How to do it…

1. Open the sequential file in read-only mode using the following code:

    fp = fopen (argv [1],"r");

2. If the file does not exist or does not have enough permissions, an error message will be displayed and the program will terminate. Set this up using the following code:

if (fp == NULL) {
    printf("%s file does not exist\n", argv[1]);
    exit(1);
 }

3. One line is read from the file, as shown in the following code:

fgets(buffer, BUFFSIZE, fp);

4. Each character of the line is accessed and checked for the presence of periods, as shown in the following code:

for(i=0;i<n;i++)
    if(buffer[i]=='.')

5. If a period is found, the character following the period is checked to confirm whether it is in uppercase, as shown in the following code:

if(buffer[i] >=97 && buffer[i] <=122)

6. If the character following the period is in lowercase, a value of 32 is subtracted from the ASCII value of the lowercase character to convert it into uppercase, as shown in the following code:

buffer[i]=buffer[i]-32;

7. If the line is not yet over, then the sequence from step 4 onward is repeated till step 6; otherwise, the updated line is displayed on the screen, as shown in the following code:

puts(buffer);

8. Check whether the end of file has been reached using the following code. If the file is not over, repeat the sequence from step 3:

while(!feof(fp))

The preceding steps are pictorially explained in the following diagram (Figure 5.1):

Figure 5.1

The convertcase.c program for converting a lowercase character found after a period in a file into uppercase is as follows:

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

#define BUFFSIZE 255

void main (int argc, char* argv[])
{
    FILE *fp;
    char buffer[BUFFSIZE];
    int i,n;

    fp = fopen (argv [1],"r");
    if (fp == NULL) {
        printf("%s file does not exist\n", argv[1]);
        exit(1);
    }
    while (!feof(fp))
    {
        fgets(buffer, BUFFSIZE, fp);
        n=strlen(buffer);
        for(i=0;i<n;i++)
        {
            if(buffer[i]=='.')
            {
                i++;
                while(buffer[i]==' ')
                {
                    i++;
                }
                if(buffer[i] >=97 && buffer[i] <=122)
                {
                    buffer[i]=buffer[i]-32;
                }
            }
        }
        puts(buffer);
    }
    fclose(fp);
}

Now, let's go behind the scenes.