------->a="<<a<<"<---------"<<endl;
}
cout<<"Enter the two numbers again!"<<endl;
cout<<"a="<<endl;
cin>>a;
cout<<"b="<<endl;
cin>>b;

if(b<a)
{
for(i=b;i<a;i++)
{
b++;
}
cout<<"--------->b="<<b<<"<---------"<<endl;
}
else if(b>a)
{
for(i=b;i>a;i--)
{
b--;
}
cout<<"--------->b="<<b<<"<---------"<<endl;
}
getch();
}
/*OUTPUT
a=
12
b=
16
--------->a=16<---------
Enter the two numbers again!
a=
12
b=
16
--------->b=12<---------
*/

Yep...cheekier!

 

[Daniel Duffy]

Does this code even compile?? iostream.h??

http://members.gamedev.net/sicrane/articles/iostream.html

The examples are naf IMO.

 

[Hongchi Zhang]

Re: C++ puzzles - Have fun and learn. You may get a job too



These are not difficult but a person with no programming knowledge might struggle. These are my answers. I haven't tried them but they should work.
1)

#include <algorithm>
...
   std::swap(a, b);
...

2) This is a little more conversome but it's also straight forward:

#include <iostream>
...
void printForward(int i, int max)
{
    if (i > max) {
        return;
    } else {
        std::cout << i << std::endl;
        printForward(i + 1, max);
    }
}

void printBackward(int i, int min)
{
    if (i < min) {
        return;
    } else {
        std::cout << i << std::endl;
        printForward(i - 1, max);
    }
}

int main()
{
    printForward(1, 100);
    printBackward(100, 1);
    return 0;
}

I did the following for number 1:

void swap (int& x, int& y)
{
x=x+y;
y = x- y;
x = x -y;
}

 

[sssagar]

#include<iostream>
using namespace std;
int main()
{
static int a = 1;
static int b = 0;
if(b == 0)
{
if(a <= 100)
{
cout<<a++<<" ";
main();
}
}
if(a >= 100)
{
b = 1;
}
if(b == 1)
{
if(a > 1)
{
cout<<--a<<" ";
main();
}
}
}

 

[sssagar]

#include<iostream>
using namespace std;
int main()
{
cin>>a>>b;
a = b + a - (b = a);
cout<<a<<" "<<b;
}

 

[Daniel Duffy]

#include<iostream>
using namespace std;
int main()
{
static int a = 1;
static int b = 0;
if(b == 0)
{
if(a <= 100)
{
cout<<a++<<" ";
main();
}
}
if(a >= 100)
{
b = 1;
}
if(b == 1)
{
if(a > 1)
{
cout<<--a<<" ";
main();
}
}
}

What's the question for this C code?

BTW static variables are considered harmful.