努力了解如何在scanf中呼叫指向指標的指標-有解無憂

我正在用 C 語言構建一個程式來為我的博士研究運行一些模擬。基本上，程式使用一個函式來讀取具有一些重要值的輸入檔案，然后將這些值分配給main函式中的變數。其中一些值需要分配給陣列，x并且par在輸入檔案中也宣告了大小。為此，我需要在函式內部使用一些動態分配的記憶體，它指向指標的地址*x和*par.

我設法使用二維陣串列示法為這些指標分配值，但我不完全理解為什么需要這樣，因為函式指向的指標被初始化為NULL指標。

下面以最小功能代碼為例：

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

void read_input(char *name, int *dim, int *npar, int *np, int *ndiv, double *t, double **par, double **x);

int main(void) {
    
    int DIM;                        
    int nP;                         
    int nDiv;                       
    int nPar;                       

    // Assign values for program parameters, system parameters and initial conditions
    char *input_filename = "input.txt";
    double t;
    double *par = NULL;
    double *x = NULL;
    read_input(input_filename, &DIM, &nPar, &nP, &nDiv, &t, &par, &x);
 
    for (int i = 0; i < DIM; i  ) {
        printf("x[%i] = %.15lf\n", i, x[i]);
    }
    for (int i = 0; i < nPar; i  ) {
        printf("par[%i] = %lf\n", i, par[i]);
    }

    free(x);
    free(par);
}

void read_input(char *name, int *dim, int *npar, int *np, int *ndiv, double *t, double **par, double **x) {
    // Open input file
    FILE *input = fopen(name, "r");
    if (input == NULL) {
        printf("Input File Not Found...\n");
        return;
    }
    // Read and assign system constants
    fscanf(input, "%i", dim);
    fscanf(input, "%i", npar);
    // Read and assign program parameters
    fscanf(input, "%i %i", np, ndiv); 
    // Read and assign initial time
    fscanf(input, "%lf", t);
    // Allocate memory for x[dim] and par[npar] vectors
    *x = malloc((*dim) * sizeof(double));
    *par = malloc((*npar) * sizeof(double));
    // Security check for pointers
    if (*x == NULL || *par == NULL) {
        free(*x);
        free(*par);
        printf("Memory allocation for *x or *par did not complete successfully");
        return;
    }
    // assign IC to x[dim] vector
    for (int i = 0; i < *dim; i  ) {
        fscanf(input, "%lf ", &x[i][i]);     
    }

    for (int i = 0; i < *dim; i  ) {
        for (int j = 0; j < *dim; j  ) {
            printf("x[%i][%i] = %lf (address: %p)\n", i, j, x[i][j], &x[i][j]);
        }
    }
    printf("==============================\n");
    // Assign values to par[npar] vector
    for (int i = 0; i < *npar; i  ) {
            fscanf(input, "%lf\n", &par[i][i]);
    }
    for (int i = 0; i < *npar; i  ) {
        for (int j = 0; j < *npar; j  ) {
            printf("par[%i][%i] = %lf (address: %p)\n", i, j, par[i][j], &par[i][j]);
        }
    }
    printf("==============================\n");
        
    // Close input file
    fclose(input);
}

另外，我檢查了雙指標x[i][j]和par[i][j]指向的地址，它們對于 , 和的每個組合都是相同[i][0], ..., [i][4]的i = 0 to 4：

x[0][0] = 0.707107 (address: 0000020B01C33FC0)
x[0][1] = 0.000000 (address: 0000020B01C33FC8)
x[1][0] = 0.707107 (address: 0000020B01C33FC0)
x[1][1] = 0.000000 (address: 0000020B01C33FC8)
==============================
par[0][0] = 1.000000 (address: 0000020B01C34580)
par[0][1] = 0.150000 (address: 0000020B01C34588)
par[0][2] = 0.025000 (address: 0000020B01C34590)
par[0][3] = -0.500000 (address: 0000020B01C34598)
par[0][4] = 1.000000 (address: 0000020B01C345A0)
par[1][0] = 1.000000 (address: 0000020B01C34580)
par[1][1] = 0.150000 (address: 0000020B01C34588)
par[1][2] = 0.025000 (address: 0000020B01C34590)
par[1][3] = -0.500000 (address: 0000020B01C34598)
par[1][4] = 1.000000 (address: 0000020B01C345A0)
par[2][0] = 1.000000 (address: 0000020B01C34580)
par[2][1] = 0.150000 (address: 0000020B01C34588)
par[2][2] = 0.025000 (address: 0000020B01C34590)
par[2][3] = -0.500000 (address: 0000020B01C34598)
par[2][4] = 1.000000 (address: 0000020B01C345A0)
par[3][0] = 1.000000 (address: 0000020B01C34580)
par[3][1] = 0.150000 (address: 0000020B01C34588)
par[3][2] = 0.025000 (address: 0000020B01C34590)
par[3][3] = -0.500000 (address: 0000020B01C34598)
par[3][4] = 1.000000 (address: 0000020B01C345A0)
par[4][0] = 1.000000 (address: 0000020B01C34580)
par[4][1] = 0.150000 (address: 0000020B01C34588)
par[4][2] = 0.025000 (address: 0000020B01C34590)
par[4][3] = -0.500000 (address: 0000020B01C34598)
par[4][4] = 1.000000 (address: 0000020B01C345A0)
==============================
x[0] = 0.707106781186547
x[1] = 0.000000000000000
par[0] = 1.000000
par[1] = 0.150000
par[2] = 0.025000
par[3] = -0.500000
par[4] = 1.000000

