当我 fprintf 时，它写入了错误的变量

Question

When I use fprintf , it actually saves the wrong content.

Usually fprintf outputs what I want, but this time it's outputting seemingly irrelevant numbers, and I don't know what the problem is.

My Inputs & Expected output

1 2 3
4 5 6
7 8 9

Actual Output

3 0 4199352 
1 11 0 
9 0 2 

I used a for loop to output a two-dimensional array

My code

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

void filein(int x, int y) {
    FILE *fin;
    int s[x][y];
    int i, j;
 
    if ((fin = fopen("test.txt", "r")) == NULL) {
        printf(" can't open");
    }
    else {
        printf("opening...\n");
    }

    for (i = 0; i < x; i++) {
        for (j = 0; j < y; j++) {
            fscanf(fin, "%d", &s[i][j]);
        }
  
        fscanf(fin, "\n");
    }
  
    fclose(fin);
  
    for (i = 0; i < x; i++) {
        for (j = 0; j < y; j++) {
            printf("%d ", s[i][j]);
            fflush(stdout);//输出
        }
      
        printf("\n");
    }
}

void fileout(int x, int y) {
    FILE *fout;
    int s[x][y];
    int i, j;
    char outname[50];
    printf("please input the name of output file (no include\".txt\"):\n");
    fflush(stdout);
    scanf("%s", outname);
    strcat(outname, ".txt\0");
    fout = fopen(outname,"w");
    
    if (fout == NULL) {
        printf("Error!");
        fflush(stdout);
    }
    else {
        printf("Writing....\n");
        fflush(stdout);
    }
    
    for (i = 0; i < x; i++) {
        for (j = 0; j < y; j++) {
            fprintf(fout, "%d ", s[i][j]);
        }

        fprintf(fout,"\n");
    }
    
    fclose(fout);
}

int main() {
    int x, y;
    printf("what is m?\n");
    fflush(stdout);
    scanf("%d", &x);
    printf("what is n?\n");
    fflush(stdout);
    scanf("%d", &y);
    filein(x, y);
    fileout(x, y);
    printf("finish");
    fflush(stdout);

    return 0;
}

Answer 1

The problem is that your fileout function doesn't have access to the data you read in your filein function. You read it into an array on the stack (the local variable s[x][y]), and then returned - that information is now gone. Then when fileout runs, the new s[x][y] is at some other place on the stack and you are just printing out whatever happens to be in memory there.

That's why your debugging printouts in filein show that you've read the data correctly, but the output is garbage.

You'll need to malloc your space for the numbers in main and pass it around, or call a helper function between main and filein and fileout that allocates s on the stack as you've already done (but only once) and then passes s into the other functions.

Answer 2

Your 2d array s in fileout function is allocated garbage numbers in the memory that's why you are getting different numbers from the numbers inside test.txt.

so I have solved that for you by defining a struct int_array2d_t* that has x and y of type size_t and array variables that could be accessed whenever needed.

The code that I added has comments that explain it, however, let me know if you do not understand sth!

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

/* define a struct that has x, y of type size_t and array */
typedef struct
{
  size_t x;
  size_t y;
  int array [];
} int_array2d_t;


/* initialize pointer of type struct int_array2d_t */
int_array2d_t* array2d = NULL;

void filein(int x, int y) {
    FILE *fin;
    int s[x][y];
    int i, j;
 
    if ((fin = fopen("test.txt", "r")) == NULL) {
        printf(" can't open");
    }
    else {
        printf("opening...\n");
    }

    for (i = 0; i < x; i++) {
        for (j = 0; j < y; j++) {
            fscanf(fin, "%d", &s[i][j]);
        }
  
        fscanf(fin, "\n");
    }
  
    fclose(fin);

    double count = 0.0;
    for (i = 0; i < x; i++) {
        for (j = 0; j < y; j++) {
            /* copy each value in s to array2d */
            array2d->array[i*y + j] = s[i][j];
            printf("%d ", array2d->array[i*y + j]);
            fflush(stdout);//输出
        }
      
        printf("\n");
    }
}

void fileout(int x, int y) {
    FILE *fout;
 
    int i, j;
    char outname[50];
    printf("please input the name of output file (no include\".txt\"):\n");
    fflush(stdout);
    scanf("%s", outname);
    strcat(outname, ".txt\0");
    fout = fopen(outname,"w");
    
    if (fout == NULL) {
        printf("Error!");
        fflush(stdout);
    }
    else {
        printf("Writing....\n");
        fflush(stdout);
    }
    
    for (i = 0; i < x; i++) {
        for (j = 0; j < y; j++) {
            /* write value in array2d->array to the file stream */
            fprintf(fout, "%d ", array2d->array[i*y + j]);
        }

        fprintf(fout,"\n");
    }
    
    fclose(fout);
}

int main() {
    int x, y;
    printf("what is m?\n");
    fflush(stdout);
    scanf("%d", &x);
    printf("what is n?\n");
    fflush(stdout);
    scanf("%d", &y);

    /* allocate array2d with the new size after we get x and y */
    array2d = malloc(sizeof(*array2d) + sizeof(double[x][y]));
    array2d->x = x;
    array2d->y = y;

    filein(x, y);
    fileout(x, y);
    printf("finish");
    fflush(stdout);

    return 0;
}