问题描述
对编程相对较新，被分配到创建 C 程序以编码和解码 2 ppm 图像以接收秘密消息的任务。我想出了代码，但它没有通过 getPPM 函数中的 if 语句的图像格式。关于我可以对代码进行哪些更改的任何提示？同样在我的 main() 中，我是否传递了正确的参数？
typedef struct NODE {
char *val;
struct NODE *next;
} NODE;
/*
Stores the RGB values of a pixel
*/
typedef struct Pixel {
int red;
int green;
int blue;
} Pixel;
/*
Stores the contents of a PPM file
*/
typedef struct PPM {
char *format;
NODE *comments;
int width,height;
int max;
Pixel *arr;
} PPM;
NODE *copy(NODE *first)
{
NODE *second = NULL,*prevIoUs = NULL;
while (first != NULL) {
NODE *temp = (NODE *)malloc(sizeof(NODE));
temp->val = first->val;
temp->next = NULL;
if (second == NULL) {
second = temp;
prevIoUs = temp;
} else {
prevIoUs->next = temp;
prevIoUs = temp;
}
first = first->next;
}
return second;
}
//copy the PPM File Returning Pointer
struct PPM* createPPM(PPM *old)
{
PPM* new = (PPM *)malloc(sizeof(PPM));
strcpy(new->format,old->format);
new->comments = copy(old->comments);
new->height = old->height;
new->width = old->width;
new->max = old->max;
new->arr = (Pixel *)malloc(old->height * old->width * sizeof(Pixel));
memcpy(new->arr,old->arr,old->height * old->width * sizeof(Pixel));
return new;
}
/*
Returns the PPM struct from the file fd
*/
PPM *getPPM(FILE *fd) {
PPM *image = (PPM *)malloc(sizeof(PPM));
image->format = (char *)malloc(MAX_LEN);
fgets(fd,MAX_LEN,image->format);
sscanf(fd,"%s",image->format);
if (strcmp(image->format,"P3") != 0) {
printf("Invalid Image Type");
exit(0);
}
char c = getc(fd);
image->comments = (NODE*)malloc(sizeof(NODE));
NODE *temp = image->comments;
while ((c = getc(fd)) == '#') {
fseek(fd,-1,SEEK_CUR);
char str[50];
fgets(str,50,fd);
temp->val = (char*)malloc(strnlen(str,50));
strcpy(temp->val,str);
temp->val[strlen(temp->val) - 1] = 0;
temp->next = (NODE*)malloc(sizeof(NODE));
temp = temp->next;
temp->next = NULL;
}
fseek(fd,SEEK_CUR);
fscanf(fd,"%d",&image->width);
fscanf(fd,&image->height);
fscanf(fd,&image->max);
image->arr = (Pixel*)malloc(image->height * image->width * sizeof(Pixel));
int t = 0;
int j = 0;
while (j < image->height * image->width) {
t = fscanf(fd,&image->arr[j].red);
t = fscanf(fd,&image->arr[j].green);
t = fscanf(fd,&image->arr[j].blue);
j = j + 1;
}
return image;
}
PPM *loadPPMFromFile(char *filename) {
FILE *file;
/* Todo: step 1,open the file */
file = fopen("C:\\Users\\Olivia\\source\\repos\\f28hs-2020-21-cwk1-c\\PPMfiles","r");
/* step 2: Check that file has been loaded correctly; show error otherwise*/
if (file != NULL) {
PPM *ppm;
/* odo: step 3,get the PPM data from the file with the getPPM function */
getPPM(file);
if (ppm == NULL) {
/* Todo: step 4,display error if file cannot been parsed into a PPM struct*/
printf("File cannot be parsed.\n");
}
/* Todo: step 5,close the file */
fclose(file);
/* step 6: return the PPM */
return ppm;
} else {
fclose(file);
return NULL;
}
}
/*
Prints a PPM struct in the format of a PPM file
*/
void showPPM(PPM *ppm) {
printf("%s\n",ppm->format); //print format
//print comments
NODE *n = ppm->comments;
while (n->next != NULL) {
printf("%s\n",n->val);
n = n->next;
}
//print width,height and max
printf("%d %d\n%d\n",ppm->width,ppm->height,ppm->max);
//print the array containing the pixels
int j;
for (j = 0; j < ppm->height * ppm->width; ++j) {
printf("%d %d %d\n",ppm->arr[j].red,ppm->arr[j].green,ppm->arr[j].blue);
}
return;
}
/*
Encodes text into red field of PPM
Returns the encoded PPM
*/
PPM *encode(char *text,PPM *i) {
PPM *str = createPPM(i);
int random;
srand((unsigned)time(NULL));
int randMax = (i->height * i->width) / (strlen(text) + 1);
random = rand() % randMax;
if (random < 1) {
random = 1;
}
int k = 0;
int j = random;
//Red fields swapped with ASCII int
while (k < strlen(text)) {
if (str->arr[j].red == text[k]) {
j = j + 1; // if the values are the same we encode in the next pixel.
