Iterations Explanation Basics for app.codility.com in C Plus Plus
Foreword: In programming, iterating means repeating some part of the code to obtain different results. This involves looping.
By: Chrysanthus Date Published: 21 Jan 2024
Introduction
A loop is a compound statement whose body can be executed again and again. The body of the loop has a number of statements. A loop needs an initial state or initial statement, from which the loop will execute the first time, before repeating. Repeating, means all the statements in the body of the loop are re-executed, in order, again and again. In order for the loop to repeat after the first pass or any pass, there has to be a statement that will cause it to repeat. In order for a loop to stop repeating, there has to be a condition that will cause the loop not to repeat.
This tutorial presents basic programming constructions that allow iterations to be performed in C++. The names of the different loops are: the do-while loop, the while-loop and the for-loop.
This article is one of the articles written by the author, to enable the reader pass the tests at app.codility.com. The tutorials at app.codility.com are given in Python, but this tutorial is given in C++.
Do-while Loop Syntax
The syntax for the do-while loop construct is:
//initial statement here
do {
//statements
//cause for next iteration
} while (condition);
This construct should be read as follows: Considering the initial statement, do all the statements in the loop, while the condition allows it. The initial statement ends with a semicolon. The do-compound statement itself, also ends with a semicolon. Note that \93while\94 here, is a reserved word.
There are three main loops in C++: the do-while loop, the while-loop and the for-loop. This tutorial (lesson ) explains the do-while loop, the while-loop and the for-loop.
do-while Loop Example
Using the above syntax, an example of a do-while loop coding, is in the following program:
#include <iostream>
using namespace std;
int main(int argc, char **argv)
{
int myInt = 0;
do {
cout << myInt << ' ';
myInt++;
} while (myInt < 5);
cout << endl;
return 0;
}
The output is:
0 1 2 3 4
The complete construct begins with \93int myInt = 0;\94 and ends at \93while (myInt < 5);\94. There are two simple statements in the braces. The first statement in the braces, prints out the value of the integer, myInt. The second statement increments myInt, by adding 1 to it. The condition is \93while (myInt < 5)\94. So, while myInt is less than 5, the compound statement is re-executed.
This loop has just one main simple statement, which is to print the value of myInt. The second simple statement is to cause the next iteration. The curly brackets can have more than one main simple statement. The following do-while loop has two main simple statements. The first one adds 2 to myInt, and the second one prints the result of the addition:
#include <iostream>
using namespace std;
int main(int argc, char **argv)
{
int myInt = 0;
do {
myInt = myInt + 2;
cout << myInt << ' ';
myInt++;
} while (myInt < 13);
cout << endl;
return 0;
}
The output is:
2 5 8 11 14
This output needs explanation. First of all, note that the while condition has been changed to \93while (myInt < 13)\94.
When myInt is 0, 2 is added to it, and myInt becomes 2. Two is printed. The increment adds 1 to myInt and it becomes 3, at the beginning of the next pass. In the next iteration (pass) myInt is 3. Two is added to it again and it becomes 5. The increment adds 1 to myInt and it becomes 6. In the next iteration myInt is 6. 2 is added to it again and it becomes 8. The increment adds 1 to myInt and it becomes 9. In the next iteration myInt is 9. 2 is added to it again and it becomes 11. The increment adds 1 to myInt and it becomes 12. In the next iteration myInt is 12. 2 is added to it again and it becomes 14. The increment adds 1 to myint and it becomes 15.
After each iteration, the while condition is checked. At this point, that the while condition is checked, the myInt is 15, above 13, after 14 has been printed. The condition results in false, and the repetition of the block, stops.
while-loop
//initial statement here
while (condition) {
//statements
//cause for next iteration
}
The main difference between the do-while loop and the while-loop is, that for the while-loop, the condition is checked first, before the block is executed. With the do-while loop, the block is executed first, before the condition is checked. Note that the while-loop construct, does not end with a semicolon.
The following program repeats the first program above, but with a while-loop:
#include <iostream>
using namespace std;
int main(int argc, char **argv)
{
int myInt = 0;
while (myInt < 5) {
cout << myInt << ' ';
myInt++;
}
cout << endl;
return 0;
}
The output is the same as for the first program above, that is:
0 1 2 3 4
The following program repeats the second program above, but with a while-loop:
#include <iostream>
using namespace std;
int main(int argc, char **argv)
{
int myInt = 0;
while (myInt < 13) {
myInt = myInt + 2;
cout << myInt << ' ';
myInt++;
}
cout << endl;
return 0;
}
The output is the same as for the second program above, that is:
2 5 8 11 14
for-loop
for (initial-state; while-condition; increment) {
//statements
}
The for-loop is a concise, though more restrictive form, of the do-while loop or while-loop. The for-loop has a pair of parentheses and a block. Here, the initial statement has been removed from outside and above the construct, into the parentheses. The while-condition, is the second statement in the parentheses. The cause-for-next-iteration (increment) is the last (third) statement in the parentheses.
Note that the for-loop construct, does not end with a semicolon. The following program repeats the first program above, but with a for-loop:
#include <iostream>
using namespace std;
int main(int argc, char **argv)
{
for (int myInt = 0; myInt < 5; myInt++) {
cout << myInt << ' ';
}
cout << endl;
return 0;
}
The block of the for-loop does not end with a semicolon. The output is the same as for the first program above, that is:
0 1 2 3 4
There is no semicolon after the increment statement (last statement) in the parentheses.
The following program repeats the second program above, but with a for-loop:
#include <iostream>
using namespace std;
int main(int argc, char **argv)
{
for (int myInt = 0; myInt < 13; myInt++) {
myInt = myInt + 2;
cout << myInt << ' ';
}
cout << endl;
return 0;
}
The output is the same as for the second program above, that is:
2 5 8 11 14