I'm trying to read from stdin the commands for a stack data structure, the valid command are "push [number]" and "pop", here's my code:
while(getline(&input, &len, stdin) > 0){
while ((n = sscanf(input,"%64s%d%n",cmd,&num,&offset)) > 0){
if (n == 1){
if (!strcmp("pop",cmd)){
pop(&head);
} else if (!strcmp("push",cmd)){
//Doing something
} else if (isNumeric(cmd) && need_num){
push(&head, num);
} else{
//error
}
}
else if (n == 2){
if (!strcmp("push",cmd)){
push(&head, num);
} else {
//error
}
}
else {
//error
}
input += offset;
}
}
This Program has many flaws, of course, since I'm not familiar with sscanf with loops. The first problem is that if I read in a series of commands in a line such as:
push 1 push 2 pop push 3
It will actually cause error, since there's number following push but no number after pop, in this way, this line of code is wrong for "pop" command:
input += offset;
But I don't know how to fix that. The other problem is that if I suppose split a "push [number]" command into 2 lines is acceptable:
push 5 pop push
4
I don't know if there's any easier way other than what I did to decide whether the line end up with push and the following line start with a number:
else if (!strcmp("push",cmd)){
//Doing something
} else if (isNumeric(cmd) && need_num){
push(&head, num);
}
Any help would be appreciated!
Well there were not any glaring errors in your use, but there were a number of subtle errors that caused you problems.
The primary problem you were faced with was taking only a single offset after both the string and integer were converted. In the case of "pop", a matching failure occurs with %d before you ever reach %n in your format string causing n to remain unset (retaining the last set value -- causing your increment of offset to be too large in that case)
Instead, you need to employ two such checks in your format string, e.g.:
while ((rtn = sscanf (input, "%63s %n%d %n",
cmd, &off1, &num, &off2)) > 0) {
That way if "pop" is read, you have a proper offset in off1 and if "push num" is read, you have the proper offset in off2. (the additional whitespace before each %n is optional (but suggested). With both %s and %d leading whitespace is consumed -- but best to get in the habit of accounting for it, as %[...] or %c do not consume leading whitepace.
Putting it together in a short example, you could do something similar to the following:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAXCMD 64 /* max characters for command */
int main (void) {
char *buf = NULL; /* buffer for getline */
size_t n = 0; /* alloc size (0 getline decides) */
ssize_t nchr = 0; /* getline returns (no. of chars read */
while ((nchr = getline (&buf, &n, stdin)) > 0) { /* read each line */
char cmd[MAXCMD] = "", /* buffer for command */
*input = buf; /* pointer to advance */
int num = 0, /* number to push */
off1 = 0, /* offset if single conversion */
off2 = 0, /* offset if double conversion */
rtn = 0; /* sscanf return */
while ((rtn = sscanf (input, "%63s %n%d %n",
cmd, &off1, &num, &off2)) > 0) {
switch (rtn) { /* switch on sscanf return */
case 1: /* handle "pop" case */
if (strcmp (cmd, "pop") == 0) {
printf ("pop\n");
input += off1; /* set offset based on off1 */
}
else
fprintf (stderr, "error: invalid single cmd '%s'.\n",
cmd);
break;
case 2: /* handle "push num" case */
if (strcmp (cmd, "push") == 0) {
printf ("push %d\n", num);
input += off2; /* set offset based on off2 */
}
else
fprintf (stderr, "error: invalid single cmd '%s'.\n",
cmd);
break;
default:
fprintf (stderr, "error: invalid input.\n");
break;
}
}
}
free (buf); /* free memory allocated by getline */
return 0;
}
(note: I have used a switch() statement, but you are free to use if, else if, else if you prefer)
Example Use/Output
Validation cases:
$ echo "pop pop push 1 push 2 pop push 3 pop" | ./bin/sscanfpushpop
pop
pop
push 1
push 2
pop
push 3
pop
$ echo "push 5 pop push 6 push 7 pop push 8" | ./bin/sscanfpushpop
push 5
pop
push 6
push 7
pop
push 8
Look things over and let me know if you have any further questions.