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Iterative Constructs
ITERATION / LOOPS
C provides three loop structures: the for-loop, the while-loop,
and the do-while-loop. Each uses the four parts of a loop a little
differently, giving different loop options to the programmer.
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The for Statement
The for-loop has the following form.
for (initialization; condition; update)
Statement
where expression1 is an assignment expression (called the initial
condition), expression2 is a relational, equality, or logical expression
(called the test condition), and expression3 is another assignment
expression (called the loop operation). All three expressions usually
have a common variable.
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Everytime C encounters a for statement, it
1. executes the initial condition.
2. If the test condition is false, the for loop ends (it does
not execute statement).
3. If the test condition is true, it executes statement.
4. It then executes the loop operation and repeats step 2.
Example:
for (j = 1; j <= 10; ++j)
printf (“Hello!\n”);
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PROGRAMMING EXERCISES:
1.
2.
3.
Write a C program that prompts the user to enter several integers
and calculates the average.
Write a C program that prints all even numbers between 1 and 50.
Write a C program that prints all odd numbers between 1 and 50.
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INFINITE LOOPS
Infinite loops are also possible in using for statements.
Examples:
1.
for (j = 1; ; ++j)
printf (“Hello!\n”);
In this example, the for statement does not have a test
condition. Therefore, there is no condition for the loop to terminate.
2.
for (j = 1; j <= 10; )
printf (“Hello!\n”);
In this example, the for statement does not have a loop
operation. Therefore, the test condition will never become false.
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The while Statement
The while-loop has the following form.
initialization
while (condition) {
body update
}
where expression is the conditional expression which represents a
relational, equality, or logical expression.
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Everytime C encounters a while statement, it
1.
determines whether expression is true or false.
2.
If expression is true, then C executes statement and control
passes back to the beginning of the while loop. If expression is
still true, then C executes statement again. This process is
repeated while the expression is true.
3.
If expression is initially false, then C ignores the remainder of the
while statement.
Example:
sum = 0; counter = 1;
while (counter <= 10) {
sum += 1;
++counter
}
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Counting, adding, searching, sorting, and other tasks often
involve doing something over and over.
The while statement in C is very useful in performing repetitive
actions.
Example:
Write a program to compute 25 using the while statement.
Algorithm:
25 = 2 * 2 * 2 * 2 * 2 = 32
25 = multiply 2 to itself 5 times
Let i be a counter to monitor how many times the multiplication
has taken place.
y = 1 (y will eventually hold the final answer)
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i=1
y=y*2 =1*2=2
i =i+1 =1+1=2
i=2
y=y*2= 2*2= 4
i=i+1 = 2+1=3
i=3
y=y*2=4*2=8
i=i+1 =3+1=4
i=4
y = y * 2 = 8 * 2 = 16
i=i+1 =4+1=5
i=5
y = y * 2 = 16 * 2 = 32
i=i+1 =5+1 =6
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The algorithm requires that after initializing variables y and i to 1,
the statements
y=y*2
i=i+1
be repeated 5 times or while i is less than or equal to 5.
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INFINITE LOOPS
The programmer must always make sure that the conditional
expression in a while loop must sooner or later become false.
Otherwise, the program will enter in an infinite loop. In other words, the
loop will not terminate.
Example:
sum = 0;
counter = 1;
while (counter <= 10) {
sum += 1;
}
In this example, the conditional expression of the while loop
(counter <= 10) will never become false since counter will always be
equal to 1.
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Exercises:
1.
2.
3.
4.
5.
PROGRAM TO COMPUTE 25
PROGRAM TO COMPUTE baseexp
Write a C program that prompts the user to enter an integer
between 5 and 10. The program will then output the number
typed in by the user. However, the program should make sure
that the number typed in is indeed between 5 and 10.
Repeat the previous exercise except that the user is limited to
three (3) trials only. If the user exceeds this limit, the program
terminates.
Write a C program that prompts the user to enter several integers
and then outputs the one with the highest value.
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The do-while Statement
The do statement is a variant of the while statement. Instead of
making its test at the top of the loop, it makes it at the bottom.
The general format of the do statement is:
do
statement
while (expression)
where expression is the conditional expression which
represents a relational, equality, or logical expression.
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Everytime C encounters a do statement, it
1.
2.
3.
executes statement.
The program then evaluates expression. If expression is true,
then control passes back to the beginning of the do loop and the
process repeats itself.
If expression is false, then C executes the statement after the do
loop.
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Example:
# include <stdio.h>
main()
{
int num;
do {
printf ("\n\nEnter an integer between 5 and 10 > ");
scanf ("%d", &num);
if (num < 5 || num > 10) {
printf ("\n\nInvalid number!!! Please try again.");
printf ("\n\nEnter an integer between 5 and 10 > ");
scanf ("%d", &num);
}
} while (num < 5 || num > 10);
printf ("The number you entered is %d.", num);
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}
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THE break AND continue STATEMENTS
To interrupt the normal flow of control within a loop (a while or
for loop), the programmer can use either the break or continue
statement.
The break statement causes an exit from an enclosing loop. In
other words, encountering the break statement causes immediate
termination of the loop. Program control picks up at the code following
the loop.
Example:
for (ctr = 1; ctr <= 100; ++ctr) {
printf (“%d\n”, ctr);
if (crt = = 50)
break;
}
This program segment will normally print all numbers between
1 to 100. But because of the break statement, only the numbers
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between 1 to 50 will be printed.
Think Possibility
#include <stdio.h>
main ()
{
int x;
for ( ; ; ) {
printf ("\nGuess the secret number > ");
scanf (“%d”, &x);
if (x = = 5)
break;
else
printf ("\nSorry!!!");
}
printf("\n\nCongratulations!!!");
}
This program will simply ask the user to continuously enter a
number until the input is equal to 5.
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The continue statement works in a somewhat similar way to the
break statement. But instead of forcing loop termination, continue
forces the next iteration for the loop to take place, skipping any code in
between.
Example:
for (ctr = 1; ctr <= 100; ++ctr) {
if (x%2 ! = 0)
continue;
printf (“%d\n”, ctr);
}
This program segment will print all even numbers between 1 to
100.
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