Thread: Unexpected Printf Behavior

Hey all, first post.

I'm working on a program that does some basic file I/O. As of right now I'm just trying to read some integers from a text file into an array. In addition to enlightening me as to why my code is wrong, I am open to better implementations (although I chose this implementation as it is complicated and I want to fully grasp the fine-grained.... I understand that there most likely exists a much better way such as some fscanf loop).

So the actual problem. Inside the main function, inside the while loop under the comment "read the numbers from a the file, ...", I am simply attempting to print out an integer array as a char array. But the printf statement only prints out the first digit of the string. I have examined the array contents at run-time with gdb and the contents are as expected.

Example: With a file with a single line entry of "16", only the number 1 gets printed. Upon examination, the contents of the array is a[0] = 49, a[1] = 54, and a[2] = 0. I don't understand why printf won't print out the entire array.

The code (note: i excluded erroneous code.... let me know if something is missing that you need to see):

Code:

int main(int argc, char *argv[])
{
    /* TODO: Complete the main method according to the steps outlined */


    /* Open a file for reading */
    if(argc != 2){
        fprintf(stderr , "usage: %s [input file]" , argv[0]);
        exit(1);
    }


    FILE * fp;
    int i = 0, num_count = 0, len, num_list[MAX_INTS], buf[MAX_INT_LENGTH];


    if((fp = fopen(argv[1], "r")) == NULL){
        fprintf(stderr , "error: could not open file %s for reading" , argv[1]);
        exit(2);
    }


    /* Read the numbers from the file, into an array */


    while((len = fetch_line(buf, MAX_INT_LENGTH, fp)) > 0){
        // *(num_list + i++) = (int) strtol((char *)buf, NULL, DECIMAL);
        printf("%s\n", (char *)buf);
        num_count++;
    }

int fetch_line( int * s , int limit , FILE * file_read ){
        int c , i;
        for( i = 0; i < limit - 1 && ( c = fgetc( file_read ) ) != NEWLINE && c != EOF; ++i )
                s[ i ] = c;
        s[ i ] = '\0';
        return i;
}

Please post actual, complete code (including headers), preferably something you just ran and then copy/pasted. This code does not compile as it is missing headers and defines, and doesn't even have a complete main. You haven't even bothered to show an example input file.

Anyway, your problem is simply that ints are not chars. How do you expect printf to print an int array as chars? Think about it. You read a char (1 byte) into an int (4 bytes). Most of the int's bytes will be 0. As soon as the printf hits one of those it considers the string to be ended and stops printing.

Why are you reading chars into ints? Why are you trying to print them as chars? What are you actually trying to do?

Code:

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


#define MAX_INTS       1000
#define MAX_INT_LENGTH 10
#define DECIMAL        10
#define NEWLINE           '\n'


/* A node in the linked list */
struct node {
    int data;
    struct node *next;
};


struct node* create_list(int intarray[], int len);


struct node* add_item_at_start(struct node *head, int data);


int search_array(int integers[], int numints, int element);


int search_list(struct node *head, int element);


struct node* create_sorted_list(struct node *head);


struct node* add_item_sorted(struct node *head, int data);


int copy_list_to_array(struct node *head, int *array);


void print_list(struct node *head);


void print_array(int integers[], int len);


int fetch_line(int *, int, FILE *);


int main(int argc, char *argv[])
{
    /* TODO: Complete the main method according to the steps outlined */


    /* Open a file for reading */
    if(argc != 2){
        fprintf(stderr , "usage: %s [input file]" , argv[0]);
        exit(1);
    }


    FILE * fp;
    int i = 0, num_count = 0, len, num_list[MAX_INTS], buf[MAX_INT_LENGTH];


    if((fp = fopen(argv[1], "r")) == NULL){
        fprintf(stderr , "error: could not open file %s for reading" , argv[1]);
        exit(2);
    }


    /* Read the numbers from the file, into an array */


    while((len = fetch_line(buf, MAX_INT_LENGTH, fp)) > 0){
        // *(num_list + i++) = (int) strtol((char *)buf, NULL, DECIMAL);
        printf("%s\n", (char *)buf);
        num_count++;
    }


    /* Print the array */
/*
    for(i = 0; i < num_count; i++)
        printf("List Entry %d -> %d\n" , i + 1, num_list[i]);
*/
    /* Create a linked list with the integers from the array */
    
    /* Print the linked list */




    /* Repeatedly prompt the user for a number to be searched.
    *  Search the array for the number and print out the result as shown in the specs.
    *  Search the linked list for the number and print out the result as shown in the specs.
    *  Stop prompting when the user enters 'q' (just the character q without the single quotes).
    */


    /* Create a sorted list(in ascending order) from the unsorted list */


    /* Print the sorted list */


    /* Copy the sorted list to an array with the same sorted order */


    /* Print out the sorted array */    


    /* Print the original linked list again */


    /* Print the original array again */




    /* Open a file for writing */


    /* Write the sorted array to a file named "sorted_numbers.txt" */


    /* Print out the number of integers written to the file */


}


struct node* create_list(int intarray[], int len)
{
    struct node * head;
    int i;


    for(i = 0; i < len; i++ )
        head = add_item_at_start( head, *(intarray + i) );


    return head;
}




struct node* add_item_at_start(struct node *head, int data)
{
    struct node * np = malloc( sizeof(struct node) );
    if(np == NULL)
        return head;
    np->data = data;
    np->next = head;
    return np;
}


int search_list(struct node *head, int element)
{
    /* TODO: Complete this function */
}


int search_array(int integers[], int numints, int element)
{
    int i;
    for(i = 0;i < numints; i++)
        if( *(integers + i) == element )
            return i;
    return -1;
}


int copy_list_to_array(struct node *head, int *array)
{
    /* TODO: Complete this function */
}


struct node* create_sorted_list(struct node *head)
{
    /* TODO: Complete this function */
}


struct node* add_item_sorted(struct node *sorted_head, int data)
{
    /* TODO: Complete this function */
}


void print_list(struct node *head)
{
    if (head == NULL) {
        printf("Linked List is Empty.\n");
    } else {
        struct node *temp = head;
        printf("head->");
        while (temp != NULL) {
            printf("|%d|->", temp->data);
            temp = temp->next;
        }
        printf("NULL\n");
    }
}


void print_array(int integers[], int len)
{
    int i;
    for (i = 0; i < len; i++) {
        printf("| %d ", integers[i]);
    }
    printf("|\n");
}


int fetch_line( int * s , int limit , FILE * file_read ){
        int c , i;
        for( i = 0; i < limit - 1 && ( c = fgetc( file_read ) ) != NEWLINE && c != EOF; ++i )
                s[ i ] = c;
        s[ i ] = '\0';
        return i;
}

Here is an example input file.

Code:

16

My thought was that casting the int * to a char * forces the compiler to interpret the entries as chars and not integers. I know that in C, chars are often stored in int variables in order to deal with EOF properly.

I'm working on a homework assignment.

Found the problem.... I was reusing the fetch_line function that I had modified for something else. The array to store the number(buf) as characters should be a char *, not an int * as pointed out above (thanks!).