从文件中读取一个数字，然后在 C 中读取一个数组

Question

How can I read from a file a number and after an array. i mean my file looks like that: 3 7 8 9

3 is the number of components, 7, 8 9 the other components of the array, arr[1], arr[2], arr[3].

Answer 1

one way to perform the desired functionality is:

First, open the file for reading:

FILE *fp = fopen( "filename.txt" );

Then check that the call to fopen() was successful and handle any error:

if( ! fp )
{
    perror( "fopen to read filename.txt failed" );
    exit( EXIT_FAILURE );
}

Note: perror() outputs both your error message and the text reason the system thinks the error occurred to stderr . which is where error messages should be output.

reserve a variable to hold the count of following values:

int maxLoops;

then read the first number and use that number as the max iterations of a loop, of course, checking for errors

if( fscanf( fp, "%d", &maxLoops ) != 1 )
{
    fprintf( stderr, "fscanf to read loop count failed\n" );
    exit( EXIT_FAILURE );
}

Note: the scanf() family of functions does not set errno when some input format specifier (in this case %d ) fails, so need to output an error message using something like fprinf() .

Note: the scanf() family of functions returns the number of successful input format conversions (or EOF)

Note: exit() and EXIT_FAILURE are exposed via:

#include <stdlib.h>

then, reserve an array for the following entries in the file, using the Variable Array Length feature of C

int dataArray[ maxLoops ];

Now, set up the loop that will read the rest of the data

for( int i = 0; i < maxLoops; i++ )
{

for each pass through the loop read another entry into the array, of course, checking for errors

    if( fscanf( fp, "%d", &dataArray[i] ) != 1 )
    {
        fprintf( stderr, "fscanf for data value failed\n" );
        exit( EXIT_FAILURE );
    }
}  // end the loop

then, cleanup before doing anything else:

fclose( fp );

What you do with the data is up to you. You might want to print out each of the data values with a loop, similar to:

for( int i = 0; i < maxLoops; i++ )
{
    printf( "entry %d = %d\n", i, dataArray[i] );
}

Note: when calling printf() no need to obtain the address of a variable (unless that is what you want to print). However, when inputting a variable, as when calling fscanf() need the address of the variable.