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Note: Updated! See
top of introduction page
c1.html
for using
a C++ .Net compiler for these standard C++ tutorials.
Repetition structures are an integral part of any programming
language, and C++ is no different. Repetitive operations
are referred to as "iterations" in C++.
Iterations involves doing the same thing again and again.
Iteration's principal
method is the loop, and these
are enormously powerful structures in any programming language.
Many complex
problems can be solved by repeating simple steps over and over.
Loops are efficient, since by reducing larger problems into
small, repetitive tasks, a small
amount of code can accomplish a great deal of input, processing
and output (IPO). The basic repetition structures in C++
are:
goto, while, while(true), do/while, and for. In many
instances, a block or two of source code is worth a thousand
words. So in this section, various repetition structures
will be listed. Copy and paste the code into your
compiler. Compile, link and execute each program until you
become familiar with how C++ repetition structures function.
Definitions and keywords will be highlighted and in bold.
"There
are only two ways to live your life. One is as
though nothing is a miracle.
The other
is as though everything is a miracle."
- Albert Einstein |
1.
Goto
-
The
"goto" structure can be employed in C++, but it is considered poor structure
by many programmers and so rarely used. Goto is
used with a "label" in C++, which is simply a name of your
choice (make sure it's not a keyword) followed by a semicolon.
#include <iostream.h>
int main()
{
int counter = 0; // initialize counter
loop: counter ++; // top of the loop
cout << "counter: " << counter << "\n";
if(counter < 5) // test the value
goto loop; // jump to the top
cout << "Complete. Counter: " << counter << ".\n";
return 0;
}
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2.
While
-
Unlike do/while, a block of code in a while loop may never occur if
the condition is
not met.
#include <iostream.h>
int main()
{
int counter = 0; // initialize the condition
while(counter < 5) // test condition still true
{
counter++; // body of the loop
cout << "counter: " << counter << "\n";
}
cout << "Complete. Counter: " << counter << ".\n";
return 0;
}
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3.
Break and Continue
-
The keyword "break" can be used to forcibly exit a loop before a
condition is met. You should avoid hard coding breaks
into your loops unless absolutely necessary, as they can be
error-prone and difficult to debug. The keyword "continue"
forces a loop back into iteration.
#include <iostream.h>
int main()
{
unsigned short small;
unsigned long large;
unsigned long skip;
unsigned long target;
const unsigned short MAXSMALL=65535;
cout << "Enter a small number: ";
cin >> small;
cout << "Enter a large number: ";
cin >> large;
cout << "Enter a skip number: ";
cin >> skip;
cout << "Enter a target number: ";
cin >> target;
cout << "\n";
// set up 3 stop conditions for the loop
while(small < large && large > 0 && small < MAXSMALL)
{
small++;
if (small % skip == 0) // skip the decrement?
{
cout << "skipping on " << small << endl;
continue;
}
if (large == target) // exact match for the target?
{
cout << "Target reached!";
break;
}
large-=2;
} // end of while loop
cout << "\nSmall: " << small << " Large: " << large << endl;
return 0;
}
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4.
While True
-
The while true loop will continue until a condition becomes
false or a break is encountered.
#include <iostream.h>
int main()
{
int counter = 0;
while(true)
{
counter ++;
if(counter > 10)
break;
}
cout << "Counter: " << counter << "\n";
return 0;
}
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5.
Do /
While
-
Unlike a while loop, a do/while loop executes
its code first, then it evaluates its test condition. This
guarantees that the block of
code in the do/while executes at least once, even if the test
condition is not met.
#include <iostream.h>
int main()
{
int counter;
cout << "How many hellos? ";
cin >> counter;
do
{
cout << "Hello\n";
counter--;
}
while(counter > 0);
cout << "Counter is: " << counter << endl;
return 0;
}
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6.
For
Loops
-
For loops are some of the most useful and flexible loops in C++.
They are comprised of three basic parts:
1 - initialization - A variable is initialized with a value to
start the loop.
