8  Comparing Values in Objective-C with an If Statement

As long as the condition is a value other than zero/nil/NULL, the code

inside the if statement will run.



An if statement that has an "otherwise" clause in it is known as if-else-statement, and

it's format is:

if (condition){

/* Your code to get executed if the condition is met */

} else {

/* Code to get executed if condition is not met */

}



The else clause of the if-else-statement can also contain it's own if statement! That

might sound strange, but consider this scenario. In real life, you can say something

similar to this: "I will go get a cup of coffee, if the place is open, I will get a tall latte, if

it's closed and the other place is open, I will get a cappuccino, otherwise, I will just

come back home and make tea for myself". The part where we said "...if it's closed and

the other place is open..." is an else statement with an if statement embedded in it. Here

is how you would implement that in Objective-C:

if (Coffee place A is open){

Get a Tall Latte from coffee place A

} else if (Coffee place B is open){

Get a Cappuccino from coffee place B

} else {

Come back home and make tea

}



The condition for an if statement, regardless of whether it is a standalone if statement

(like the first condition in the last example) or embedded inside an else-statement, must

resolve to a boolean value. A boolean value is either YES or NO. For instance, the following

code will always get executed, regardless of which condition/device you run it on:

if (YES){

/* This code will get executed whenever the app gets to it */

} else {

/* The app will NEVER get here */

}



The reason behind this is that the condition for the if statement in this example is always

met as long as the YES is the condition. To make things more exciting, you can do

comparisons in the condition supplied to an if statement, like so:

NSInteger integer1 = 123;

NSInteger integer2 = 456;

if (integer1 == integer2){

NSLog(@"Integers are equal.");

} else {

NSLog(@"Integers are not equal.");

}



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We use the double-equal sign in a conditional because the result of a

double-equal is a boolean value, whereas the single-equal sign might

confuse the compiler and usually returns the value/object on the left

side of the equal sign. It is best to avoid using single-equal signs in a

conditional like an if statement. Instead, compare your values using

double-equal signs.



If you are comparing objects, it is best to use the isEqual: instance method of the

NSObject class:

NSObject *object1 = [[NSObject alloc] init];

NSObject *object2 = object1;

if ([object1 isEqual:object2]){

NSLog(@"Both objects are equal.");

} else {

NSLog(@"Objects are not equal.");

}



For now you don't have to worry about what objects are. This will be

explained in detail in other recipes in this chapter.



Some objects, such as strings, however have their own comparison methods, changing

the way we compare two strings. For instance, you can have two string objects that

contain the same characters. If you compare them using their isEqual: instance method, you will get the result NO, because they are different objects. However, they might

still contain the exact same characters. Because of this, different classes expose their

own comparison methods in Objective-C. For more information about classes, please

refer to Recipe 1.11. To learn more about objects, refer to Recipe 1.14.

An if statement and its else statement can be written with or without curly braces. Using

the former syntax (with curly brackets), you can execute multiple lines of code after

the condition is satisfied. However, without curly brackets, you can write only one line

of code for each condition. Here is an example of the latter syntax without curly braces:

NSString *shortString = @"Hello!";

if ([shortString length] == 0)

NSLog(@"This is an empty string");

else

NSLog(@"This is not an empty string.");



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Be extra careful with logging and if statements without curly-brackets.

Often, when a product goes to production, a production manager might

attempt to comment out all your NSLog methods simply by replacing all

occurrences of NSLog with //NSLog. If you have if statements without

curly brackets, as in our last example, the production manager's commented-out code will look like this:

NSString *shortString = @"Hello!";

if ([shortString length] == 0)

//NSLog(@"This is an empty string");

else

//NSLog(@"This is not an empty string.");



This will break the code and people in the company will not be happy.

It doesn't matter whether they are not happy at you or not happy at the

production manager. That would be a team effort gone wrong, so you

will all be to blame. To avoid this, make sure that you always write your

if statements with curly braces.



See Also

XXX



1.9 Implementing Loops with For Statements

Problem

You want to implement a code that repeats a certain number of times, perhaps applying

the same procedure to every element in an array or some other changing values.



Solution

Use the for statement. The format of this statement is:

for (code to execute before loop;

condition to be met for the loop to terminate;

code to execute in every iteration of the loop){

}



All three clauses of the for loop are optional. In other words, you can

have a for loop that looks like this:

for (;;){ YOUR CODE HERE }



This is known as an infinite-loop or a loop that has no condition to

terminate and will run forever. This is a very bad programming practice

indeed and you should avoid using it by all means.



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Discussion

Loops are useful in programming because you will often need to start a loop from one

place to another, from one index to another, or from start to stop. For instance, you

might want to loop through all characters inside a string and count how many "A"

characters you can find in it. Another example is a loop that finds all files in a directory.

This is a loop that finds the number of files and then starts from the first one until it

gets to the last one.

Usually, programmers require a counter in their loops. For instance, you might want

to read all the characters inside a C-String. For this, you will need the index of each

character. If your string is 10 characters long, you will need to go from index 0 to 9. If

your string is 20 characters long, you have to read from index 0 to 19. Since the length

of your string is a variable, you can put it as the exit-conditional of your loop. Here is

an example:

char *myString = "This is my string";

NSUInteger counter = 0;

for (counter = 0;

/* Start from index 0 */

counter < strlen(myString); /* Exit loop when we reach last character */

counter++){ /* Increment the index in every iteration */

char character = myString[counter];

NSLog(@"%c", character);

}



The code that gets executed before the loop (as noted in the Solution section of this

recipe) is obviously optional. In fact, all three main parts of a for loop are optional, but

it is recommended that you think about how you intend to use your loops and use the

three main parts of the for statement accordingly.

Let's have a look at where you would want to skip the first statement of your for loop.

As you could see in the previous section, our counter variable was set to 0 before we

even started our loop. However, we are setting it to 0 again once our loop is about to

start. This is unnecessary in this example, but there is nothing wrong with that approach. If you feel you don't need the redundant code, simply remove it:

char *myString = "This is my string";

NSUInteger counter = 0;

for (; /* empty section */

counter < strlen(myString); /* Exit loop when we reach last character */

counter++){ /* Increment the index in every iteration */

char character = myString[counter];

NSLog(@"%c", character);

}



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