C 中 fscanf textfile.txt 的变量定义

Question

我有一个 textfile.txt 逐行包含变量定义，例如长度 = 1.1m

使用fscanf 我想在我的变量double length 中存储值1.1

谁能给我一些提示，在这种情况下如何使用 fscanf 有一个 '=' 分隔符？

提前致谢！

编辑： 下面是一个示例 inputfile.txt，显示了它的结构：

[elements]
Number of elements = 2
length_1 = 1.5m
length_2 = 2.1m
velocity_1 = 0.35m/s
velocity_2 = 0.11m/s

[cells]
Number of cells = 2
cell_1 = 0.0m
cell_2 = 1.3m

我想将两个部分的参数提取到两个不同的结构中。

Answer 1

一个读取格式化文件的简单程序：

#include <stdio.h>

int main(void) {
    FILE *fp = fopen("data.txt", "r");
    float length;

    if (fp == NULL) {
        printf("The file was failed to open.\n");
        return -1;
    }

    while (fscanf(fp, "%*s %*s %f", &length) == 1)
        printf("%f", length);

    return 0;
}

注意我们已经使用了

%*s %*s %f

format %*s string 跳过该单词的阅读。

文件data.txt 看起来像：

length = 5.324325m

然后你会得到一个输出：

5.324325

您现在可以清楚地看到字符串被截断并且值已成功分配给变量。

Answer 2

这里有一个办法，两个部分可以任意顺序给出，也可以只给出一个，甚至不给出：

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

typedef struct Element {
  double length;
  double velocity;
} Element;

void * read_section(FILE * fp, const char * what, int * n, size_t sz)
{
  if (*n != 0) {
    fprintf(stderr, "invalid file, several sections [%s]\n", what);
    exit(-1);
  }

  char fmt[64];
  
  sprintf(fmt, " Number of %s = %%d", what); /* notice space at beginning */

  if (fscanf(fp, fmt, n) != 1) {
    fprintf(stderr, "invalid file, expected 'Number of %s = <n>'\n", what);
    exit(-1);
  }
  
  if (*n <= 0) {
    fprintf(stderr, "number of %s must be > 0\n", what);
    exit(-1);
  }
  
  void * r = malloc(*n * sz);
  
  if (r == NULL) {
    fprintf(stderr, "not enough memory for %d %s\n", *n, what);
    exit(-1);
  }
  
  return r;
}

void read_var(FILE * fp, const char * var, const char * unity, int rank, double * v)
{
  char fmt[64];
  
  sprintf(fmt, " %s_%d = %%lg%s", var, rank, unity); /* notice space at beginning */
  
  if (fscanf(fp, fmt, v) != 1) {
    fprintf(stderr, "invalid file, expected '%s_%d = <val>%s'\n", var, rank, unity);
    exit(-1);
  }
}

int main(int argc, char ** argv)
{
  if (argc != 2) {
    fprintf(stderr, "Usage: %s <file>\n", *argv);
    exit(-1);
  }
  
  FILE * fp = fopen(argv[1], "r");
  
  if (fp == NULL) {
    perror("cannot open file");
    exit(-1);
  }
  
  int nelts = 0;
  Element * elts = NULL;
  int ncells = 0;
  double * cells = NULL;
  char line[32];
  int i;
  
  while (fscanf(fp, " %31s", line) == 1) { /* notice space at beginning of format */
    if (!strcmp(line, "[elements]")) {
      elts = read_section(fp, "elements", &nelts, sizeof(*elts));
      for (i = 0; i != nelts; ++i)
        read_var(fp, "length", "m", i+1, &elts[i].length);
      for (i = 0; i != nelts; ++i)
        read_var(fp, "velocity", "m/s", i+1, &elts[i].velocity);
    }
    else if  (!strcmp(line, "[cells]")) {
      cells = read_section(fp, "cells", &ncells, sizeof(*cells));
      for (i = 0; i != ncells; ++i)
        read_var(fp, "cell", "m", i+1, &cells[i]);
    }
    else {
      fputs("invalid file, section header expected\n", stderr);
      exit(-1);
    }
  }
  
  fclose(fp);
  
  /* to check */
  
  printf("%d elements:\n", nelts);
  for (i = 0; i != nelts; ++i)
    printf("%d) length=%g, velocity=%g\n", i, elts[i].length, elts[i].velocity);
  
  printf("\n%d cells:", ncells);
  for (i = 0; i != ncells; ++i)
    printf(" %g", cells[i]);
  putchar('\n');
  
  free(elts);
  free(cells);
  
  return 0;
}

当需要绕过换行符时，请注意格式开头的空格（以及可能的其他空格）。

用你的例子编译和执行：

pi@raspberrypi:/tmp $ gcc -g -Wall x.c
pi@raspberrypi:/tmp $ 
pi@raspberrypi:/tmp $ cat f1
[elements]
Number of elements = 2
length_1 = 1.5m
length_2 = 2.1m
velocity_1 = 0.35m/s
velocity_2 = 0.11m/s

