问题scanf函数和与fgets [英] Problem with scanf and fgets

问题描述

这是一个家庭作业的一些特定字符串进行排序。我提示字符串数量的用户，他们希望与 scanf函数来排序，根据这个数字分配一个数组，然后获取字符串本身与与fgets 。

This is for a homework assignment to sort some given strings. I'm prompting the user for the number of strings they'd like to sort with scanf, allocating an array based on that number, and then getting the strings themselves with fgets.

一切工作正常，如果字符串的数量是很难codeD，但 scanf函数的除了让用户决定螺丝的东西了。这里的code：

Everything works fine if the number of strings is hardcoded, but the addition of scanf to let the user decide screws things up. Here's the code:

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

#define LENGTH  20 // Maximum string length.

int main(void)
{
    int index, numStrings = 0;
    char **stringArray;
    printf("Input the number of strings that you'd like to sort: ");
    assert(scanf("%d", &numStrings) == 1);
    stringArray = (char **)malloc(numStrings * sizeof(char *));

    for (index = 0; index < numStrings; index++)
    {
        stringArray[index] = (char *)malloc(LENGTH * sizeof(char));
        assert(stringArray[index] != NULL);
        printf("Input string: ");
        assert(fgets(stringArray[index], LENGTH, stdin) != NULL);
    }

    // Sort strings, free allocated memory.

    return 0;
}

和这里的控制台的样子：

And here's what the console looks like:

Input the number of strings that you'd like to sort: 3
Input string: Input string: foo
Input string: bar

有跳过循环的第一次迭代，从而在所述阵列的开头空字符串。我的问题是，为什么会这样，我怎么能解决这个问题？

It skips over the first iteration of the loop, resulting in an empty string at the beginning of the array. My question is, why does it do that, and how can I fix it?

下面就是控制台看上去与格式字符串％d个\\ N传递给 scanf函数：

Here's what the console looks with the format string "%d\n" passed to scanf:

Input the number of strings that you'd like to sort: 3
foo
Input string: Input string: bar
Input string: baz

所以，我可以输入所有的字符串，而是一个字符串的第一个提示是在错误的地方。

So, I can input all of the strings, but the first prompt for a string is in the wrong place.

推荐答案

真正的答案（愚见，但不断所谓正确的舆论：P）是不使用 scanf函数。使用与fgets 来读的第一行（即号码），然后自己分析该字符串，比如的sscanf 或 strtoul将。这样，你有当有人确实在一个不错的格式不输入数据处理错误的能力，你不必砍周围 scanf函数的缺乏强劲空白的处理。

The real answer (in my humble but ever-so-correct opinion :P) is not to use scanf. Use fgets to read the first line (i.e. the number) and then parse that string yourself with, say, sscanf or strtoul. That way you have the ability to handle errors when someone doesn't input the data in a nice format, and you don't have to hack around scanf's lack of robust whitespace handling.

另外，除非你希望有很多的阵列与-4长度从未使用过的 INT 存储大小。该标准规定了无符号类型为size_t 为无符号类型，它大到足以存储对象的大小和数组索引。使用任何其他类型并不保证工作。

Also, never used an int to store sizes unless you expect to have a lot of arrays with -4 length. The standard specifies the unsigned type size_t as an unsigned type that is large enough to store object sizes and array indices. Using any other type isn't guaranteed to work.

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