I am trying to understand how to use recursion in C, and I can't get how return
works in it.
Please consider the following code:
int recur(int i)
{
printf("recur: i = %d\n", i);
if (i < 3)
{
recur(i + 1);
return 10;
}
else if (i < 5)
recur(i + 1);
return i;
}
int main(void)
{
int i = 0;
i = recur(i);
printf("i = %d\n", i);
return 0;
}
The output is:
recur: i = 0
recur: i = 1
recur: i = 2
recur: i = 3
recur: i = 4
recur: i = 5
i = 10
What does the last return, return i
, do? Does this code even make sense?
The recursive calls of the function do not influence on the returned value. Only the first return
met in the first instance of your recursive function will return a value to the parent function. Any other return
met will just stop the function's instance the program is currently in.
Thus as the function was called in main with the argument 0
int i = 0;
i = recur(i);
The first return
met is located inside of an if
statement:
if (i < 3)
{
recur(i + 1);
return 10;
}
In this case, the recur
function is called before returning a value to main
. It will create another instance of recur
which will do some stuff, but after this instance of recur
has ended, the main instance of recur
will continue and, in this case, will return 10 to the function main
.
To know what your recursive function will return to the main
function, you can simply comment all calls to a new instance of the function:
int recur(int i)
{
if (i < 3)
{
//recur(i + 1);
return 10;
}
else if (i < 5)
{
//recur(i + 1);
}
return i;
}
In this case, this is what the program will read:
int recur(int i)
{
if (i < 3)
return 10;
return i;
}