The file becomes NULL after printing 254 rows in .txt-CodePudding

I am trying to solve the problem of recursion. The problem is : Given a number n, print out all the possible combinations of set {a, b, ..., a n-1}. Output the answer in "output-n.txt". 1<=n<=9.

Here's my code:

#define _CRT_SECURE_NO_WARNINGS
#include<stdio.h>
#include<stdlib.h>

void comb(int n,int curlen,int j,int num[],int flag) {
    int i,k;
    char filename[20] = "output-0.txt";
    filename[7] = n   '0';
    char ans[20] = { 0 };

    FILE* fptr;
        fptr = fopen(filename, "a");


        if (fptr != NULL) {
            if (curlen < n && flag != 0) {
                for (i = 0; num[i] != 0; i  ) {
                    ans[i] = num[i]   '`';
                }
                for (k = 0; k <= curlen; k  ) {
                    fprintf(fptr, "%c", ans[k]);
                }
                fprintf(fptr, " ");
                
                if (curlen == 0 && num[0] == n) {
                    fclose(fptr);
                }
                else {
                    comb(n, curlen   1, j   1, num, 0);
                }

            }
            else {
                for (; j <= n; j  ) {
                    num[curlen] = j;
                    comb(n, curlen, j, num, 1);
                }
            }
        }
        

}

int main() {
    int n,flag=0,num[20] = { 0 };
    
    scanf("%d", &n);
    
    comb(n, 0, 1, num, flag);
    

    system("pause");
    return 0;
}

My program is doing fine while 1<=n<=7, but at n=8 and n=9, the FILE pointer becomes NULL in the middle and can only output part of the answers. I looked closely to the output .txt file and realized that both of the files can only output to the 254th row.I wonder did I make any mistake. Thanks for answering.

CodePudding user response：

You have hit the limit for the number of open files.

You keep opening the same file for append and don't always close it. When I rearranged the code so that it opens the file once in main in "w" mode, it works correctly.

#include <stdio.h>
#include <stdlib.h>

void comb(FILE *fptr, int n,int curlen, int j, int num[], int flag) {
    int i,k;
    char ans[20] = { 0 };
    if (curlen < n && flag != 0) {
        for (i = 0; num[i] != 0; i  ) {
            ans[i] = num[i]   '`';
        }
        for (k = 0; k <= curlen; k  ) {
            fprintf(fptr, "%c", ans[k]);
        }
        fprintf(fptr, " ");
        
        if (curlen == 0 && num[0] == n) {
            //fclose(fptr);
        }
        else {
            comb(fptr, n, curlen   1, j   1, num, 0);
        }
    }
    else {
        for (; j <= n; j  ) {
            num[curlen] = j;
            comb(fptr, n, curlen, j, num, 1);
        }
    }
}

int main() {
    int n,flag=0,num[20] = { 0 };
    char filename[20] = "output-0.txt";
    if(scanf("%d", &n) != 1 || n < 1 || n > 9)  // sanity check
        return 1;
    filename[7] = n   '0';
    FILE *fptr = fopen(filename, "w");          // open once
    if(fptr != NULL) {
        comb(fptr, n, 0, 1, num, flag);         // pass the file pointer
        fclose(fptr);
    }
    return 0;
}