Iteration
Iteration, repetition, or looping, they all mean the same thing. When you want to repeat some
portion of the program, you will use some form of iteration. Looping is a very common term to describe
this.
The most basic looping construct is the while statement. The form is identical to the
if statement, but uses the keyword while instead:
| Single statement | | Multiple statements (braces required) |
|---|
while ( expression )
statement
|
|
while ( expression )
{
statements
}
|
Whereas the if statement caused statement to be executed exactly once if expression
was true, the while statement causes statement to be executed repeatedly as long as
expression remains true. If expression becomes false, then the repetition stops.
Simple example:
int count = 5;
int i = 0;
while (i < count) /* controlling expression */
i++; /* body of the loop */
Example with output (and a review of expressions). Note that, like the if statement, the braces are only
required with multiple (compound) statements:
| Version #1 (braces required) | Version #2 (braces optional) | Version #3 (braces optional) |
|---|
int count = 5;
int i = 0;
while (i < count)
{
i++;
printf("i is %i\n", i);
}
|
int count = 5;
int i = 0;
while (i < count)
{
printf("i is %i\n", ++i);
}
|
int count = 5;
int i = 0;
while (i++ < count)
printf("i is %i\n", i);
|
The output from all 3 examples:
i is 1
i is 2
i is 3
i is 4
i is 5
Bart Simpson could have used C to make his punishment less painful:
int i = 1;
while (i <= 100)
{
printf("I will include the the course number in email correspondence with the teacher.\n");
i++;
}
(Partial) output:
I will include the course number in email correspondence with the teacher.
I will include the course number in email correspondence with the teacher.
I will include the course number in email correspondence with the teacher.
I will include the course number in email correspondence with the teacher.
I will include the course number in email correspondence with the teacher.
I will include the course number in email correspondence with the teacher.
I will include the course number in email correspondence with the teacher.
I will include the course number in email correspondence with the teacher.
I will include the course number in email correspondence with the teacher.
I will include the course number in email correspondence with the teacher.
I will include the course number in email correspondence with the teacher.
I will include the course number in email correspondence with the teacher.
I will include the course number in email correspondence with the teacher.
I will include the course number in email correspondence with the teacher.
I will include the course number in email correspondence with the teacher.
I will include the course number in email correspondence with the teacher.
I will include the course number in email correspondence with the teacher.
I will include the course number in email correspondence with the teacher.
I will include the course number in email correspondence with the teacher.
I will include the course number in email correspondence with the teacher.
|
 |
Notes:
- The body of the loop is not guaranteed to execute at all.
- The controlling expression:
- is executed once before the body of the loop is executed.
- is executed again after each repetition of the body.
- usually it will eventually evaluate to false, to stop the repetition.
- If the controlling expression never evaluates to false, the while loop
is called an infinite loop, because it never stops.
| Infinite loop #1 | Infinite loop #2 | Infinite loop #3 |
|---|
int i = 1;
while (i != 10)
i += 2;
|
int i = 0;
while (i < 10)
printf("i is %i\n", i);
|
while (1)
printf("This loop never ends...\n");
|
Also sometimes referred to as the do...while statement. The basic format is:
| Single statement | Multiple statements |
|---|
do
statement
while ( expression );
|
do
{
statements
}
while ( expression );
|
The primary difference between the while statement and the do statement is that the
body of the do statement is guaranteed to execute at least once. This is simply because the
controlling expression is executed after the first iteration of the loop body:
| Body executes 0 or more times | Body executes 1 or more times |
|---|
while ( expression )
statement
|
do
statement
while ( expression );
|
Example:
int number;
int choice;
do
{
printf("Enter a number: ");
scanf("%d", &number);
printf("You entered %i\n", number);
printf("Enter another number? (1=yes,0=no) ");
scanf("%d", &choice);
}
while (choice);
Sample run:
Enter a number: 23
You entered 23
Enter another number? (1=yes,0=no) 1
Enter a number: 10
You entered 10
Enter another number? (1=yes,0=no) 1
Enter a number: 5
You entered 5
Enter another number? (1=yes,0=no) 3
Enter a number: 12
You entered 12
Enter another number? (1=yes,0=no) 0
There really isn't much difference between while statement and the do statement. If you need
the loop to execute at least once, then the do statement is the one to use.
In computer science, there are 3 general kinds of repetitions regarding the number of
repetitions and the technique used:
| Repetitions | Keyword |
|---|
| 0 or 1 time | if |
| 0 or more times | while |
| 1 or more times | do...while |
Technically, the if isn't repeating anything. However, keep these keywords
in mind when you have to do something in one of those three ways.
Now we get to the most complex of the looping mechanisms: the for statement. The general
form is:
for ( expression1 ; expression2 ; expression3 )
statement
and the compound version:
for ( expression1 ; expression2 ; expression3 )
{
statements
}
The meaning of this is a little involved:
- First, evaluate expression1. This is executed and evaluated exactly once at the beginning of the loop.
- Evaluate expression2.
- If expression2 is true, execute statement. (If it's not true, jump out of the loop.)
- Evaluate expression3.
- Goto step 2.
This process can be written using an equivalent while statement:
expression1;
while ( expression2 )
{
statements
expression3;
}
This also means that any for loop can be written as a while loop and vice-versa.
