printf scanf执行顺序正在交换

Question

printf("Enter the value\n"); 
scanf("%d ",&data); 

但是scanf在printf之前执行。可能是什么原因？

这是我的整个程序

#include<stdio.h> 
#include<stdlib.h> 
struct Queue{ 
    int front,rear; 
    int capacity; 
    int *array; 
}; 

void enqueue(struct Queue *q){ 

    int data; 
    if(q->front == (q->rear+1)%q->capacity){ 
     printf("Queue is full\n"); 
     return; 
    }else{ 
     printf("Enter the value\n"); 
     scanf("%d ",&data); 
     q->rear = (q->rear+1) % q->capacity; 
     q->array[q->rear] =data; 

     if(q->front == -1) 
      q->front = q->rear; 
    } 

} 
void dequeue(struct Queue *q){ 

    if(q->front == -1){ 
     printf("Queue is Empty\n"); 
     return; 
    }else{ 
     printf("value dequeued %d \n",q->array[q->front]); 
     if(q->front == q->rear) 
      q->front = q->rear = -1; 
     else 
      q->front = (q->front+1)% q->capacity; 
    } 
} 
void printQueue(struct Queue *q){ 

    if(q->front == -1){ 
     printf("Queue is Empty\n"); 
     return; 
    }else{ 
     int f = q->front, r =q->rear; 

     while((f% (q->capacity)) != r) 
     { 
      printf ("\t %d", q->array[f]); 
      f++; 
     } 
    printf("\t %d",q->array[q->rear]); 
    printf("\n"); 
    } 
} 


main(){ 

    struct Queue *q = malloc(sizeof(struct Queue)); 
    if(!q){ 
     printf("Memory Error\n"); 
     exit; 
    } 
    q->capacity = 3; 
    q->front= q->rear = -1; 
    q->array = malloc(q->capacity * sizeof(int)); 
    if(!q->array){ 
     printf("Memory Error\n"); 
     exit; 
    } 

    int option; 
    char ch ='a'; 
    while(1) { 
     printf("Enter the choice 1.Enqueue or 2.Dequeue 3.print\n"); 
     fflush(stdout); 
     scanf("%d",&option); 
     switch(option){ 
     case 1: 
      enqueue(q);     
      break; 
     case 2: 
      dequeue(q); 
      break; 
     case 3: 
      printQueue(q); 
      break; 
     default: 
      printf("Wrong choice\n"); 
     } 


    printf("Do you want to continue? press y to continue\n"); 
    scanf("%s", &ch); 
    if(ch != 'y')break; 
    } 
} 

当我排队等待的输入，将printf语句被执行后，我的元素。 e你想继续吗？ 理想的情况下，当我进入元素排队并按回车应该问我，你想继续，然后它应该等待输入，即“Y”。但是，只要我输入排队元素，它就等待输入'y'，输入'y'后，'你要继续'执行

Answer 1

重做了问题代码。最大的问题是使用scanf（），它在stdin中留下未读值（例如回车符）。未读字符会干扰scanf（）的下一次使用。通过增加%*c的格式字符串改变了scanf（）的吞噬从标准输入多余字符：

#include<stdio.h> 
#include<stdlib.h> 
struct Queue{ 
    int front,rear; 
    int capacity; 
    int *array; 
}; 

void enqueue(struct Queue *q){ 

    int data; 
    if(q->front == (q->rear+1)%q->capacity){ 
     printf("Queue is full\n"); 
     return; 
    }else{ 
     printf("Enter the value\n"); 
     scanf(" %d%*c",&data); 
     q->rear = (q->rear+1) % q->capacity; 
     q->array[q->rear] =data; 

     if(q->front == -1) 
      q->front = q->rear; 
    } 

} 
void dequeue(struct Queue *q){ 

    if(q->front == -1){ 
     printf("Queue is Empty\n"); 
     return; 
    }else{ 
     printf("value dequeued %d \n",q->array[q->front]); 
     if(q->front == q->rear) 
      q->front = q->rear = -1; 
     else 
      q->front = (q->front+1)% q->capacity; 
    } 
} 
void printQueue(struct Queue *q){ 

    if(q->front == -1){ 
     printf("Queue is Empty\n"); 
     return; 
    }else{ 
     int f = q->front, r =q->rear; 

     while((f% (q->capacity)) != r) 
     { 
      printf ("\t %d", q->array[f]); 
      f++; 
     } 
    printf("\t %d",q->array[q->rear]); 
    printf("\n"); 
    } 
} 


int main() 
    { 
    int option; 
    char ch ='a'; 

    struct Queue *q = malloc(sizeof(struct Queue)); 
    if(!q) 
     { 
     printf("Memory Error\n"); 
     goto CLEANUP; 
     } 

    q->capacity = 3; 
    q->front= q->rear = -1; 
    q->array = malloc(q->capacity * sizeof(int)); 
    if(!q->array) 
     { 
     printf("Memory Error\n"); 
     goto CLEANUP; 
     } 

    while(1) 
     { 
     printf("Enter the choice 1.Enqueue or 2.Dequeue 3.print\n"); 
     scanf(" %d%*c",&option); 
     switch(option) 
     { 
     case 1: 
      enqueue(q); 
      break; 

     case 2: 
      dequeue(q); 
      break; 

     case 3: 
      printQueue(q); 
      break; 

     default: 
      printf("Wrong choice\n"); 
      break; 
     } 


    printf("Do you want to continue? press y to continue\n"); 
    scanf(" %c%*c", &ch); 
    if(ch != 'y') 
     break; 
    } 

CLEANUP: 

    return(0); 
    }