Functions messes up numbers - c

Question

When I add workers thru main in for loop, everything is fine. But when I add them with one of my functions insert_one_with_pointers or insert_multiple_workers, and check all workers with my third function display_all_workers, I see that something is wrong.
Their id values are not correct and sometimes the program crashes or it doesn't display them all of them, just the ones thet were inserted in the beginning in main.
I just started working with structures and functions so I'm not sure what exactly is causing this, but I think that it has to do with my probably incorrect usage of i. I've been trying to fix this for quite a while, but unsuccessfully.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct Worker {
int id;
char name[40];
int age;
};
void insert_one_with_pointers(struct Worker *niz, int i) {
niz[i].id = i + 1;
printf("\nWhat is name of %d. worker: ", i + 1);
scanf("%s", niz[i].name);
printf("What is age of %d. worker: ", i + 1);
scanf("%d", &niz[i].age);
}
void insert_multiple_workers(struct Worker niz[], int i, int number) {
int j;
for (j = i; j < number; j++) {
niz[j].id = j + 1;
printf("\nWhat is name of %d. worker: ", j + 1);
scanf("%s", niz[j].name);
printf("What is age of %d. worker: ", j + 1);
scanf("%d", &niz[j].age);
}
}
void display_all_workers(struct Worker niz[], int i) {
printf("\nAll workers:\n");
for (int j = 0; j < i; j++) {
printf("\nID: %d", niz[j].id);
printf("\nName: %s", niz[j].name);
printf("\nAge: %d", niz[j].age);
}
}
int main() {
int choice, i, m, number;
printf("Add a number of workers and then their info: ");
scanf("%d", &i);
struct Worker niz[i];
for (int j = 0; j < i; j++) {
niz[j].id = j + 1;
printf("\nWhat is name of %d. worker: ", j + 1);
scanf("%s", niz[j].name);
printf("What is age of %d. worker: ", j + 1);
scanf("%d", &niz[j].age);
}
while(1) {
printf("\nChoose a function:\n1 - Add a worker using pointers\n2 - Add n workers\n3 - Display all workers\n4 - Leave\n");
scanf("%d", &choice);
switch(choice) {
case 1:
insert_one_with_pointers(niz, i); i++;
break;
case 2:
printf("\nHow many workers do you want to insert? ");
scanf("%d", &m);
number = i + m;
insert_multiple_workers(niz, i, number);
break;
case 3:
display_all_workers(niz, i);
break;
case 4:
return 0;
}
}
return 0;
}
Edit: I followed everything that replies to my post said, which fixed most of it.
The final change was that I added i = i + m; below the call for function insert_more_workers in main. Now everything works as it should.

Answer 1

Continuing from my comment, the primary problem you face is that you set the number of workers (i) at the beginning of your program and you use that value to size your niz VLA (Variable Length Array). Once set, it cannot be changed. So when you attempt to add additional workers later using you menu, you attempt to write beyond the end of your array (resulting in Undefined Behavior) causing the problem output you see.
Your alternatives are either to dynamically allocate storage for your workers and keep track of how many you have allocated and how many you have filled, and when filled == allocated you reallocate more storage. Another option is simply to declare some reasonable max number of workers you will have and then declare an array of that many, keeping track of the number added and when the number of workers added equals the array size -- just indicate the array is full. (though you can't expand the number of workers here)
An additional note about using VLAs. Beginning with the C11 standard, compiler support for VLAs was made optional -- though I don't know of any compilers that don't continue to support them.
Weaknesses In Your Code
The most glaring weakness is your failure to check the return of scanf() when a numeric conversion is required. That invites problems. Why? scanf() can fail in two ways (1) a matching-failure where the input provided doesn't match the conversion specified. (e.g. user enters "twenty-one" for age instead of 21). When a matching failure occurs, character extraction from the input buffer ceases at the point of failure -- leaving "twenty-one" unread in the input buffer -- just waiting to bite you on your next attempted input.
In fact if a matching-failure occurs before you enter you menu while(1) loop -- your code locks into an infinite loop.
(2) scanf() can fail with an input-failure where EOF is reached before the first valid conversion takes place. (a user can cancel input by generating a manual EOF with Ctrl + d, or `Ctrl + z on windows) If you don't check the return, you cannot handle either case.
