用scanf（）从括号中读取逗号分隔的值

Question

I have the following input:

((1828,299),(2729,2553),(2797,2929),(2200,1383),(2894,876))

and following struct:

struct x{
int a;
int b;
}

How can I read the input with scanf() to create an array of my structs?

I have tried

scanf("%[^, ()],%d", &arr);
    if (i % 2 == 0){
        arr[i].x = scanf("%d");
    }else
        arr[i].y = scanf("%d");

But when I tried to print these values out I got some strange characters

Answer 1

Incorrect use of scanf()

1. scanf("%[^, ()],%d", ...) expects to scan and form a string ( "%[^, ()]" ) and an int ( "%d" ). Code only provided for a place to save the int .

2. Code did not check the return value form scanf() , so code if not aware of any scanning issues.

When data is a line , recommend to use fgets() , then parse it. Could use strtok() strtol() sscanf() . Various pros/cons to each. Example:

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

#define MAXN (5)
// maximum size needed to print and `int`
#define INTSIZE (sizeof (int )* CHAR_BIT  / 3 + 3)
// Expect buffer size needed
#define EXPECTEDSIZE (1+(2*INTSIZE + 4) + 3)

void fooo() {
  int i;

  struct x {
    int a;
    int b;
  } xx[MAXN];
  char buf[EXPECTEDSIZE * 2]; // I favor 2x size buffers
  while (fgets(buf, sizeof buf, stdin)) {
    char *p = buf;
    if (*p++ != '(') {
      exit(EXIT_FAILURE);
    }
    for (i = 0; i < MAXN; i++) {
      int n; // use %n to locate scan completion
      int cnt = sscanf(p, " (%d ,%d ) %n", &xx[i].a, &xx[i].b, &n);
      fprintf(stderr, "cnt = %d '%s'\n", cnt, p);
      if (cnt != 2) {
        exit(EXIT_FAILURE);
      }
      p += n;
      if (p[0] != ',') {
        if (p[0] == ')') {
          i++;
          break; // Successfully reached the end
        }
        exit(EXIT_FAILURE);
      }
      p++;
    }
    int j;
    for (j=0; j< i; j++) {
      printf("%d  (%d ,%d )\n", j, xx[j].a, xx[j].b);
    }
  }
}

Answer 2

The reason you got strange characters is that scanf() does not return the scanned value, it returns the number of items that matched the format.

You are invoking scanf() in a way that invokes undefined behavior. It expects a pointer to an integer as a parameter when you use the "%d" specifier, so the correct way would be

if (scanf("%d", &arr[i].x) == 1)
    /* succesful */
else
    /* error */

The *scanf() family of functions, are not capable of matching the pattern you want, either use a regular expressions library, or parse the string by splitting the (value, value) and then their contents separately.

One way that comes to my mind is to use strtok() with ")," but that wouldn't count the last element and would easily fail if there is a white space between the ")" and "," so, a state machine and parsing the string one character at a time might be the best way to do it.

This is an example of what I mean, I enjoyed writing this

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

struct point
 {
    int x;
    int y;
 };

struct stack
 {
    char  *top;
    char **stack;
    size_t count;
    size_t size;
 };

void
stackinit(struct stack *stack)
 {
    if (stack == NULL)
        return;
    stack->top   = NULL;
    stack->stack = NULL;
    stack->count = 0;
    stack->size  = 0;
 }

void
stackfinish(struct stack *stack)
 {
    if (stack == NULL)
        return;
    free(stack->stack);

    stack->stack = NULL;
 }

char *
stacktop(struct stack *stack)
 {
    if ((stack == NULL) || (stack->count == 0))
        return NULL;
    return stack->stack[stack->count - 1];
 }

void
stackpush(struct stack *stack, char *value)
 {
    void *pointer;
    if (stack == NULL)
        return;
    if (stack->size == stack->count)
     {
        pointer = realloc(stack->stack, (stack->size + 100) * sizeof(char *));
        if (pointer == NULL)
            return;
        stack->stack = pointer;
        stack->size += 100;
     }

    if (stack->stack == NULL)
        return;

    stack->stack[stack->count] = value;
    stack->count              += 1;
 }

void
stackpop(struct stack *stack)
 {
    if ((stack == NULL) || (stack->count <= 0))
        return;
    stack->count              -= 1;
    stack->stack[stack->count] = NULL;
 }

void
extractpoint(char *string, struct point **points, size_t *count)
 {
    struct point point;
    void        *pointer;
    char        *tail;
    if ((string == NULL) || (points == NULL) || (count == NULL))
        return;
    tail = strchr(string, ')');
    if (tail == NULL)
        return;
    if (sscanf(string, "%d,%d", &point.x, &point.y) != 2)
        return;
    pointer = realloc(*points, (1 + count[0]) * sizeof(*points));
    if (pointer == NULL)
        return;
    points[0]             = pointer;
    points[0][count[0]++] = point;
 }

void
parse(char *input, struct point **points, size_t *count)
 {
    struct stack stack;

    stackinit(&stack);
    while (*(input++) != '\0')
     {
        char *top;
        switch (*input)
         {
        case '(':
            stackpush(&stack, input + 1);
            break;
        case ')':
            stackpop(&stack);
            break;
        case ',':
            top = stacktop(&stack);
            if (top == NULL)
                continue;
            extractpoint(top, points, count);
            break;
        default:
            break;
         }
     }
    stackfinish(&stack);

    return;
 }

int
main(void)
 {
    char          input[] = "((1828,299),((2729,2553),(2797,2929),(2200,1383),(2894,876))";
    size_t        count   = 0;
    struct point *points  = NULL;
    size_t        index   = 0;

    parse(input, &points, &count);
    for (index = 0 ; index < count ; ++index)
        fprintf(stdout, "%zu: %d, %d\n", index, points[index].x, points[index].y);
    free(points);

    return 0;
 }

Answer 3

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

struct x{
    int a;
    int b;
};

int count(const char *s){
    if(s == NULL ||  *s != '(')
        return 0;//bad
    int n, a, b, count = 0;
    for(;;){
        n = -1;
        if(2!=sscanf(++s, "(%d,%d)%n", &a, &b, &n) || n < 0){
            return 0;
        } else {
            s += n;
            ++count;
            if(*s == ',')
                continue;
            else if(*s == ')')
                return count;
            else
                return 0;
        }
    }
}
void set(struct x *a, const char *s){
    int n, c = 0;
    while(2==sscanf(++s, "(%d,%d)%n", &a[c].a, &a[c].b, &n)){
        s += n;
        ++c;
    }
}

int main(void) {
    char *input = "((1828,299),(2729,2553),(2797,2929),(2200,1383),(2894,876))";
    int i, n = count(input);
    if(n == 0){
        printf("invalid format!\n");
        exit(EXIT_FAILURE);
    }
    struct x arr[n];
    set(arr, input);
    for(i = 0; i < n; ++i){
        printf("(%4d, %4d)\n", arr[i].a, arr[i].b);
    }

    return 0;;
}