Question

我有一个学校作业，我应该创建三个功能。函数是printFirstWord（），skipWords（）和printWord（）。

虽然写得不完美，但我设法使printFirstWord功能正常工作，其他两个功能大部分工作正常。

但是，在skipWords（）中，如果您希望跳过的单词数量大于您输入的字符串中的单词数量，我应该返回指针值NULL。这是我的代码：

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

//Call the functions
void printFirstWord(char inputString[]);
char* skipWords(char sentence[], int words);
void printWord(char sentence[], int wordNumber);

int main()
{
    printWord("I like to eat bunnies", 0); //Prints I
    printWord("I like to eat bunnies", 1); //Prints like
    printWord("I like to eat bunnies", 2); //etc
    printWord("I like to eat bunnies", 3);
    printWord("I like to eat bunnies", 4);
    printWord("I like to eat bunnies", 5); //This should return NULL
    printWord("I like to eat bunnies", 6); //This should return NULL

    return 0;
}

//Function that prints only the first word of a string
void printFirstWord(char inputString[])
{
    int i = 0;

    //Removes initial non-alpha characters, if any are present
    while (!isalpha(inputString[i]))
        i++;

    //Checks if the next input is alphabetical or is the character '-'
    while (inputString[i] != ' ')
    {
        printf("%c", inputString[i]);
        i++;
    }

}

char* skipWords(char sentence[], int words)
{
    int i = 0, wordCount = 0;

    for(i = 0; wordCount < words; i++)
    {
        if(sentence[i] == ' ')
        {
            wordCount++;
        }
    }

    //Can't get this to work, not sure how to return NULL in a function
    if (words >= wordCount)
        return NULL;
    else
        return &sentence[i];
}

void printWord(char sentence[], int wordNumber)
{
    char *sentencePointer;
    sentencePointer = skipWords(sentence, wordNumber);

    if (sentencePointer != NULL)
        printFirstWord(sentencePointer);
    else if (sentencePointer == NULL)
        printf("\nError. Couldn't print the word.\n");
}

最初我的问题是程序不断崩溃，但我添加了printWord函数的最后一部分，它停止了崩溃。我期待这个输出：

Iliketoeatbunnies

Error. Couldn't print the word.

Error. Couldn't print the word.

这是我收到的输出：

Error. Couldn't print the word.

Error. Couldn't print the word.

Error. Couldn't print the word.

Error. Couldn't print the word.

Error. Couldn't print the word.

Error. Couldn't print the word.

指针是我的弱点，我觉得我错过了一些至关重要的东西，我一直在网上看，我还没有找到任何适合我的解决方案，或者至少我认为不适合我。

Answer 1

您的代码中存在很少的错误。更正将是

componentDidMount() {
    this.setState({
        thing: [
            {
                status: "running",
                test: "testing"
            }
        ]
    });
}

render() {
    return (
        <div>
            {this.state.thing.length > 0? <h1 className="mt-5 mb-5">{ this.state.thing[0].status }</h1>: null
}
            <button className="mt-5" onClick={ this.handleUpdate } value="not running">
                Click to change
            </button>
        </div>
    );
}

另一个是

for(i = 0; sentence[i] && wordCount < words; i++)

最后一次

while (inputString[i] !=' ' && inputString[i]!='\0')

第一个的解释是 - 你不会检查超过字符串的结尾。否则你有未定义的行为。

有些情况下，当你到达字符串的末尾但仍然没有获得空格。为避免这种情况，您还需要考虑if (words > wordCount)案例。

如果\0那么只有你应该抛出错误。如果它们相等则应该打印该值。

Answer 2

skipWords中的此循环是问题所在。它将遍历字符串并超越字符串，因为您没有检查字符串的结尾。

for(i = 0; wordCount < words; i++)
{
    if(sentence[i] == ' ')
    {
        wordCount++;
    }
}

