CODEWITHSUNDEEP
cfunction
yaylitzis
yaylitzis

Reputation: 5534

add numbers with recursive functions

I am learning C and i come up with this example

     #include <stdio.h>

     int MyAdd(int);
     main ()
     {
          int i;
          int c = 0;
          c = MyAdd(5); 
          printf("%d\n", c);
     }

     int MyAdd(int a)
     {
          if(a > 0)
               return a + MyAdd(--a);
          else
               return 0;
     }

I run this by my self and i calculate 15. (5 +4+3+2+1) but when i run it, i get 10... why??? At the 1st time, dont we get 5+ (do the func again) and etc..?

Upvotes: 0

Views: 151

Answers (2)

Jo&#227;o Mendes
Jo&#227;o Mendes

Reputation: 1425

When used in expressions, side effect operators do funny, unexpected things, because you're basically at the mercy of the compiler.

In this case, your compiler is evaluating the second operand of a + MyAdd(--a) before the first one. So, you're decrementing the variable before using it in the addition.

In any case, you don't really need the decrement operator. I would suggest rewriting the line as return a + MyAdd(a - 1);.

Upvotes: 6

Pandrei
Pandrei

Reputation: 4951

Doing something like:

foo(a++) or
foo(++a)

Is not very safe because you depend on Compiler implementation - meaning if it reads arguments from left to right or right to left.

Lets consider the first case:

foo(a++)

If the compiler reads the arguments left to right the result will be call foo(a) and then a++. If the arguments are read right to left, the result will be call f(a+1)

Upvotes: 0

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