Looping Under the for Statement
The general form of the for statement is
for (expression1; expression2; expression3) {
statement1;
statement2;
.
.
.
}
You see from this example that the for statement uses three expressions (expression1, expression2, and expression3) that are separated by semicolons.
Several statements, such as statement1 and statement2, are placed within the braces ({ and }). All the statements and the braces form a statement block that is treated as a single statement. (You learned about this in Hour 3, "The Essentials of C Programs.")
In the preceding for statement format, the beginning brace ({) is put on the same line of the for keyword. You can place the beginning brace on a separate line beneath the for keyword.
The for statement first evaluates expression1, which usually initializes one or more variables. In other words, expression1 is only evaluated once when the for statement is first encountered.
The second expression, expression2, is the conditional part that is evaluated right after the evaluation of expression1 and then is evaluated after each successful looping by the for statement. If expression2 returns a nonzero value, the statements within the braces are executed. Usually, the nonzero value is 1. If expression2 returns 0, the looping is stopped and the execution of the for statement is finished.
The third expression in the for statement, expression3, is not evaluated when the for statement is first encountered. However, expression3 is evaluated after each looping and before the statement goes back to test expression2 again.
In Hour 5, "Reading from and Writing to Standard I/O," you saw an example (in Listing 5.5) that converts the decimal numbers 0 through 15 into hex numbers. Back then, conversions made for each number had to be written in a separate statement. Now, with the for statement, we can rewrite the program in Listing 5.5 in a very efficient way. Listing 7.1 shows the rewritten version of the program.
TYPE
Listing 7.1. Converting 0 through 15 to hex numbers.
1: /* 07L01.c: Converting 0 through 15 to hex numbers */
2: #include <stdio.h>
3:
4: main()
5: {
6: int i;
7:
8: printf("Hex(uppercase) Hex(lowercase) Decimal\n");
9: for (i=0; i<16; i++){
10: printf("%X %x %d\n", i, i, i);
11: }
12: return 0;
13: }
OUTPUT
After creating the executable file 07L01.exe and running it by typing in 07L01 from a DOS prompt, I obtain the same output as the one from 05L05.exe:
C:\app> 07L01
Hex(uppercase) Hex(lowercase) Decimal
0 0 0
1 1 1
2 2 2
3 3 3
4 4 4
5 5 5
6 6 6
7 7 7
8 8 8
9 9 9
A a 10
B b 11
C c 12
D d 13
E e 14
F f 15
C:\app>
ANALYSIS
Now, let's have a look at the code in Listing 7.1. As you know, line 2 includes the header file stdio.h for the printf() function used later in the program.
Inside the body of the main() function, the statement in line 6 declares an integer variable, i. Line 8 displays the headline of the output on the screen.
Lines 9_11 contain the for statement. Note that the first expression in the for statement is i=0, which is an assignment expression that initializes the integer variable i to 0.
The second expression in the for statement is i<16, which is a relational expression. This expression returns 1 as long as the relation indicated by the less-than operator (<) holds. As mentioned earlier, the second expression is evaluated by the for statement each time after a successful looping. If the value of i is less than 16, which means the relational expression remains true, the for statement will start another loop. Otherwise, it will stop looping and exit.
The third expression in the for statement is i++ in this case. This expression is evaluated and the integer variable i is increased by 1 each time after the statement inside the body of the for statement is executed. Here it doesn't make a big difference whether the post-increment operator (i++) or the pre-increment operator (++i) is used in the third expression.
In other words, when the for loop is first encountered, i is set to 0, the expression
i<16
is evaluated and found to be true, and therefore the statements within the body of the for loop are executed. Following execution of the for loop, the third expression i++ is executed incrementing i to 1, and i<16 is again evaluated and found to be true, thus the body of the loop is executed again. The looping lasts until the conditional expression i<16 is no longer true.
There is only one statement inside the for statement body, as you can see in line 10. The statement contains the printf() function, which is used to display the hex numbers (both uppercase and lowercase) converted from the decimal values by using the format specifiers, %X and %x.
Here the decimal value is provided by the integer variable i. As explained, i contains the initial value of 0 right before and during the first looping. After each looping, i is increased by 1 because of the third expression, i++, in the for statement. The last value provided by i is 15. When i reaches 16, the relation indicated by the second expression, i<16, is no longer true. Therefore, the looping is stopped and the execution of the for statement is completed.
Then, the statement in line 12 returns 0 to indicate a normal termination of the program, and finally, the main() function ends and returns the control back to the operating system.
As you see, with the for statement, you can write a very concise program. In fact, the program in Listing 7.1 is more than 10 lines shorter than the one in Listing 5.5, although the two programs can do exactly the same thing.
Actually, you can make the program in Listing 7.1 even shorter. In the for statement, you can discard the braces ({ and }) if there is only one statement inside the statement block.
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