c - 结构指针运算符

Question

代码的目的是创建一个指向学生结构的指针数组，以便在其他函数中使用该指针数组。我不确定如何在二元函数中使用箭头运算符。它不会返回找到 id 的索引值。

typedef struct{
    int IDno;
    char name[20];
    int project;
    int exam;
    double final;
} student;

student **create_class_list(char*filename, int *sizePtr);
void print_list(student**list,int *sizePtr);
int find_binsrch(int idNo, student **list, int size,int low, int high);

int main(void){

    int i, n; 
    student **listPtr;
    listPtr = create_class_list("student.txt", &n);
    print_list(listPtr,&n); 
    index2 = find_binsrch(searchID, listPtr, n, 1200, 4580);     
}

student **create_class_list(char *filename, int *sizeptr){

    int n,i;
    FILE *fptr; 
    fptr=fopen(filename,"r");
    if(fptr==NULL)
        printf("The file could not be opened.\n");
    else
        fscanf(fptr, "%d",sizeptr);

    n=*sizeptr;

    student **list;
    list = (student**)calloc(1, sizeof(student*));


    for(i=0;i<n;i++){
        list[i]=(student*)calloc(n,sizeof(student));
        fscanf(fptr,"%d %[^\n]s", &(list[i]->IDno),(list[i]->name));
    }

    return list;
}

void print_list(student**list,int *sizePtr){
    int i; 
    for(i=0; i<*sizePtr; i++){
        printf("%d %s\n",&(list[i]->IDno),(list[i]->name));
    }
}

int find_binsrch(int idNo, student **list, int size, int low, int high){
    int middle, i;

    while(low<=high){
        middle =(low+high)/2;

        printf("%d\n", middle); 

        if(idNo==list[middle]->IDno)
            return list[i]->IDno;

        if(idNo<list[middle]->IDno)
            high = middle -1;
        else
            low = middle +1;
    return -1;    
    }
}

Answer 1

您必须学会做的是每次编译时启用警告。这使得编译器能够识别代码中需要注意的许多区域。您不应该接受编译时带有警告的代码。只有在非常非常罕见的情况下，才可以接受编译时带有警告的代码(您在编程的第一年可能不会遇到这种情况)，因此请始终启用 -Wall -Wextra 作为一部分你的编译字符串。 (您还可以启用 -pedantic 来查看其他警告以及一些特定的警告请求，但对于一般用途，-Wall -Wextra 即可)

如果您编译时带有警告，您会看到:

students3.c: In function ‘main’:
students3.c:23:5: error: ‘index2’ undeclared (first use in this function)
    index2 = find_binsrch(searchID, listPtr, n, 1200, 4580);
    ^
students3.c:23:5: note: each undeclared identifier is reported only once for each function it appears in
students3.c:23:27: error: ‘searchID’ undeclared (first use in this function)
    index2 = find_binsrch(searchID, listPtr, n, 1200, 4580);
                        ^
students3.c:19:9: warning: unused variable ‘i’ [-Wunused-variable]
    int i, n;
        ^
students3.c: In function ‘print_list’:
students3.c:53:9: warning: format ‘%d’ expects argument of type ‘int’, but argument 2 has type ‘int *’ [-Wformat=]
        printf("%d %s\n",&(list[i]->IDno),(list[i]->name));
        ^
students3.c: In function ‘find_binsrch’:
students3.c:57:48: warning: unused parameter ‘size’ [-Wunused-parameter]
int find_binsrch(int idNo, student **list, int size, int low, int high){
                                                ^
students3.c: In function ‘main’:
students3.c:24:1: warning: control reaches end of non-void function [-Wreturn-type]
}
^
students3.c: In function ‘find_binsrch’:
students3.c:74:1: warning: control reaches end of non-void function [-Wreturn-type]
}
<snip>

简单地解决警告/错误并重新编译(以及解决修复第一个列表中披露的新警告/错误)将允许您系统地更正您的代码。采取这些基本步骤将允许您更正代码，使其可以在没有警告的情况下进行编译:

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

typedef struct{
    int IDno;
    char name[20];
    int project;
    int exam;
    double final;
} student;

student **create_class_list(char*filename, int *sizePtr);
void print_list(student**list,int *sizePtr);
int find_binsrch(int idNo, student **list, int size,int low, int high);

int main(void){

    int n, index2, searchID = 2; 
    student **listPtr = NULL;
    listPtr = create_class_list("student.txt", &n);
    if (!listPtr) {
        fprintf (stderr, "error: create_class_list failed.\n");
        return 1;
    }
    print_list(listPtr,&n); 
    index2 = find_binsrch(searchID, listPtr, n, 1200, 4580);

    if (index2) {} /* stub to eliminate unused warning */

    return 0;
}

student **create_class_list(char *filename, int *sizeptr){

    int n,i;
    FILE *fptr; 
    fptr=fopen(filename,"r");
    if(fptr==NULL)
        printf("The file could not be opened.\n");
    else
        fscanf(fptr, "%d",sizeptr);

    n=*sizeptr;

    student **list;
    list = (student**)calloc(n, sizeof(student*));


    for(i=0;i<n;i++){
        list[i]=(student*)calloc(n,sizeof(student));
        fscanf(fptr,"%d %[^\n]s", &(list[i]->IDno),(list[i]->name));
    }

    return list;
}

void print_list(student**list,int *sizePtr){
    int i; 
    for(i=0; i<*sizePtr; i++){
        printf("%d %s\n",list[i]->IDno, list[i]->name);
    }
}

int find_binsrch(int idNo, student **list, int size, int low, int high)
{
    int middle;

    if (size) {}  /* stub to eliminate unused warning */

    while(low<=high){
        middle =(low+high)/2;

        printf("%d\n", middle); 

        if(idNo==list[middle]->IDno)
            return list[middle]->IDno;

        if(idNo<list[middle]->IDno)
            high = middle -1;
        else
            low = middle +1;
    }
    return -1;    
}

注意:它是否正确运行确实是一个不同的问题，这取决于您的数据和消除任何逻辑错误。