Class 34: Reading Lines (2)

Held: Tuesday, 1 April 2003

Summary: Today we continue our investigation of the steps involved in reading a line from a file. We also consider other issues from exam 1.

Related Pages:

• EBoard.
• readLine.h.
• testrl.c.
• readLine.c.
• Notes on exam 1.

Overview:

• Testing readLine, continued.
• Implementing readLine.
• An efficient exponentiation algorithm.
• strncpy.

readLine, Continued

Testing readLine, Continued

• What should the body of our test look like?
• What are some particular situations we should test?
• Can we write something that steps through a number of related tests?

A first readLine

• Here's a hybrid readLine, based on some different things various of you did. We'll critique it together and then try to fix it.

  int 
  readLine(FILE *file, char string[], int max) {
    char c;
    int i;
    for(i = 0; ((c=getc(file))!=EOF) && (c != '\n') && (i < max-1); i++)
      string[i] = c;
    string[i++] = '\0';
    if (i >= max) 
      return 0;
    else if (c == EOF)
      return 1;
    else if (c == '\n')
      return 1;
    else
      return 0;
  } /* readLine(FILE *, char[], int) */
  

A New Version

• At some point, we may decide to stop patching and start writing again.
• What's our overall strategy?
• In what order should we do tests within the loop?
• What are the requirements?

Efficient Exponentiation

• Key ideas:
  • Revised recurrence relation
  • Accumulate the extra multiplications
  • Carefully use preconditions and postconditions

strncpy

• Requirements?
• Use pointers or array indices?
• When do you stop?