c passing pointer to recursive function

Question

So here i am struggling with this program, i was trying to find out how can I use a array of pointers declared into main, in a recursive function to memorize data, the question that arises here is if it's the same approach as for a single pointer, what about for a struct type ? what is the best way to pass by reference a variable/array to a recursive function ?

#include <stdio.h> #include <stdlib.h> #define N 1 void f(int i,int j,int *cnt); int j=0; int main(int argc, char *argv[]) { int *cnt=0; f(0,++j,&cnt); printf("------ %d ---- \n",cnt); system("PAUSE"); return 0; } void f(int i,int j,int *cnt){ if(i>N){ printf("---if --- %d ---- %d \n",i,j); (*cnt)++; return; } (*cnt)++; printf("---bg --- %d ---- %d \n",i,j); f(i+1,++j,cnt); f(i+1,++j,cnt); }

Another thing i'd like to know is how does the recursive functions handle the ++i and i++ and i+1 increments (when passed as parameters),

This code won't compile. And you haven't initialised cnt to point at any memory. — Oliver Charlesworth
– Oliver Charlesworth, Commented Jan 31, 2012 at 0:49

John · Accepted Answer · 2012-01-31 01:03:12Z

int main(int argc, char *argv[]) { int *cnt=0; f(0,++j,&cnt); printf("------ %d ---- \n",(*cnt)); system("PAUSE"); return 0; }

needs to be

int main(int argc, char *argv[]) { int intStorage = 0;//<---- As Oli said. int *cnt= &intStorage; f(0,++j,cnt);//<-------AMPERSAND removed, overly dereferenced. printf("------ %d ---- \n",(*cnt)); system("PAUSE"); return 0; }

++i and i++ and i+1 (when passed as parameters):

++i: i + 1 is passed and is also the value i takes afterwards.
i++: i is passed and i = i + 1 after the call.
i+1: i + 1 is passed but i remains as just i afterwards.

I'll try and fix your function a little too:

void f(int i,int j,int *cnt){ if(i>N){ printf("---if --- %d ---- %d \n",i,j); return; } (*cnt)++; printf("---bg --- %d ---- %d \n",i,j); if ( i < 50 && j < 50 ) { f(i+1,++j,cnt); f(i+1,++j,cnt); } }

Still a lot of recursion but without the danger of not stopping.

I didn't realise the recursive functions needed work too. Oh yeah they just stack right up in infinite recursion.
You should be getting a compilation warning - type mismatch on argument 3 of call to f() in main(). You are passing an int ** where the function expects an int *.

Jonathan Leffler · Accepted Answer · 2012-01-31 01:02:57Z

A simple way of handling the pointer into your function is:

#include <stdio.h> #include <stdlib.h> #define N 1 void f(int i, int j, int *cnt); int j = 0; int main(void) { int cnt = 0; f(0, ++j, &cnt); printf("------ %d ----\n", cnt); return 0; } void f(int i, int j, int *cnt) { // Having a local variable j and a global j is likely to confuse someone! if (i > N) { printf("---if --- %d ---- %d\n", i, j); return; } (*cnt)++; printf("---bg --- %d ---- %d\n", i, j); f(i+1, ++j, cnt); f(i+1, ++j, cnt); }

This code produces the following output with no crash:

---bg --- 0 ---- 1 ---bg --- 1 ---- 2 ---if --- 2 ---- 3 ---if --- 2 ---- 4 ---bg --- 1 ---- 3 ---if --- 2 ---- 4 ---if --- 2 ---- 5 ------ 3 ----

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