[cells]
Number of cells = 2
cell_1 = 0.0m
cell_2 = 1.3m
pi@raspberrypi:/tmp $ ./a.out f1
2 elements:
0) length=1.5, velocity=0.35
1) length=2.1, velocity=0.11

2 cells: 0 1.3
pi@raspberrypi:/tmp $ 

交换部分顺序

pi@raspberrypi:/tmp $ cat f2
[cells]
Number of cells = 2
cell_1 = 0.0m
cell_2 = 1.3m
[elements]
Number of elements = 2
length_1 = 1.5m
length_2 = 2.1m
velocity_1 = 0.35m/s
velocity_2 = 0.11m/s
pi@raspberrypi:/tmp $ ./a.out f2
2 elements:
0) length=1.5, velocity=0.35
1) length=2.1, velocity=0.11

2 cells: 0 1.3
pi@raspberrypi:/tmp $ 

只有元素

pi@raspberrypi:/tmp $ cat f3
[elements]
Number of elements = 2
length_1 = 1.5m
length_2 = 2.1m
velocity_1 = 0.35m/s
velocity_2 = 0.11m/s

pi@raspberrypi:/tmp $ ./a.out f3
2 elements:
0) length=1.5, velocity=0.35
1) length=2.1, velocity=0.11

0 cells:
pi@raspberrypi:/tmp $ 

只有单元格

pi@raspberrypi:/tmp $ cat f4
[cells]
Number of cells = 2
cell_1 = 0.0m
cell_2 = 1.3m
pi@raspberrypi:/tmp $ ./a.out f4
0 elements:

2 cells: 0 1.3
pi@raspberrypi:/tmp $ 

空输入文件

pi@raspberrypi:/tmp $ ./a.out /dev/null
0 elements:

0 cells:
pi@raspberrypi:/tmp $ 

更多元素和单元格

pi@raspberrypi:/tmp $ cat f
[elements]
Number of elements = 3
length_1 = 1.5m
length_2 = 2.1m
length_3 = 1.1m
velocity_1 = 0.35m/s
velocity_2 = 0.11m/s
velocity_3 = 0.33m/s

[cells]
Number of cells = 5
cell_1 = 0.0m
cell_2 = 1.3m
cell_3 = 2.3m
cell_4 = 3.3m
cell_5 = 4.3m
pi@raspberrypi:/tmp $ ./a.out f
3 elements:
0) length=1.5, velocity=0.35
1) length=2.1, velocity=0.11
2) length=1.1, velocity=0.33

5 cells: 0 1.3 2.3 3.3 4.3
pi@raspberrypi:/tmp $ 

并且检查了可能的错误情况，我鼓励您永远不要假设一切都正常，但要检查它

如果您在 Linux/Unix 下完成，请检查您在 valgrind 下运行的程序，例如：

pi@raspberrypi:/tmp $ valgrind ./a.out f
==13981== Memcheck, a memory error detector
==13981== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==13981== Using Valgrind-3.15.0 and LibVEX; rerun with -h for copyright info
==13981== Command: ./a.out f
==13981== 
3 elements:
0) length=1.5, velocity=0.35
1) length=2.1, velocity=0.11
2) length=1.1, velocity=0.33

5 cells: 0 1.3 2.3 3.3 4.3
==13981== 
==13981== HEAP SUMMARY:
==13981==     in use at exit: 0 bytes in 0 blocks
==13981==   total heap usage: 5 allocs, 5 frees, 5,560 bytes allocated
==13981== 
==13981== All heap blocks were freed -- no leaks are possible
==13981== 
==13981== For lists of detected and suppressed errors, rerun with: -s
==13981== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)
pi@raspberrypi:/tmp $ 

Answer 3

假设你的文本文件是这样的：

length = 1.2m
length = 3.4m
length = 12.4m

然后你就可以用这段代码来阅读了：

FILE* filePtr = fopen("textfile.txt","r"); 
if (filePtr==NULL) 
{ 
    printf("no such file."); 
    return 0; 
} 
  
float num; 
while (fscanf(filePtr,"length = %f", &num)==1) 
    printf("%f\n", num);
fclose(filePtr);

有关从文件读取的更多信息，请查看link