Here is a sample program that uses a variety of the important string commands provided by C. The program solves the following problem:

Problem: You are given a list of names that are separated by the word "end". For example:

Yifan Tang end
Brad Vander Zanden end
Nels Blank
Vander Zanden end

Your program should print:

the longest name (i.e., the name with the longest length)
the first name in alphabetical order

In the above input, "Nels Blake Vander Zanden" is the longest name and "Brad Vander Zanden" is the first name in alphabetical order. Note that names are typically comprised of multiple words separated by blank spaces, so you will need to use strcat to concatenate the words together to form a name. You will need to use the strlen function to determine the length of the longest string, strcmp to find the first name in alphabetical order and to determine when you have encountered an "end" token, and strcpy to save a copy of the longest length word and a copy of the first word in alphabetical order.

The Program

I have included some commentary at the end of this program.

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

#define MAX_SINGLE_NAME 20
#define MAX_NAME 100

main() {
  char word[MAX_SINGLE_NAME];
  char name[MAX_NAME] = "";
  char longest[MAX_NAME] = "";
  char smallest[MAX_NAME] = "";
  
  /* read an initial word into name. I do not want to wait to initialize
     name until I am in the loop, because the first strcat
     statement will add a blank space to the front of the name. */
  scanf("%s", name);

  /* read words until EOF */
  while (scanf("%s", word) != EOF) {
    if (strcmp(word, "end") == 0) {
      if (strlen(name) > strlen(longest)) {
	strcpy(longest, name);
      }
      /* determine if the current name is the new smallest name */
      if (strcmp(smallest, "") == 0) {
 	strcpy(smallest, name);
      }
      else if (strcmp(name, smallest) < 0) {
	strcpy(smallest, name);
      }
      /* I used to copy an empty string to name to re-initialize it,
	 but I discovered that this was causing me to have a space
	 before the start of the first name, because strcat starts
	 by appending a space to the name. I thus changed the code
	 so that it now reads the next word directly into name. This
	 assumes that there will not be an empty name (i.e., there will
	 not be two consecutive "end" tokens in the input) */
      scanf("%s", name); /* start the next name */
    }
    /* concatenate words into a bigger name until I see an "end" token */
    else {
      strcat(name, " ");
      strcat(name, word);
    }
  }

  /* print longest name */
  printf("longest = %s\n", longest);
  /* print first name in alphabetical order */
  printf("smallest = %s\n", smallest);
  return(0);
}

Commentary

Here are some issues that I encountered or thought about as I developed the program:

I initially developed the program without worrying about boundary conditions. Boundary conditions are special cases that occur at the start or end of the program (e.g., how do you initialize a variable), or that involve special cases in the input (e.g., how do you handle an empty file). By initially ignoring boundary conditions, I was able to focus on the main logic of the program, without worrying about the annoying, but important details of boundary conditions.
Once I had written the initial code, I knew that I had not initialized many of the important variables, such as longest, smallest, and name. It is important to get variable initialization correct and as you will see, I did not get it entirely correct on my first try. I took a different approach to initializing each of these three variables:
- longest: I knew that every string would be longer than 0 characters, so I initialized longest to be the empty string. I initially used a special test to determine whether or not longest had been initialized. However, after I wrote the program I realized that I did not need a special test to see whether or not longest had been initialized, because I knew that the first name would be longer than the empty string stored in longest, and hence longest would be immediately set to the first name I encountered. Hence I cleaned up the code by eliminating the test of whether or not longest had been initialized.
- smallest: Unlike longest, it is hard to find a reasonable string that will be always be last in alphabetical order. I therefore decided to leave a "marker" in smallest that indicated that it was not yet initialized. A good marker is the empty string, since a name cannot be the empty string. In my code, I checked to see whether or not smallest contained the empty string, before checking whether or not my current name was before smallest in alphabetical order. This check ensures that the very first name I encounter will be used to initialize smallest.
- name: I initially thought I could initialize name to be the empty string, and then simply concatenate words to name. However, when I tested the program, I discovered that all my names started with a blank space. For example, instead of "Brad Vander Zanden" I was getting " Brad Vander Zanden". The problem turned out to be that the strcat code for appending words to names first appends a space. So if I initialize name to the empty string, then what happens is that before the first word is added, a space is appended to the empty string, thus creating a name with a leading space. This is an example of a "boundary" condition causing a problem. In this case the boundary condition is what happens with the first word added to the name. I realized that I could overcome this problem by simply reading a word from the input stream into name whenever I needed to start a new name. This works only if I am sure that a name cannot be empty (i.e., there cannot be two consecutive "end" words). If I need to check for empty names, my code would be a bit more complicated. My trick of reading a word into name works even when I've reached EOF. The read will fail and hence will leave the old name in name. The program will then continue back to the top of the while loop. The while loop will try another scanf, and the scanf will again fail, thus causing the loop to exit. I will never use name again, and hence it does not matter that name has an invalid value.