2 - test condition - A condition is tested for to end the loop.
3 - action - What to do each time the loop iterates (usually
increment or decrement a variable).
The three parts are each separated by a semicolon.
Example:
#include <iostream.h>
int main()
{
int counter;
for(counter = 0; counter < 5; counter++)
{
cout << "Looping! ";
}
cout << "\nCounter: " << counter << ".\n";
return 0;
}
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For loop with multiple
expressions:
//For loop with multiple expressions
#include <iostream.h>
int main()
{
for(int i=0, j=0; i<3; i++, j++)
{
cout << "i: " << i << " j: " << j << endl;
}
return 0;
}
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For loop with null
statement:
#include <iostream.h>
int main()
{
int counter = 0;
for( ; counter < 5; )
{
counter++;
cout << "Looping! ";
}
cout << "\nCounter: " << counter << ".\n";
return 0;
}
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For loop that’s empty – infinite loop without break:
//For loop that’s empty – infinite loop without break
#include <iostream.h>
int main()
{
int counter=0; // initialization
int max;
cout << "How many hellos?";
cin >> max;
for(;;) // an infinite for loop that doesn't end
{
if(counter < max) // test
{
cout << "Hello!\n";
counter++; // increment
}
else
break;
}
return 0;
}
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For loop with NULL:
#include <iostream.h>
int main()
{
for(int i = 0; i<5; cout << "i: " << i++ << endl)
{ ; }
return 0;
}
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Nested For Loops:
//Nested For loops
#include <iostream.h>
int main()
{
int rows, columns;
char theChar;
cout << "How many rows? ";
cin >> rows;
cout << "How many columns? ";
cin >> columns;
cout << "What character? ";
cin >> theChar;
for(int i = 0; i<rows; i++)
{
for(int j = 0; j<columns; j++)
cout << theChar;
cout << "\n";
}
return 0;
}
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Drawing Shapes with for loops:
#include <iostream.h>
//Function prototypes
void DrawRectangle();
void DrawTriangle1();
void DrawTriangle2();
int main()
{
cout << endl << endl;
DrawRectangle();
DrawTriangle1();
DrawTriangle2();
cout << endl << endl << "End of the program!";
cout << endl << endl;
return 0;
}
//Function definitions
void DrawRectangle() {
for(int row = 1; row <= 5; ++row ) {
for(int column = 1; column <= 15 ; ++column ) {
cout << "*";
} //ends 2nd for loop
cout << endl;
} //ends 1st for loop
cout << endl << endl << endl;
} //end function
void DrawSquare() {
for(int row = 1; row <= 15; ++row ) {
for(int column = 1; column <= 15 ; ++column ) {
cout << "*";
} //ends 2nd for loop
cout << endl;
} //ends 1st for loop
cout << endl << endl << endl;
} //end function
void DrawTriangle1() {
int i, j ;
for(i = 1 ; i <= 5 ; i++ )
{
for(j = 1 ; j <= i ; j++)
{
cout << "*";
} //ends 2nd for loop
cout << "\n";
} //ends 1st for loop
} //end function
void DrawTriangle2() {
int i, j;
cout << endl << endl;
for(i = 1 ; i <= 5 ; i++ )
{
for(j = 1 ; j <= 5 ; j++)
{
if(i > 5 - j)
{
cout << "*";
}
else
{
cout << " ";
}
} //ends 2nd for loop
cout << endl;
} //ends 1st for loop
} //end function
void DrawDiagonalO() {
int i, j ;
for(i = 1 ; i <= 5 ; i++ )
{
for(j = 1 ; j <= 5 ; j++)
{
if(i == j)
{
cout << "O";
}
else
{
cout << "*";
}
} //ends 2nd for loop
cout << endl;
} //ends 1st for loop
} //end function
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Output:
***************
***************
***************
***************
*************** |
*
**
***
****
***** |
*
**
***
****
*****
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Recursion, nested For
loops and iteration for Fibonacci numbers:
Calculating the
Fibonacci series is a very popular example in programming
tutorials. It is a fun mathematical construction to play
with, and it illustrates the effectiveness of looping, iteration
and recursion to solve a more complex problem by reducing it
into a set of smaller problems and solving them. The
Fibonacci series appears as a pattern where every number is the
sum of the two preceding numbers that come before it. This
pattern appears throughout the observable universe as both
microcosmic spiraling fractal patterns, such as those in DNA,
bacteria, crystal structures, snail shells, leaves, and tree
branches, and in macrocosmic spiral constructions such as
galaxies and super clusters. It is amazing how many
patterns can be observed to conform to the Fibonacci series in
nature, perhaps due to a balancing act of the four forces, and
as a result it is sometimes referred to as the "golden mean".