Another weakness with your approach is using scanf() for user-input. Why? Because in the matching-failure case, or after any use, characters are left in stdin unread. In the normal case the '\n' is left unread, and if your next input isn't taken with scanf() and you don't extract the unread character(s), your next input will fail. This is one of the (many) pitfalls associated with using scanf(). Instead, as a general proposition, all user-input should be taken using fgets() and then any conversions should be handled by sscanf() from the buffer filled by fgets(). That way, with a sufficiently sized buffer, you are guaranteed to consume each line of input regardless of whether the subsequent conversion fails. No characters are left unread to impact the next input attempt.
Alternatives Approach
While the two different input functions may be a homework requirement, there is no need for different input functions to read one, and multiple workers. Just write a single function that can handle both. Since your input function can succeed or fail, you need to provide a meaningful return to the caller, so the success or failure of your input function can be validated at the point it was called. void return type is only sufficient for functions that have no bearing on the continued operation of your code (like print functions, etc..).
In your switch() statement, you should handle menu entries that are out-of-range. What if the user slips and enters 5 or -32 or "foo" for that matter? We addressed the matching-failure above, but to handle input that is out-of-range, simply providing a default: case can suffice. For example:
default: fputs (" error: selection out of range.\n", stderr);
break;
Is all that is needed to cover:
Choose a function:
1 - Add a worker using pointers
2 - Add n workers
3 - Display all workers
4 - Leave
5
error: selection out of range.
To make your code more readable, understand that adjacent string-literals are joined together during compilation. This allows you to provide much more readable code. For example, your menu can be written as:
printf ("\nChoose a function:\n"
" 1 - Add a worker using pointers\n"
" 2 - Add n workers\n"
" 3 - Display all workers\n"
" 4 - Leave\n");
Your display_all_workers() function could also benefit from that approach reducing the number of calls to printf() from 3-to-1.
Putting all the suggestions above together, and using a standard array of 200 elements for your workers array, you could rewrite your code as:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAXWRKRS 200 /* if you need a constant, #define one (or more) */
#define MAXC 1024
struct Worker
{
int id;
char name[40];
int age;
};
int insert_workers (struct Worker *workers, int *nworkers, int toadd)
{
char buf[MAXC] = "";
int i = 0, start_index = *nworkers;
size_t len = 0;
while (i < toadd) { /* loop nworkers times */
int current = i + start_index;
if (current == MAXWRKRS) { /* check array bounds */
puts ("(workers array full)");
return i;
}
printf ("\nWhat is name of %d. worker: ", current + 1);
if (!fgets (buf, MAXC, stdin)) { /* read all input into buf */
return i;
}
buf[(len = strcspn (buf, "\n"))] = 0; /* trim '\n', save length */
memcpy (workers[current].name, buf, len + 1);
for (;;) { /* loop continually until valid integer entered */
printf ("What is age of %d. worker: ", current + 1);
if (!fgets (buf, MAXC, stdin)) { /* read all input into buf */
return i;
}
/* parse needed information from buf with sscanf() */
if (sscanf (buf, "%d", &workers[current].age) != 1) { /* check return */
fputs (" error: invalid integer input.\n", stderr);
continue;
}
else {
workers[current].id = current + 1; /* set worker ID, done */
*nworkers += 1; /* all conditions met - update worker count */
break;
}
}
i++;
}
return i; /* return number of workers added (can validate in caller) */
}
void display_all_workers (struct Worker *workers, int nworkers)
{
puts("\nAll workers:");
for (int j = 0; j < nworkers; j++) {
printf ("\n ID : %d\n"
" Name : %s\n"
" Age : %d\n",
workers[j].id, workers[j].name, workers[j].age);
}
}
int main()
{
char buf[MAXC] = ""; /* buffer for all user input */
int i, nwrkrs = 0; /* tmp counter & number of workers */
struct Worker workers[MAXWRKRS]; /* array of workers */
printf("Add a number of workers and then their info: ");
if (!fgets (buf, MAXC, stdin)) {
puts ("(user canceled input)");
return 0;
}
if (sscanf (buf, "%d", &i) != 1) {
fputs ("errur: invalid integer input.\n", stderr);
return 1;
}
printf ("%d workers added\n\n", insert_workers (workers, &nwrkrs, i));
while(1) {
int choice;
printf ("\nChoose a function:\n"
" 1 - Add a worker using pointers\n"
" 2 - Add n workers\n"
" 3 - Display all workers\n"
" 4 - Leave\n");
if (!fgets (buf, MAXC, stdin)) {
puts ("(user canceled input)");
return 0;
}
if (sscanf (buf, "%d", &choice) != 1 ) {
fputs ("error: invalid integer input.\n", stderr);
continue;
}
switch (choice) {
case 1 : insert_workers (workers, &nwrkrs, 1);
break;
case 2 : printf("\nHow many workers do you want to insert? ");
if (!fgets (buf, MAXC, stdin)) {
puts ("(user canceled input)");
return 0;
}
if (sscanf (buf, "%d", &i) != 1) {
fputs ("error: invalid integer input.\n", stderr);
break;
}
insert_workers (workers, &nwrkrs, i);
break;
case 3 : display_all_workers (workers, nwrkrs);
break;
case 4 : return 0;
default: fputs (" error: selection out of range.\n", stderr);
break;
}
}
return 0;
}
(note: how the number of workers nworkers is passed as a pointer to the insert_workers() function so that the number of workers can be updated within the function when a valid name and age has been entered)
One immediate benefit of the use of fgets() for input is you can now enter names containing spaces. Another is that errors in numeric input are easily handled, e.g.
