在 C 中跳过了循环

Question

This is my code, I wanted to display all of the lines stored inside of a text file and at the same time replace that particular line by getting input from user of what line to replace.

///Get Customer ID
char c_id[100];
printf("Enter Customer ID: ");
scanf("%s", &c_id);

FILE* fPtr;
FILE* fTemp;
char temp_line[100];
char buffer[100];
char newline[100];
int i, arr_n, line, count;

/* Remove extra new line character from stdin */
fflush(stdin);
fflush(stdout);

/// Open all required files
fPtr = fopen(c_id, "r");
fTemp = fopen("replace.txt", "w");

/// Print the content
i = 1;
arr_n = 0;

printf("\nOptions:\n");

while (fgets(buffer, BUFFER_SIZE, fPtr))
{
    printf("%d. %s\n", i, buffer);
    i = i + 1;
}

printf("Enter your options: ");
scanf("%d", &line);

printf("Enter the updated information: ");
scanf("%s", &newline);

count = 0;

while (fgets(temp_line, BUFFER_SIZE, fPtr))
{
    count++;
    if (count == line)
    {
        switch (line)
        {
        case 1:
            fprintf(fTemp, "Name: %s", newline);
            break;
        case 2:
            fprintf(fTemp, "Date of birth: %s", newline);
            break;
        case 3:
            printf("It did got here");
            fprintf(fTemp, "Address: %s", newline);
            break;
        case 4:
            fprintf(fTemp, "Contact 1: %s", newline);
            break;
        case 5:
            fprintf(fTemp, "Contact 2: %s", newline);
            break;
        default:
            printf("Invalid answer given");
        }
    }
    else
    {
        fputs(temp_line, fTemp);
    }
}

/// Close all files to release resource
fclose(fPtr);
fclose(fTemp);


/// Delete original source file 
remove(c_id);

/// Rename temporary file as original file 
rename("replace.txt", c_id);

printf("\nSuccessfully updated the information");

It's the last while loop that got skipped. Why would it skip that while loop, I can't seem to find the reason it skips. I tried to change to another buffer to temp_line, and it's still the same.

I tried to remove few unnecessary line that's not related to it first and try to resolve the problem but still can't find the exact reason the while loop got skip.

Is the because the switch case?

Or is there something wrong with buffer?

Answer 1

You first loop reaches end of file. Function fgets returns a pointer to the buffer or a null pointer if end of file is reached. Since you are using the pointer value as a boolean in the while loop second loop is skipped because your file is already exhausted and fgets returns NULL. while(NULL) {... } never enters the while loop code block.

Use function fseek or rewind to reset the file pointer to the top of the file before the second loop.

Answer 2

i said that the while loop is skipped because whenever I put input of line for example 3, it should've run into case 3 and print "It did got here". However it didn't even print those out and then I tried to re run again by putting another printf at the start of while and nothing got printed out again.

while (fgets(temp_line, BUFFER_SIZE, fPtr))
{
    printf("successfully got into while loop");
    count++;
    if (count == line)
    {
        switch (line)
        {
        case 1:
            fprintf(fTemp, "Name: %s", newline);
            break;
        case 2:
            fprintf(fTemp, "Date of birth: %s", newline);
            break;
        case 3:
            printf("It did got here");
            fprintf(fTemp, "Address: %s", newline);
            break;
        case 4:
            fprintf(fTemp, "Contact 1: %s", newline);
            break;
        case 5:
            fprintf(fTemp, "Contact 2: %s", newline);
            break;
        default:
            printf("Invalid answer given");
        }
    }
    else
    {
        fputs(temp_line, fTemp);
    }
}