} else {
str->arr[j].red = text[k];
k = k + 1;
j = j + random;
}
}
return str;
}
/*
Compares 2 PPM structs and decodes the message hidden within
Returns the decoded message if the images have the same dimensions
Returns NULL otherwise
*/
char *decode(PPM *i1,PPM *i2) {
int i = 0;
int j = 0;
char *str = (char*)malloc(sizeof(char));
while (i < i1->height * i1->width) {
if (i1->arr[i].red != i2->arr[i].red) {
str[j] = i2->arr[i].red;
j = j + 1;
}
i = i + 1;
}
str = realloc(str,i);
return str;
printf("%s",str);
}
/* Todo: Question 3 */
int main(int argc,char *argv[]) {
/* check arguments */
switch (argc) {
case 2:
/* Todo: not enough arguments,print an error message */
printf("Not enough arguments. \n");
break;
case 3:
if (strcmp(argv[1],"e") == 0) { //Argument "e" - encode PPM
PPM *ppm = loadPPMFromFile(argv[2]);
createPPM(ppm);
/*Check that PPM is valid; continue if it is,exit otherwise */
if (ppm != NULL) {
PPM *encodedPPM;
/* Todo: encode the ppm with the text and assign to encodedPPM */
encodedPPM = encode(argv[2],encodedPPM);
/*Check that no error occured*/
if (encodedPPM == NULL) {
return EXIT_FAILURE;
} else {
/* Todo: print a confirmation that encoding was successful */
printf("Encoding successful. \n");
/* Todo: print the PPM to standard output with showPPM */
showPPM(encodedPPM);
return EXIT_SUCCESS;
}
} else {
return EXIT_FAILURE;
}
} else {
printf("Unrecognised or incomplete command line.\n");
return EXIT_FAILURE;
}
break;
case 4:
if (strcmp(argv[1],"d") == 0) { //Argument "d" - decode PPM
PPM *comparisonPPM;
PPM *encodedPPM;
/* Todo: get comparison file filename from argv,load it with
loadPPMFromFile then assign to comparisonPPM */
comparisonPPM = loadPPMFromFile(argv[2]);
/* Todo: get encoded file filename from argv,load it with
loadPPMFromFile then assign to encodedPPM */
encodedPPM = loadPPMFromFile(argv[2]);
/*Check both PPMs are valid; continue if so,exit otherwise */
if (comparisonPPM != NULL && encodedPPM != NULL) {
char *decodedMsg;
/* Todo: decode the encodedPPM with the comparisonPPM and assign to decodedMsg */
decodedMsg = decode(encodedPPM,comparisonPPM);
/*Check that the message has been decoded*/
if (decodedMsg != NULL) {
/* Todo: print a confirmation message that the message was decoded */
printf("Decoded message:\n");
/* Todo: print the decoded message */
printf("%p",decodedMsg);
return EXIT_SUCCESS;
} else {
return EXIT_FAILURE;
}
} else {
return EXIT_FAILURE;
}
} else {
fprintf(stderr,"Unrecognised or incomplete command line.\n\n");
return EXIT_FAILURE;
}
break;
default:
fprintf(stderr,"Unrecognised or incomplete command line.\n\n");
return EXIT_FAILURE;
}
}
解决方法
“也在我的 main() 中，我是否传入了正确的参数？”
是的，他们是正确的。
“getPPM 函数中的 if 语句没有通过图像格式”
getPPM 有几个问题。
在以下语句中覆盖 c：
char c = getc(fd);
已包含在评论中。
以下内容：
sscanf(fd,"%s",image->format);
if (strcmp(image->format,"P3") != 0)
{
因为如果有新行，sscanf() 会选择新行，strcmp() 将无法与 "P3" 进行比较。
建议执行以下一项或多项操作：
- 在调试器中运行，在
if处放置一个断点并查看image->format的值 - 将
if(image->format != NULL) printf("%s",image->format)放在sscanf()之后 - 如果您怀疑
\n或\r\n可能在缓冲区中，请使用image->format[strcspn(image->format,"\r\n")] = 0;删除它们。
关于以下内容，建议您阅读有关每个 C 函数的文档，例如 fgets()：
这个：
fgets(fd,MAX_LEN,image->format);
scanf(...
应该
if(fgets(image->format,fd) != NULL)
{
scanf(...
（参数位置已更改，并在尝试使用缓冲区之前进行测试。）
旁白：因为这是 C，所以遵循 malloc 语句
PPM* image = (PPM*)malloc(sizeof(PPM));
image->format = (char*)malloc(MAX_LEN);
更correctly写成
PPM* image = malloc(sizeof(*image));
image->format = malloc(MAX_LEN);
（从 malloc() 中删除 Cast 并在第一条语句中注意 sizeof argument。在整个代码中适用。