Loops are used to do repetitive actions
Branches are used to make decisions
Loops and Branches are quite similar to C, Java, Fortarn, Basic, etc
Special attention will be needed for cin to handles input
Doing repetitive tasks
In C++, we have
- for loop
- while loop
- do while loop
In general, we have to do the following:
Setting a value initially
Performing a test to see if the loop should continue
Executing the loop actions
Updating value(s) used for the test
Syntax:
for (initialization; test-expression; update-expression) ... body ...
Example:
// forloop.cpp -- introducing the for loop
#include <iostream.h>
int main(void)
{
// initialize; test ; update
for (int i = 0; i < 5; i++)
cout << "C++ knows loops.\n";
cout << "C++ knows when to stop.\n";
return 0;
}
// Mixing with input and outout// num_test.cpp -- use numeric test in for loop #include <iostream.h> int main(void) { cout << "Enter the starting countdown value: "; int limit; cin >> limit; for (int i = limit; i; i--) // entry-condition test cout << "i = " << i << "\n"; cout << "Done now that i = " << i << "\n"; return 0; }// The output will be // Enter the starting countdown value: 4// i = 4 // i = 3 // i = 2 // i = 1 // Done now that i = 0
A new addition to C++
There are two version (prefix version ++x, postfix version x++)
Example:
// plus_one.cpp -- the increment operator #include <iostream.h> int main(void) { int a = 20; int b = 20; cout << "a = " << a << ": b = " << b << "\n"; cout << "a++ = " << a++ << ": ++b = " << ++b << "\n"; cout << "a = " << a << ": b = " << b << "\n"; return 0; }// The output will be // a = 20: b = 20 // a++ = 20: ++b = 21 // a = 21: b = 21More Example:
int x = 5; int y = ++x; // change x, then assign to y // y is 6, x is 6int z = 5; int y = z++; // assign to y, then change z // y is 5, z is 6
Think of it as shorthand for programming
i = i + y; // Can be rewritten as
i += y;
Example:
int k = 5;k += 3; // Now, k is set to 8
Operator Effect (L = Left operand, R = right operand) += assigns L + R to L -= assigns L - R to L *= assigns L * R to L /= assigns L / R to L %= assigns L % R to L
Can be called compound statement or block statement
for (int i = 1; i <= 5; i++) { // block starts here cout << "Value " << i << ": "; cin >> number; sum += number; }
It is kind of a trick
Might be wise to stay away from it
j++, i-- // This count as one expression instead of two
int i, j; // One expression as well
Operator Actual Meaning < is less than <= is less than or equal to == is equal to > is greater than >= is greater than or equal to != is not equal to
Example:
for (x = 20; x > 5; x--) // continue while x is greater than 5
for (x = 1; y != x; x++) // continue while y is not equal to x
for (cin.get(c); c == ' '; cin.get(c)) // continue while c is a space
Note:
myint = 4; // Assignment
myint == 4 // Logical cmparsion
A for loop stripped off the initialization and update parts. It is more general for application programmers
Syntax:
while (test-condition) bodyExample:
// A while loop repeats itself until a certain condition // is met. This loop adds the integers from 1 to ... // until the sum is greater than 50. sum = 0; int m = 1; while (sum <= 50) { cout << "Adding " << m << " to the sum." << endl; sum += m; ++m; } cout << "The sum is " << sum << endl;
for (init-expression; test-expression; update-expression) { statement(s); }init-expression; while (test-expression) { statement(s); update-expression; }// Counting example long wait = 0;while (wait++ < 10000) ; // Count silently
Exit condition loop
Execute at least once
Syntax:
do body while (test-expression);
Example:// dowhile.cpp -- exit-condition loop #include <iostream.h> int main(void) { int n; cout << "Enter numbers in the range 1-10 to find "; cout << "my favorite number\n"; do { cin >> n; // execute body } while (n != 7); // then test cout << "Yes, 7 is my favorite.\n" ; return 0; }
// If the input file look like this 5 ox
// If you open a file with an ifstream object, // then you can read from it the same way that // you read from cin. ifstream inputFile ("week2b.in"); // Read the first number to find out how many // occurrences we will replace. int maxReplacements; inputFile >> maxReplacements; // Read the "from" and "to" characters that // specify the replacement. char fromCh; char toCh; inputFile >> fromCh >> toCh;// Now fromCh get o toCh get x maxReplacements is 5 !
Instead of using cin >> ch
We can use cin.get(ch); // this will read the space as a character as well
Two ways of finding out whether we have a EOF or not
while (inputFile.good()) { .... }// ORwhile (cin.get(ch)) // cin.get(ch) is 0 on EOF { cout << ch; count ++; }
Use to decide whether a particular statement or block is executed or not
Syntax:
if (test-condition) statement1 else statement2Example:
// ifelse.cpp -- using the if else statement #include <iostream.h> int main(void) { char ch; cout << "Type, and I shall repeat.\n"; while (cin.get(ch)) { if (ch == '\n') cout << ch; // done if newline else cout << ++ch; // done otherwise } // try ch + 1 instead of ++ch for interesting effect cout << "Please excuse the slight confusion.\n"; return 0; }
// Example 1 if (ch == 'A') a_grade++; else if (ch == 'B') b_grade++; else soso++; // Example 2 if (ch == 'A') a_grade++; else if (ch == 'B') b_grade++; else soso++;
OR: ||
if (ch == 'y' || ch == 'Y') { // Do something }AND: &&while (i < size && sum >= 100) { // continue the loop // etc }NOT: !if (!(x>5)) { // do something } // It is the same as if (x <= 5) { // do something }// Range test if ((x > 5) && (x < 10)) { // We are in the right range // do something }
Go for big if then else statement
switch (integer-expression)
{
case label1 : statement(s)
case label2 : statement(s)
...
default : statement(s)
}