Example:
1 1 2
3 5 8 13 21 are numbers in
this series, as 1+2 = 3, 2+3 = 5,
3+5 = 8, 5+8 = 13, and
8+13 = 21.
To calculate this
series using recursion, we will create a function. We
will pass in a value to this function and attempt to
calculate the fibonacci series for it. If the number
is less than 3, we will do nothing except return a 1, as 1
is the beginning number in the Fibonacci series. If
the number passed in is greater than 3, we will perform some
recursive magic. We will create a loop that
initializes itself by taking whatever we pass in to it and
subtracting 3 from it. We will then iterate through
this loop until what we pass in is no longer greater than 0.
Each time we iterate through this loop, we will decrement
(subtract 1 from) the value we passed in. To get each
number in the Fibonacci series, we add the two before it
together. So if we pass a 5 to our function, this
would occur:
Let's start with
the lowest number in the series, 1, and add the next
number, 2. Let's repeat this until we reach 5:
1+1 = 2
1+2 = 3
2+3 = 5
If we pass 8 into
our function, we will need to repeat the process (n-3), or
(8-3), or 5 times:
Let's start with
the lowest number in the series, 1, and add the next
number, 2. Let's repeat this until we reach 21:
0+1 = 1
0+1 = 1
//Subtracting 1st 3
numbers in test condition.
1+1 = 2
//This leaves 5 numbers to reach
position 8, which is 21.
1+2 = 3
2+3 = 5
3+5 = 8
5+8 = 13
8+13 = 21
#include <iostream.h>
//Function declaration and prototype
int fib(int position);
int main()
{
int answer, position;
cout << "Which position? ";
cin >> position;
cout << "\n";
//Call the Fibonacci function
answer = fib(position);
cout << answer << " is the ";
cout << position << "th Fibonacci number.\n";
return 0;
} //end main()
//Function definition
int fib(int n)
{
int minusTwo;
int minusOne;
int answer;
minusTwo=1;
minusOne=1;
answer=2;
if(n < 3)
return 1; //ends function
for(n = n-3; n > 0; n--) //like saying for(n -= 3; n; n--)
{
minusTwo = minusOne;
minusOne = answer;
answer = minusOne + minusTwo;
}
return answer;
} //Close function
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When we pass in a
5, we will subtract 3 to account for the first three numbers
in the series, then iterate with what is left. The
first time through the loop, answer = 2 + 1, or 3. The
second time through, answer = 3 + 2, or 5. Then we
fall out of the loop. 2 loop iterations + the 3
numbers we bypassed make 5. 5 is the 5th Fibonacci
number. Example: 1 1 2
3 5
8 . If we
pass in 13, we subtract 3 to account for the first three
numbers. Then, the first time through the loop, answer
= 2 + 1, or 3. The second time, answer = 3 + 2, or 5.
The third time, answer = 5 + 3, or 8. The fourth time,
answer = 8 + 5, or 13. We looped 4 times, and 4 + the
three numbers we bypassed make 7. Therefore, 13 is the
7th Fibonacci number. Example: 1 1
2 3 5 8
13 21 .
©2004 C. Germany
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