...
Choose a function:
1 - Add a worker using pointers
2 - Add n workers
3 - Display all workers
4 - Leave
1
What is name of 6. worker: Bugs Bunny
What is age of 6. worker: dunno - really
error: invalid integer input.
What is age of 6. worker: 101
...
The error handling shown in the example code isn't intended to be exhaustive, and there is much more that can be added. However, note by using fgets() and checking if the return is NULL (e.g. if (!fgets(...))) you are handling the case where a manual EOF is generated by the user to cancel input.
If you are still stuck with your code, or if you have further questions about the changes above or why one approach has advantages over another, just drop a comment below and I'm happy to help further.
Example Use/Output
Here is a short set of input/output from testing the code:
./bin/workers_fn_new
Add a number of workers and then their info: 2
What is name of 1. worker: Mickey Mouse
What is age of 1. worker: 99
What is name of 2. worker: Minnie Mouse
What is age of 2. worker: 97
2 workers added
Choose a function:
1 - Add a worker using pointers
2 - Add n workers
3 - Display all workers
4 - Leave
3
All workers:
ID : 1
Name : Mickey Mouse
Age : 99
ID : 2
Name : Minnie Mouse
Age : 97
Choose a function:
1 - Add a worker using pointers
2 - Add n workers
3 - Display all workers
4 - Leave
1
What is name of 3. worker: Pluto (the dog)
What is age of 3. worker: 92
Choose a function:
1 - Add a worker using pointers
2 - Add n workers
3 - Display all workers
4 - Leave
3
All workers:
ID : 1
Name : Mickey Mouse
Age : 99
ID : 2
Name : Minnie Mouse
Age : 97
ID : 3
Name : Pluto (the dog)
Age : 92
Choose a function:
1 - Add a worker using pointers
2 - Add n workers
3 - Display all workers
4 - Leave
2
How many workers do you want to insert? 2
What is name of 4. worker: Daffy Duck
What is age of 4. worker: 93
What is name of 5. worker: Daisy Duck
What is age of 5. worker: 91
Choose a function:
1 - Add a worker using pointers
2 - Add n workers
3 - Display all workers
4 - Leave
3
All workers:
ID : 1
Name : Mickey Mouse
Age : 99
ID : 2
Name : Minnie Mouse
Age : 97
ID : 3
Name : Pluto (the dog)
Age : 92
ID : 4
Name : Daffy Duck
Age : 93
ID : 5
Name : Daisy Duck
Age : 91
Choose a function:
1 - Add a worker using pointers
2 - Add n workers
3 - Display all workers
4 - Leave
1
What is name of 6. worker: Bugs Bunny
What is age of 6. worker: dunno - really
error: invalid integer input.
What is age of 6. worker: 101
Choose a function:
1 - Add a worker using pointers
2 - Add n workers
3 - Display all workers
4 - Leave
3
All workers:
ID : 1
Name : Mickey Mouse
Age : 99
ID : 2
Name : Minnie Mouse
Age : 97
ID : 3
Name : Pluto (the dog)
Age : 92
ID : 4
Name : Daffy Duck
Age : 93
ID : 5
Name : Daisy Duck
Age : 91
ID : 6
Name : Bugs Bunny
Age : 101
Choose a function:
1 - Add a worker using pointers
2 - Add n workers
3 - Display all workers
4 - Leave
4