如何在c中浮点数后面紧跟字母'e'的floatf进行扫描? [英] How to scanf a float followed immediately by the letter 'e' in c?

问题描述

我正在尝试使用 fscanf 读取数据，部分输入是浮点数，后跟字母'e'，例如 41.72过去.在为 fscanf 编写strng时，我尝试使用％felapsed" ，但这是行不通的，因为％fe 是它自己的格式说明符.我将如何使用 fscanf 来阅读此内容?

I'm trying to use fscanf to read in data, and part of the input is a float followed by the letter 'e', for example, 41.72elapsed. When writing the strng for fscanf, I attempted to use "%felapsed", but this doesn't work, as %fe is its own format specifier. How would I read this in using fscanf?

这是代码:

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

#define CHAR_MAX 1024

int main(int argc, char **argv)
{
    FILE *file_in = fopen(argv[1], "r+");
    char out_name[CHAR_MAX];
    strcpy(out_name, argv[1]);
    strcat(out_name, ".csv");
    FILE *csv_out = fopen(out_name, "w");
    int minutes;
    float seconds;
    fprintf(csv_out, "Trial #, Execution Time\n");

    for (int i = 0; fscanf(file_in, "%*fuser %*fsystem %d:%felapsed %*d%%CPU (%*davgtest+%*davgdata %*dmaxresident)k\n%*dinputs+%*doutputs (%*dmajor+%*dminor)pagefaults %*dswaps\n", &minutes, &seconds) == 2; i++) {
         fprintf(csv_out, "%d, %d:%.2f\n", i, minutes, seconds);
     };
    return 0;
}

以下是一些示例输入:

283.97user 0.69system 1:13.77elapsed 385%CPU (0avgtext+0avgdata 107472maxresident)k

0inputs+4616outputs (0major+9550minor)pagefaults 0swaps

287.87user 0.35system 1:14.41elapsed 387%CPU (0avgtext+0avgdata 107328maxresident)k

0inputs+4616outputs (0major+9524minor)pagefaults 0swaps

推荐答案

这是 scanf()

FP格式将 e 引入了幂运算.由于 e 后没有数字，因此将停止扫描 float .但是 scanf()都已经准备好在 e 之后进行一次扫描，并且C不需要 scanf()可以备份更多内容.1个字符.因此，使用简单的％f" 运气不好.

FP formats like "%f" see the e as introducing exponentiation. Since the e is not followed by a number, scanning for the float stops. But scanf() has all ready scanned one past the e and C does not require for scanf() to be able to back up more than 1 character. So code is out-of-luck using a simple "%f".

某些系统将备份1个以上的字符，但是C不需要该功能.

Some systems will back up more than 1 character, but C does not require that capability.

代码需要一种新方法-以秒为单位扫描字符串

Code needs a new approach - scan in seconds as a string

char sec[20];
int cnt = fscanf(file_in, "%d:%19[0-9. ]elapsed", &minutes, sec);
if (cnt == 2) {
  seconds = atof(sec); 
  ...
}

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