C、数组中的随机元素变成'?'打印到终端时的字符

Question

我正在尝试洗牌一系列卡片结构并将它们打印到屏幕上。洗牌工作正常，但是在将数组打印到屏幕时，恰好一个对象显示为“？”字符。每次都是不同的对象。 没有改组，这是输出：

$./shuffle
2H 3H 4H 5H 6H 7H 8H 9H TH JH QH KH AH 
2C 3C 4C 5C 6C 7C 8C 9C TC JC QC KC AC 
2D 3D 4D 5D 6D 7D 8D 9D TD JD QD KD AD 
2S 3S 4S 5S 6S 7S 8S 9S TS JS QS KS AS

以下是我使用 shuffle 运行程序时终端的一些输出：

$./shuffle
6S 8C 7H 9H 5S 7S 4H 9C 9D AH 4S TD 4C 
JC 8D KH 5D JD 7D 7C 3D 2S 4D QH 3C TH 
AC 2H 3H 8H AS KD QS QD KC 3S 6D 9S 6H 
JH ? 5C TC QC 8S JS 6C 2D KS TS AD 2C
$./shuffle
QH TC 5H AD 3C QS 4S TH 9S 8S 5D 6C JC 
KH JH AS KS JD 7S AC 7H 6H 6D 3S 2C 3D 
5C ? 7D JS 4C QC 9C 3H 8C 2D TS 2S QD 
5S KC 4H 8D 8H KD AH TD 9D 7C 6S 2H 9H

编辑：问题在于尝试访问数组的第 -1 个元素会导致 C 访问数组外的数据。谢谢！

这是我的主要代码：

    /* Simulate a deck of cards (array of Card structures) and shuffle 
    the deck
         * Then the shuffled deck is printed to the terminal window. */
#include <stdio.h>
#include <math.h>
#include <stdlib.h>
#include <time.h>
#include <string.h>
#define DECK_SIZE (52) //52 cards in a deck

struct card { //define the structure of a card
  char suit[2]; //the suit on the card
  char value[2]; //the value of the card
};
typedef struct card Card; //define a Card type structure

Card deck[DECK_SIZE]; //a deck of cards is an array of 52 Card structures

#include "myheader.h" //myheader includes the shuffle() function for arrays of Cards

int main() {

  char suits[5] = "HCDS"; //the 4 suits
  char values[14] = "23456789TJQKA"; //the card values (T is 10)

  int j,k; //counting variables
  //fill the deck with the cards 
  for (j=0; j < 4; j++) {  //For each suit...
    for(k=0; k < 13; k++) { //and for each value...
      //Give the next card in the list that suit and value.
      deck[ ( j * 13 ) + ( k ) ].value[ 0 ] = values[ k ]; //assign the card's value
      deck[ ( j * 13 ) + ( k ) ].suit[ 0 ] = suits[ j ]; //assign the card's suit
    }
  }

  //Shuffle the deck (implementing Fisher-Yates Shuffle)
  int length = sizeof(deck)/sizeof(deck[0]);
  //shuffle function is found in "myheader.h"
  shuffle( &deck[0], length ); //pass a pointer to the first position of the deck

  //Print the deck to the screen (assuming 52 card deck)
  for (j=0; j < 4; j++) { //for each suit...
    for (k=0; k < 13; k++) { //for each value...
      //print the card
      printf("%s%s ", deck[ ( j * 13 ) + ( k ) ].value, deck[ ( j * 13 ) + ( k )].suit); 
    }
    //print a new line so the rows are organized into 4 rows of 13 cards each.
    printf("\n");
  }
}
//End of program.

这是我定义随机播放函数的标题：

//Requires math.h
//Requires stdlib.h
//Requires time.h

void shuffle(Card deck[], int length) { //takes an pointer to the array of type Card
  //This shuffle function implements the Fisher-Yates shuffle algorithm.
  printf("Length is: %i\n",length);
  int element;
  Card temp; //a place to temporarily store cards while shuffling them.
  srand(time(NULL)); //seed the random number generator with the current time.

  while (length) {
    //While there are things to shuffle
    //Pick a remaining element from the un-randomized elements
    element = rand() % (length); 
    length--; //decrease the "length" of the part of the array that is still unshuffled

    //Swap the element with the last element in the list.
    temp = deck[length]; //temporarily store the end object in the last slot
    deck[length] = deck[element-1];
    //place the selected element in the 'vacated' last slot
    deck[element-1] = temp;
    //put the temporarily stored object in place of the element 
  }
}
//There is a bug where one of the elements in the array gets changed to a ?

我怀疑存储在 shuffle 函数中的临时结构中的结构之一在通过编辑时会变得混乱，但我不确定为什么只有一个对象会变得混乱，因为大概是每个在随机播放循环期间，数组中的元素在某一时刻被放入 temp 中。我是 C 的初学者，我知道足够多的东西来写东西，但对实际发生的事情还不够了解，无法开始调试这样的奇怪行为。如果有人知道这个“？”在哪里？字符来自或为什么将其插入数组中，这将非常有帮助。谢谢！

Answer 1

element = rand() % (length);

给你一个介于 0 和 length-1 之间的值。

deck[length] = deck[element-1];

当元素为0时，可以访问数组外部