This question came up in an exam I had.
The answer is 120 can someone explain to me how this is?
Q.
What is the output of this program?
Code:#include <stdio.h> #include <stdlib.h> int func(int x); int main() { int x = 5; x = func(x); printf("%d \n", x); } int func(int x) { if(x > 1) return x*func(x-1); return 1; }
Trace through the recursion. Write down what happens on paper, if you need to. You will find that it is a recursive implementation of a mathematical construct that you have probably seen before.
Look up a C++ Reference and learn How To Ask Questions The Smart WayOriginally Posted by Bjarne Stroustrup (2000-10-14)
Get a pencil and paper, and mentally go through the program line by line, using the pencil and paper to keep track of the value of 'x' wherever it changes.
This might be a bit more difficult to represent on paper, since it uses a recursive function, but figuring it out how to visualize it will help your understanding.
[edit] Too slow.
One might exclaim that a certain operation relates 5 and 120.
The best way to trace a program is either on paper (in your head for more experienced programmers for this program) or using a debugger.
If you do NOT know how to use a debugger; I would suggest varying the input. In this program the value of x is the input value.
Tim S.Code:int x = 5;
"...a computer is a stupid machine with the ability to do incredibly smart things, while computer programmers are smart people with the ability to do incredibly stupid things. They are,in short, a perfect match.." Bill Bryson
Another way - to use the unit test. Another way - to use the unit test. This is similar to fitness classes. Start doing it in his youth, when no health problems. It is better than start when the health is undermined.
Test-driven development - Wikipedia, the free encyclopedia
Test driven development in C
C Testing for Embedded Applications with greatest
Code:#include "greatest.h" int func(int x) { if(x > 1) return x*func(x-1); return 1; } TEST func_1() { ASSERT_EQ(func(1), 1); PASS(); } TEST func_2() { ASSERT_EQ(func(2), 2); PASS(); } TEST func_3() { ASSERT_EQ(func(3), 6); PASS(); } TEST func_4() { ASSERT_EQ(func(4), 24); PASS(); } TEST func_5() { ASSERT_EQ(func(5), 120); PASS(); } SUITE(the_suite) { RUN_TEST(func_1); RUN_TEST(func_2); RUN_TEST(func_3); RUN_TEST(func_4); RUN_TEST(func_5); } GREATEST_MAIN_DEFS(); int main(int argc, char **argv) { GREATEST_MAIN_BEGIN(); /* command-line arguments, initialization. */ RUN_SUITE(the_suite); GREATEST_MAIN_END(); /* display results */ }
Our goals are clear, tasks are defined! Let's work, comrades! -- Nikita Khrushchev
When the requirements are known, writing a unit test is a great idea. In this case, the reader does not know what the code is supposed to do, only what the code actually does. Writing a unit test does not work because the test parameters are unknown. Reading a unit test to help understand the code will work, but that's not the scenario here.
Good idea for development, but not "another way" for understanding code with unknown requirements and no existing unit tests. Once one understands the intention of the code, then unit tests could be written.
Look up a C++ Reference and learn How To Ask Questions The Smart Way