由于离开循环的唯一方法是找到与words传入的空格一样多的空格，因此它将始终返回NULL。

可以说wordCount也应该从1开始，因为如果字符串中没有空格，除非字符串为空，否则你总是至少有一个单词。

Answer 3

skipWords()中的算法已损坏，我将通过更改算法并将int更改为size_t来实现您的功能：

#include <stdio.h>
#include <stddef.h>
#include <ctype.h>

int printFirstWord(char const *inputString);
char const *skipWords(char const *sentence, size_t words);
int printWord(char const *sentence, size_t wordNumber);

int main(void) {
  printWord("I like to eat bunnies", 0); // Prints I
  printf("\n");
  printWord("I like to eat bunnies", 1); // Prints like
  printf("\n");
  printWord("I like to eat bunnies", 2); // etc
  printf("\n");
  printWord("I like to eat bunnies", 3);
  printf("\n");
  printWord("I like to eat bunnies", 4);
  printf("\n");
  printWord("I like to eat bunnies", 5); // This should return NULL
  printf("\n");
  printWord("I like to eat bunnies", 6); // This should return NULL
  printf("\n");
}

int printFirstWord(char const *inputString) {
  int ret = 0;
  while (isblank(*inputString)) {
    inputString++;
  }
  while (!isblank(*inputString) && *inputString) {
    int tmp = printf("%c", *inputString++);
    if (tmp < 0) {
      return -1;
    }
    ret += tmp;
  }
  return ret;
}

char const *skipWords(char const *inputString, size_t words) {
  while (isblank(*inputString)) {
    inputString++;
  }
  while (words > 0) {
    words--;
    while (!isblank(*inputString)) {
      if (!*inputString) {
        return NULL;
      }
      inputString++;
    }
    while (isblank(*inputString)) {
      inputString++;
    }
  }
  return *inputString ? inputString : NULL;
}

int printWord(char const *sentence, size_t wordNumber) {
  char const *sentencePointer = skipWords(sentence, wordNumber);
  if (sentencePointer != NULL) {
    return printFirstWord(sentencePointer);
  } else {
    fprintf(stderr, "Error. Couldn't print the word.\n");
    return -1;
  }
}

Answer 4

我们初学者应该互相帮助。:)

你在这里

#include <stdio.h>
#include <ctype.h>

char * skipWords( const char *s, size_t n )
{
    if ( n )
    {
        while ( isblank( ( unsigned char )*s ) ) ++s;

        do
        {
            while ( *s && !isblank( ( unsigned char )*s ) ) ++s;
            while ( isblank( ( unsigned char )*s ) ) ++s;
        } while ( *s && --n );
    }

    return ( char * ) ( *s ? s : NULL );
}

void printFirstWord( const char *s )
{
    while ( isblank( ( unsigned char )*s ) ) ++s;
    while ( *s && !isblank( ( unsigned char )*s ) ) putchar( *s++ );
}

void printWord( const char *s, size_t n )
{
    const char *word = skipWords( s, n );

    if ( word )
    {
        printFirstWord( word );
    }
    else
    {
        printf( "%s", "Error. Could not print the word." );
    }

    putchar( '\n' );
}

int main(void) 
{
    printWord("I like to eat bunnies", 0); //Prints I
    printWord("I like to eat bunnies", 1); //Prints like
    printWord("I like to eat bunnies", 2); //etc
    printWord("I like to eat bunnies", 3);
    printWord("I like to eat bunnies", 4);
    printWord("I like to eat bunnies", 5); //This should return NULL
    printWord("I like to eat bunnies", 6); 

    return 0;
}

程序输出

I
like
to
eat
bunnies
Error. Could not print the word.
Error. Could not print the word.

至于你的代码，你通常不会检查终止零，例如

while (inputString[i] != ' ')
{
    printf("%c", inputString[i]);
    i++;
}

并忽略几个空格相互跟随的情况。

将NULL值从函数返回到C中的指针

4 个答案: