vintasoft barcode .net sdk 3: Introduction to Object-Oriented Programming in Objective-C

Creation QR Code JIS X 0510 in Objective-C 3: Introduction to Object-Oriented Programming

Count-2 produces this output:

2008-07-20 2008-07-20 2008-07-20 2008-07-20 2008-07-20 2008-07-20 2008-07-20 2008-07-20 2008-07-20 2008-07-20 2008-07-20 11:55:35.909 11:55:35.926 11:55:35.927 11:55:35.928 11:55:35.935 11:55:35.936 11:55:35.936 11:55:35.939 11:55:35.939 11:55:35.940 11:55:35.940 03.02 03.02 03.02 03.02 03.02 03.02 03.02 03.02 03.02 03.02 03.02 Count-2[18001:10b] Count-2[18001:10b] Count-2[18001:10b] Count-2[18001:10b] Count-2[18001:10b] Count-2[18001:10b] Count-2[18001:10b] Count-2[18001:10b] Count-2[18001:10b] Count-2[18001:10b] Count-2[18001:10b] The numbers from 1 to 10: 1 2 3 4 5 6 7 8 9 10

Modifying the program in this way is obviously not a very tricky change to make: you can do it with a simple search-and-replace action, and only two places need to be changed. However, doing a similar search and replace in a larger program, consisting of, say, tens of thousands of lines of code would be a lot trickier. We would have to be careful about simply replacing 5 with 10: no doubt, there would be other instances of the number five that aren t related to this and so shouldn t be changed to ten. Solving this problem is what variables are for. Rather than sticking the upper loop value (five or ten) directly in the code, we can solve this problem by putting the number in a variable, thus adding a layer of indirection. When you add the variable, instead of telling the program to go through the loop five times, you re telling it to go look in this variable named count, which will say how many times to run the loop. Now, the program is called Count-3 and looks like this:

#import <Foundation/Foundation.h> int main (int argc, const char * argv[]) { int count = 5; NSLog (@"The numbers from 1 to %d:", count); int i; for (i = 1; i <= count; i++) { NSLog (@"%d\n", i); }

The program s output should be unsurprising:

2008-07-20 2008-07-20 2008-07-20 2008-07-20 2008-07-20 2008-07-20 11:58:12.135 11:58:12.144 11:58:12.144 11:58:12.145 11:58:12.146 11:58:12.151 03.03 03.03 03.03 03.03 03.03 03.03 Count-3[18034:10b] Count-3[18034:10b] Count-3[18034:10b] Count-3[18034:10b] Count-3[18034:10b] Count-3[18034:10b] The numbers from 1 to 5: 1 2 3 4 5

The NSLog() time stamp and other information take up a lot of space, so for clarity, we ll leave that information out of future listings.

If you want to print the numbers from 1 to 100, you just have to touch the code in one obvious place:

#import <Foundation/Foundation.h> int main (int argc, const char * argv[]) { int count = 100; NSLog (@"The numbers from 1 to %d:", count); int i; for (i = 1; i <= count; i++) { NSLog (@"%d\n", i); } return (0); } // main

By adding a variable, our code is now much cleaner and easier to extend, especially when other programmers need to change the code. To change the loop values, they won t have to scrutinize every use of the number five to see if they need to modify it. Instead, they can just change the count variable to get the result they want.

Files provide another example of indirection. Consider Word-Length-1, a program that prints a list of words along with their lengths; it is in the 03.04 Word-Length-1 folder. This vital program is the key technology for your new Web 2.0 start-up, Length-o-words.com. Here s the listing:

#import <Foundation/Foundation.h> int main (int argc, const char * argv[]) { const char *words[4] = { "aardvark", "abacus", "allude", "zygote" }; int wordCount = 4; int i; for (i = 0; i < wordCount; i++) { NSLog (@"%s is %d characters long", words[i], strlen(words[i])); } return (0); } // main

The for loop determines which word in the words array is being processed at any time. The NSLog() function inside the loop prints out the word using the %s format specifier. We use %s, because words is an array of C strings rather than of @"NSString" objects. The %d format specifier takes the integer value of the strlen() function, which calculates the length of the string, and prints it out along with the word itself. When you run Word-Length-1, you see informative output like this:

aardvark is abacus is 6 allude is 6 zygote is 6 8 characters long characters long characters long characters long