以下是 input.txt 檔案的格式：

2
5

1000 1000

0.0
0.707106781186547 0.0

1.0
0.15
0.025
-0.5
1.0

我在這里想念什么？

uj5u.com熱心網友回復：

在 2D 索引中的第一個索引為 non 的所有情況下，發布的代碼都具有未定義的行為0。例如：

    fscanf(input, "%lf ", &x[i][i]);

x是函式中指標的int地址main。
x[i]僅在iis時定義0，否則您正在從記憶體中的位置讀取超出main區域變數的位置，該位置具有未定義的行為，但可能會或可能不會導致問題。
x[i][i]取消參考 value x[i]，這可能是一個無效的指標，因此很可能會導致崩潰。僅僅把這個地址當作&x[i][i]可能仍然被忽視，但是當fscanf()試圖在那里存盤一個int時，取消參考&x[i][i]將是一個問題。

令人驚訝的是，您的程式可以毫無問題地產生輸出。

此回圈的正確語法是

// assign IC to x[dim] vector
for (int i = 0; i < *dim; i  ) {
    fscanf(input, "%lf ", &(*x)[i]);
}

&(*x)[i]i是指向的陣列的第 - 個元素的地址*x。還有其他可能，但都不令人滿意：

&x[0][i]
(*x) i
*x i
x[0] i
i *x
&0[x][i]
&i[*x]
&i[0[x]]
i 0[x]

是的，以上所有都是等價的......

更好的方法是定義具有正確型別的區域變數，read_input并在成功操作后將它們傳回給呼叫者。您還可以將所有這些變數分組到一個結構中并將指標傳遞給read_input：

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

int read_input(const char *name, int *pdim, int *pnpar,
               int *pnp, int *pndiv, double *pt,
               double **ppar, double **px)
{
    // use regular variables
    int dim, npar, np, ndiv;
    double t;
    double *par = NULL;
    double *px = NULL;

    // Open input file
    FILE *input = fopen(name, "r");
    if (input == NULL) {
        printf("Cannot open file %s: %s\n", name, strerror(errno));
        return -1;
    }
    // Read and assign system constants
    if (fscanf(input, "%i %i", &dim, &npar) != 2)
        goto invalid;
    // Read and assign program parameters
    if (fscanf(input, "%i %i", &np, &ndiv) != 2)
        goto invalid;
    // Read and assign initial time
    if (fscanf(input, "%lf", &t) != 1)
        goto invalid;
    // Allocate memory for x[dim] and par[npar] vectors
    x = calloc(dim, sizeof(double));
    par = calloc(npar, sizeof(double));
    // Security check for pointers
    if (x == NULL || par == NULL) {
        printf("Memory allocation for x or par did not complete successfully\n");
        free(x);
        free(par);
        fclose(input);
        return -1;
    }
    // assign IC to x[dim] vector
    for (int i = 0; i < dim; i  ) {
        if (fscanf(input, "%lf", &x[i]) != 1)
           goto invalid;
    }

    // Assign values to par[npar] vector
    for (int i = 0; i < npar; i  ) {
        if (fscanf(input, "%lf", &par[i]) != 1)
            goto invalid;
    }
        
    // Close input file
    fclose(input);

    // update valid data to the caller
    *pdim = dim;
    *pnpar = npar;
    *pnp = np;
    *pndiv = ndiv;
    *pt = t;
    *px = x;
    *ppar = par;
    return 0;

invalid:
    printf("Invalid or missing input\n");
    free(x);
    free(par);
    fclose(input);
    return -1;
}

uj5u.com熱心網友回復：

我設法在一些文章中找到了為指標賦值的正確方法：

為此，應通過一維陣串列示法訪問指標，方法是將指標本身稱為：

scanf("%lf", &(*x)[i]);

代替

scanf("%lf", &x[i][i]);

然后，可以正確分配值。

轉載請註明出處，本文鏈接：https://www.uj5u.com/qianduan/486437.html

標籤：C 指针扫描指针对指针

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