Simple examples to print the numbers 1 through 10:
| for loop #1 | for loop #2 | while loop |
|---|
int i;
for (i = 1; i <= 10; i++)
printf("%i\n", i);
|
int i;
for (i = 0; i < 10; i++)
printf("%i\n", i + 1);
|
int i = 1;
while (i <= 10)
printf("%i\n", i++);
|
The for loops above show the typical ways in which they are used. The variable i is sometimes called
the loop control variable (or simply the counter) because it controls when the loop continues or stops.
The three expressions generally do these things:
- expression1 - initializes the loop variable (or counter)
- expression2 - compares the counter with some value
- expression3 - modifies the counter (usually add/subtract 1)
These are just typical uses of the expressions. You can do practically anything with those expressions.
| Count to 20 by 2 | Count down from 30 by 3 | Squares of 1 to 10 |
|---|
int i;
for (i = 2; i <= 20; i += 2)
printf("%i\n", i);
2
4
6
8
10
12
14
16
18
20
|
int i;
for (i = 30; i >= 3; i -= 3)
printf("%i\n", i);
30
27
24
21
18
15
12
9
6
3
|
int i;
for (i = 1; i <= 10; i++)
printf("%i\n", i * i);
1
4
9
16
25
36
49
64
81
100
|
Of course, all of these could be written as while loops as well.
| Count to 20 by 2 | Count down from 30 by 3 | Squares of 1 to 10 |
|---|
int i = 2;
while (i <= 20)
{
printf("%i\n", i);
i += 2;
}
|
int i = 30;
while (i >= 3)
{
printf("%i\n", i);
i -= 3;
}
|
int i = 1;
while (i <= 10)
{
printf("%i\n", i * i);
i++;
}
|
Note that any or all of the expressions in the for loop can be omitted:
int i = 1;
for (; i <= 10;)
printf("%i\n", i++);
Of course, this is nothing but a strange-looking while loop now.
int i = 1;
while (i <= 10)
printf("%i\n", i++);
You can even omit the second expression, but this would lead to an infinite loop, since the default empty expression
is true!
int i = 1;
for (;;)
printf("%i\n", i++);
If you want to exit from the loop prematurely, you can use the break statement:
| breaking out of infinite for | breaking out of infinite while |
|---|
int i = 1;
for (;;)
{
printf("%i\n", i++);
if (i > 10)
break;
}
|
int i = 1;
while (1)
{
printf("%i\n", i++);
if (i > 10)
break;
}
|
The break statement can be used in any of the looping mechanisms as well as the switch statement.
You can also have multiple expressions in between the semicolons:
| Using a for loop | Using a while loop |
|---|
int i;
for (i = 0, j = 0; i < 16 || j < 3; i +=2, j++)
printf("%i * %i = %i\n", i, j, i * j);
0 * 0 = 0
2 * 1 = 2
4 * 2 = 8
6 * 3 = 18
8 * 4 = 32
10 * 5 = 50
12 * 6 = 72
14 * 7 = 98
|
int i = 0;
int j = 0;
while (i < 16 || j < 3)
{
printf("%i * %i = %i\n", i, j, i * j);
i += 2;
j++;
}
|
Note the use of the comma operator used in the expressions. This operator has the form:
expression1 , expression2 , expression3 , etc...
- The expressions are evaluated in order, left to right.
- This order of evaluation is well defined and guaranteed. (Few other operators can make this guarantee).
- The result of the entire expression is the value of the last expression.
- What are the values of i, j, and c below?
Assume they have been declared as integers. (Look carefully!!) You'll need the
precedence chart for this.
| | |
|---|
i = 5;
j = 3;
c = ++i, ++j, j + i;
printf("%i, %i, %i\n",i,j,c);
|
i = 5;
j = 3;
c = (++i), (++j), (j + i);
printf("%i, %i, %i\n",i,j,c);
|
i = 5;
j = 3;
c = (++i, ++j, j + i);
printf("%i, %i, %i\n",i,j,c);
|
The continue statement is similar to the break statement in that it causes the loop to deviate from
its prescribed course. The difference is subtle, but very important.
| break statement | continue statement |
|---|
for (/* expressions */)
{
/* first statement in loop */
/* second statement in loop */
/* etc... */
break;
/* last statement in loop */
}
[break jumps to here]
/* first statement after loop */
|
for (/* expressions */)
{
/* first statement in loop */
/* second statement in loop */
/* etc... */
continue;
/* last statement in loop */
[continue jumps to here]
}
/* first statement after loop */
|
This prints the even numbers from 2 to 20:
| using for | using while | using while |
|---|
for (i = 2; i <= 20; i++)
{
if ( (i % 2) == 1 )
continue;
printf("%i\n", i);
}
2
4
6
8
10
12
14
16
18
20
|
i = 2;
while (i <= 20)
{
if ( (i % 2) == 1 )
{
i++;
continue;
}
printf("%i\n", i++);
}
|
i = 2;
while (i <= 20)
{
if ( (i++ % 2) == 1 )
continue;
printf("%i\n", i - 1);
}
|
The break and continue statements are not used all that often.
But when you need this behavior, they come in handy.