问题
I have an assignment to create a menu program that uses a linked list to store a list of phone catalog. Here is my source code:
int isempty(FILE *in)
{
return in == NULL;
}
node *makenewnode(item newitem)
{
node *newnode = (node *) malloc(sizeof(node));
newnode->info = newitem;
return newnode;
}
int countlines(FILE *datain)
{
int lines = 0;
char string[MAX];
while (fgets(string, MAX, datain))
lines++;
return lines;
}
node *push(node *listin, item newitem)
{
node *newnode = makenewnode(newitem);
if(listin == NULL)
{
newnode->next = NULL;
listin = newnode;
}
else
{
newnode->next = listin;
listin = newnode;
head = listin;
}
cur = head;
return head;
}
node *builldbytext(FILE *txt, int maxlines)
{
int i = 0;
node *result = (node *) malloc(maxlines*sizeof(node));
item *temp = (item *) malloc(maxlines*sizeof(item));
while(i < maxlines)
{
fscanf(txt, "%s %d %f %s\n", temp[i].model, &temp[i].memory, &temp[i].screen_size, temp[i].price);
i++;
}
for(i = 0; i< maxlines; i++)
result = push(result, temp[i]);
return result;
}
void tranverse(node *listprint, int maxlines)
{
int i = 0;
printf("%-20s%-15s%-20s%-30s\n", "Model", "Memory (GB)", "Screen size (Inch)", "Price");
for(i = 0; i < maxlines; i++)
{
item temp = listprint[i].info;
printf("%-20s%-15d%-20.2f%-30s\n", temp.model, temp.memory, temp.screen_size, temp.price);
}
}
int main()
{
FILE *fp = fopen("phoneDB.txt", "r");
int maxlines = countlines(fp);
fclose(fp);
node *phonelist = (node *) malloc(maxlines*5*sizeof(node));
int choice = 0;
do
{
FILE *fptxt = fopen("phoneDB.txt", "r");
printf("1. Import from Text\n3. Display List\n8. Quit\n");
printf("Your choice: ");
scanf("%d", &choice);
getchar();
switch(choice)
{
case 1:
if(isempty(fptxt))
{
printf("This file is empty\n");
goto x;
}
phonelist = builldbytext(fptxt, maxlines);
x: break;
case 3:
tranverse(head, maxlines);
break;
}
fclose(fptxt);
}while(choice != 8);
free(phonelist);
return 0;
}
Here is my text file (phoneDB.txt):
Iphone6 12 9.6 2000000
IphoneX 32 12.3 40000000
SamsungA6 16 11.3 1000000
SamsungNote6 16 12.3 12000000
Iphone5 32 9.5 6000000
Iphone5s 32 9.5 7000000
Iphone6 32 9.3 8000000
Iphone6s 32 11.3 8500000
OppoF5 32 9.3 10000000
OppoE6 32 11.3 20000000
OppoReno 16 12.6 20000000
IphoneSXmax 128 11.3 45000000
Huawei4 64 11.3 20000000
NokiaE5 16 8.6 3000000
SamsungGalaxy 32 12.3 6000000
SamsungNote7 32 12.3 8000000
Iphone7s 32 12.3 10000000
Huawei6 16 9.5 15000000
SamsungNote5 16 8.5 12500000
IphoneX 16 12.3 25000000
Iphone7 24 11.5 25100000
My problem: When I debug my program, I see that my program can't read the text file. It shows that the file is empty (function isempty() returns true). However, when I run my program, it does not show any messages (I use gcc of MinGW on Windows 10 as a compiler). Moreover, because of its unreadability, the output does not show the expected result.
When I use ftell() function to check the position of pointer, it returns 0.
Note: I have checked my input file name so the error is not there. I think it is somewhere else. The output:
Model Memory (GB) Screen size (Inch) Price
Iphone7 24 11.50 25100000
0 0.00
-2147479552 0.00
0 0.00
0 -0.42 ¨‰
917518 0.00
0 0.00
\\Very long after that but I just show a sample of it
回答1:
Much of the confusion comes from the awkward way you are approaching building your list. Set aside the menu for now -- that is left to you, let's just concentrate on building your linked list. For starters, it's impossible to tell where your failure arises because you fail to check the return of your input function. You cannot use any input function correctly (or any function critical to the continued operation of your code) unless you check the return. You also do not want to Hardcode Filenames, that is what the parameters to int main (int argc, char **argv) are there for — or you can take the filename to read as user-input.
Next, since your data is arranged in lines-of-input in your file, you should use a line-oriented input function to read from the file (such as fgets() or POSIX getline()) That way you ensure you consume an entire line of input each time and what remains unread does not depend on the format specifier used. Instead, declare a character array of sufficient size to hold each line from your file and then read into that array with fgets() and then separate into name, mem, size, and price using sscanf() (don't forget to check the return)
While you can send your open FILE* pointer to builldbytext() to read and build your list, below let's just take a simple approach and read from the file in a loop in main() and fill a temporary struct with the values from each line. Then we can pass the address of the pointer to your list along with a pointer to the temporary struct containing data to an add() function (your push()) to build the list.
You didn't provide your node definition, so for purposes of the example we will use:
#include <stdio.h>
#include <stdlib.h>
#define NAMSZ 32 /* if you need a constant, #define one (or more) */
#define MAXC 1024
typedef struct node_t { /* list node */
char name[NAMSZ];
double size;
int mem, price;
struct node_t *next;
} node_t;
While you can use forward-chaining to add the nodes, you will end up with your nodes in the reverse order from what you have in the file. You can either use a head and tail pointer to add in order using the tail pointer in the same O(1) time or you can iterate to the end and add in O(n) time.
