1. EPSII
59:006
Spring 2004

2. A Design Example Illustrates: Pointers and arrays of pointers
command line arguments
simple character string processing

3. The Objective: Develop a program that will alphabetize a list of
words typed as command line arguments
Example:
%gcc alphabetize.c –o alphabetize
%alphabetize how now brown cow
brown
cow
how
now %

4. How to approach this problem:
Will need to use:
main(int argc, char *argv[]) {
to capture the command line arguments
Will need to bubble sort the strings in argv[] into alphabetical order
Will need to figure out how to compare character strings

5. Dealing with the command line arguments:
argv[0]
‘a’
‘l’
‘p’
‘h’
‘a’
‘b’
‘e’
‘t’
‘i’
‘z’
‘e’
‘\0’
argv[1]
‘h’
‘o’
‘w’
‘\0’
argv[2]
‘n’
‘o’
‘w’
‘\0’
argv[3]
‘b’
‘r’
‘o’
‘w’
‘n’
‘\0’
argc 5
argv[4]
‘c’
‘o’
‘w’
‘\0’
Need to sort the strings argv[1] through argv[argc-1] into alphabetical order

6. argv[] after sorting argv[0] ‘a’ ‘l’ ‘p’ ‘h’ ‘a’ ‘b’ ‘e’ ‘t’ ‘i’ ‘z’
‘\0’
argv[1]
‘b’
‘r’
‘o’
‘w’
‘n’
‘\0’
argv[2]
‘c’
‘o’
‘w’
‘\0’
argv[3]
‘h’
‘o’
‘w’
‘\0’
argv[4]
‘n’
‘o’
‘w’
‘\0’
Note: We only need to sort the pointers.

7. The Algorithm First refinement: Second Refinement:
Sort the command line arguments into alphabetic order and print them
Second Refinement:
Bubblesort the strings according to increasing lexigraphical (alphabetical) order
Print out the ordered strings

8. The Algorithm--Continued
The bubblesort step:
This will be fairly straightforward
Only twist is that we will just sort the array of pointers (argv[])
We need to come up with a way to compare two character strings to determine which one is “larger”—i.e. comes later in alphabetical order

9. Comparing Character Strings
Need to use the notion of “alphabetical order” of words
Can compare individual characters based upon the values of their underlying ASCII representations( ‘a’ < ‘b’ < ‘c < … < ‘z’)
See Appendix D in the text for the complete ASCII Character set

10. Comparing Character Strings—The Cases
A < B cases:
A > B cases: A
‘x’ …
‘y’
‘\0’ A
‘x’ …
‘y’
‘z’ …
‘\0’ B B
‘x’
‘x’ …
‘y’ …
‘\0’ …
‘y’
‘\0’
‘z’
strings match in
this portion
strings match in
this portion A … A
‘x’ …
‘y’
‘x’
‘z’ …
‘\0’ …
‘y’
‘w’
‘\0’ B
‘x’ …
‘y’
‘w’ …
‘\0’ B
‘x’ …
‘y’
‘z’ …
‘\0’
strings match in
this portion
strings match in
this portion

11. Comparing Strings—The Cases:
A ==B Case:
‘x’ A …
‘y’
‘\0’ B
‘x’ …
‘y’
‘\0’
strings match in
this portion

12. String Comparison Algorithm-Pseudocode
First refinement:
Compare two character strings A and B
Second Refinement:
Starting from the left end find the first position i in which the strings
differ or the end of at least one string is reached
If string B has reached the end but not string A or if neither string
has reached the end but the ith character of A is greater than the
ith character of B, then A>B
If String A has reached the end but not string B or if neither string
has reached the end but the ith character of A is less than the ith
character of B, then A < B
Otherwise (both strings have reached the end) A ==B

13. The String Comparison Algorithm--Continued
Third refinement:
Starting from the left end find the first position i in which the strings
differ or the null termination character of at least one string is reached
If the ith character of A is greater than the ith character of B, then A>B
If the ith character of A is less than the ith character of B, then A < B
Otherwise (both strings have reached the end) A ==B
Note: This refinement takes advantage of the fact the the null termination
charater ‘\0’ is the “smallest character in the ASCII character set.

14. the compareStrings function
int compareStrings(char x[], char y[]) {
//returns -1 if string x[] is less than string y[]
// returns 0 if string x[] equals string y[]
//returns 1 if string x[] > string y[]
int i = 0;
while ((x[i] == y[i])&&(x[i] != '\0')&&(y[i] != '\0')) i++;
if (x[i] > y[i]) return 1; // first string is larger
else if (x[i] < y[i]) return -1; // second string is larger
else return 0; // strings are equal
} //end of compareStrings()

15. The main program for(pass=1; pass<argc; pass++) {
for(j=1; j<argc-1; j++) {
if (compareStrings(argv[j], argv[j+1]) ==1) {
hold = argv[j];
argv[j] = argv[j+1];
argv[j+1] = hold; }
for (j=1; j<argc; j++) {
printf("%s\n", argv[j]);
} // end of main
#include <stdio.h>
#include <string.h>
#define TRUE 1
#define FALSE 0
int compareStrings(char [], char [] );
int main(int argc, char *argv[]) {
int pass, j;
char *hold;

16. Another version of compareStrings(), using pointer notation:
int compareStrings(char *x, char *y) {
//returns -1 if string *x is less than string *y
// returns 0 if string *x equals string *y
//returns 1 if string *x > string *y
while ((*x == *y)&&(*x != '\0')&&(*y != '\0')) {
x++;
y++; }
if (*x > *y) return 1; // first string is larger
else if (*x < *y) return -1; // second string is larger
else return 0; // strings are equal
} //end of compareStrings()

17. Design Example—Final Notes
Note that our alphabetize program acts funny with respect to upper case letters—e.g. it considers the string “Zzzzz” to come before “aAAAA”. Why????
The C strings library has a built-in function called strcmp that does the same thing as our compareStrings function.
The strings library has many powerful string processing functions. This will be the next topic that we cover in this course

18. Have a Great
Spring Break!!!