c中的for循环返回错误值

Question

image of issue with output

I'm reading a set of numbers from a file (1 2 3 4 5 6 7) and when I print them out in the while loop it returns the correct numbers. In the for loop directly below it, it's returning random numbers (under loop through array). Anyone know what is going on?

#include <stdio.h>

int main(int argc, char* argv[])
{
  if(argc != 2){
      printf("%s\n", "Wrong number of arguments");
  }
  else{
    char* filename = argv[1];
    printf("%s\n", filename);
    FILE* fp = fopen(filename, "r");
    int arrSize;
    int array[arrSize];
    int i = 0;

    fscanf(fp, "%d\n", &arrSize);

    printf("%d\n", arrSize);

    while (fscanf(fp, "%d", &array[i]) == 1) {
      printf("%d\n", array[i]);
      i = i+1;
    }

    
    printf("%s\n", "Loop through array");
    for (int j = 0; j < arrSize; j++) {
      printf("%d", array[j]);
      printf("\n");
    }

    fclose(fp);
  }
}

Answer 1

You have:

int arrSize;
int array[arrSize];
int i = 0;

fscanf(fp, "%d\n", &arrSize);

When you define array , you have no idea what value is in arrSize ; it is uninitialized.

You need:

int arrSize;
int i = 0;

fscanf(fp, "%d\n", &arrSize);

if (arrSize <= 0 || arrSize > MAX_ARRAY_SIZE)
    …deal with error condition…

int array[arrSize];

You might want to think about writing the first loop as:

for (int i = 0; i < arrSize && fscanf(fp, "%d", &array[i]) == 1; i++)
    printf("%d\n", array[i]);

This avoids overflowing the bounds of the array you have allocated. You then don't need the separate declaration of i either. The braces are optional; I wouldn't use them for a single, simple statement like the printf() call, but many people always use them. The braces become necessary, of course, if the loop is revised to:

for (int i = 0; i < arrSize; i++)
{
    if (fscanf(fp, "%d", &array[i]) != 1)
        break;
    printf("%d\n", array[i]);
}

It's good that you test for fscanf(…) == 1 ; people often mistakenly test against EOF.

Answer 2

Look at this piece of code.

int arrSize;
int array[arrSize];
int i = 0;

fscanf(fp, "%d\n", &arrSize);

What do you think what will happen? You are declaring an array without defining its size.

int arrSize;
fscanf(fp, "%d\n", &arrSize);
int array[arrSize];