A simple example of adding the node at the end iterating each time would be:
/** add node at end of list */
node_t *add (node_t **head, node_t *tmp)
{
node_t **ppn = head, /* pointer to pointer to head */
*pn = *head, /* pointer to head */
*node = malloc (sizeof *node); /* allocate new node */
if (!node) { /* validate allocation */
perror ("malloc-node");
return NULL;
}
*node = *tmp; /* assign tmp struct values */
node->next = NULL; /* set next pointer NULL */
while (pn) { /* iterate to end of list */
ppn = &pn->next;
pn = pn->next;
}
return *ppn = node; /* assign & return new node */
}
(note: by using a pointer-to-pointer there is no special treatment required for adding the first or subsequent nodes)
A simple traversal prn() and a function to delete all nodes in the list del_list() when done could be:
/** print all nodes in list */
void prn (node_t *l)
{
if (!l) {
puts ("list-empty");
return;
}
for (node_t *n = l; n; n = n->next)
printf ("%-16s %3d %5g %d\n", n->name, n->mem, n->size, n->price);
}
/** delete all nodes in list */
void del_list (node_t *l)
{
node_t *n = l;
while (n) {
node_t *victim = n;
n = n->next;
free (victim);
}
}
Finally, all that is needed to take your filename to read as the first argument to the program, or read from stdin if no argument is provided, to fill the list and then traverse and free all allocated memory could be:
int main (int argc, char **argv) {
char buf[MAXC]; /* buffer to hold each line */
node_t *list = NULL; /* pointer to list (must initialize NULL) */
/* use filename provided as 1st argument (stdin by default) */
FILE *fp = argc > 1 ? fopen (argv[1], "r") : stdin;
if (!fp) { /* validate file open for reading */
perror ("file open failed");
return 1;
}
while (fgets (buf, MAXC, fp)) { /* read each line from file */
node_t tmp; /* temporary struct to hold values */
/* if name, mem, size, price successfully parsed from line */
if (sscanf (buf, "%s %d %lf %d",
tmp.name, &tmp.mem, &tmp.size, &tmp.price) == 4)
if (!(add (&list, &tmp))) /* add node to list/validate */
break;
}
if (fp != stdin) /* close file if not stdin */
fclose (fp);
prn (list);
del_list (list);
}
(note: the is_empty() functionality is handled in each function that receives the list simply by validating that the first node in the list isn't NULL)
Example Use/Output
With your data in the file dat/phones.txt, you would receive:
$ ./bin/lls_phones dat/phones.txt
Iphone6 12 9.6 2000000
IphoneX 32 12.3 40000000
SamsungA6 16 11.3 1000000
SamsungNote6 16 12.3 12000000
Iphone5 32 9.5 6000000
Iphone5s 32 9.5 7000000
Iphone6 32 9.3 8000000
Iphone6s 32 11.3 8500000
OppoF5 32 9.3 10000000
OppoE6 32 11.3 20000000
OppoReno 16 12.6 20000000
IphoneSXmax 128 11.3 45000000
Huawei4 64 11.3 20000000
NokiaE5 16 8.6 3000000
SamsungGalaxy 32 12.3 6000000
SamsungNote7 32 12.3 8000000
Iphone7s 32 12.3 10000000
Huawei6 16 9.5 15000000
SamsungNote5 16 8.5 12500000
IphoneX 16 12.3 25000000
Iphone7 24 11.5 25100000
If you had used forward-chaining to add the nodes, then they would print in the reverse order.
Memory Use/Error Check
In any code you write that dynamically allocates memory, you have 2 responsibilities regarding any block of memory allocated: (1) always preserve a pointer to the starting address for the block of memory so, (2) it can be freed when it is no longer needed.
It is imperative that you use a memory error checking program to ensure you do not attempt to access memory or write beyond/outside the bounds of your allocated block, attempt to read or base a conditional jump on an uninitialized value, and finally, to confirm that you free all the memory you have allocated.
For Linux valgrind is the normal choice. There are similar memory checkers for every platform. They are all simple to use, just run your program through it.
$ valgrind ./bin/lls_phones dat/phones.txt
==17133== Memcheck, a memory error detector
==17133== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==17133== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==17133== Command: ./bin/lls_phones dat/phones.txt
==17133==
Iphone6 12 9.6 2000000
IphoneX 32 12.3 40000000
SamsungA6 16 11.3 1000000
SamsungNote6 16 12.3 12000000
Iphone5 32 9.5 6000000
Iphone5s 32 9.5 7000000
Iphone6 32 9.3 8000000
Iphone6s 32 11.3 8500000
OppoF5 32 9.3 10000000
OppoE6 32 11.3 20000000
OppoReno 16 12.6 20000000
IphoneSXmax 128 11.3 45000000
Huawei4 64 11.3 20000000
NokiaE5 16 8.6 3000000
SamsungGalaxy 32 12.3 6000000
SamsungNote7 32 12.3 8000000
Iphone7s 32 12.3 10000000
Huawei6 16 9.5 15000000
SamsungNote5 16 8.5 12500000
IphoneX 16 12.3 25000000
Iphone7 24 11.5 25100000
==17133==
==17133== HEAP SUMMARY:
==17133== in use at exit: 0 bytes in 0 blocks
==17133== total heap usage: 24 allocs, 24 frees, 6,848 bytes allocated
==17133==
==17133== All heap blocks were freed -- no leaks are possible
==17133==
==17133== For counts of detected and suppressed errors, rerun with: -v
==17133== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)
Always confirm that you have freed all memory you have allocated and that there are no memory errors.
Look things over and let me know if you have questions. If you would like to change to forward-chaining or using a tail pointer, let me know and I'm happy to help further.
来源：https://stackoverflow.com/questions/61166837/can-not-read-text-file-in-c