Conditional statements

If then / If then else statements

Format
if condition
    thenstatement

if condition
    thenstatement
else
    elsestatement2

for condition

an expression followed by a relational operator followed by an expression (x+1 < y+2) or
a condition followed by a logical operation followed by a condition (x<y && y<z)

relational operators: < > <= >= == !=
logical operators: ! && ||

NOTES:

Parentheses are REQUIRED around the entire condition.
A semicolon ALWAYS follows the statement preceding else.
Braces are required if the thenstatement or the elsestatement is composed of more than one C++ statement.

All operators have precedence. Of those considered thus far, the precedence is:
()
++,--,!,+,-
*,/,%
+,- (binary operators)
< > <= >= == !=
&&
||

Examples

if (v1 < v2)
v2 = v2 - v1;

if (v1 < v2)
v2 = v2 - v1;
else
v1 = v1 - v2;

if (v1 != v2) {
t = v1;
v1 = v2;
v2 = t;
}

if (v1<v2 && v2<v3)
t ++;
else
t--;

Example programs

Simple if-then-else

#include <iostream.h>

main () {
/* Determine whether a given integer is negative.*/
int i;
cout << "Give value.\n";
cin >> i;
if (i < 0)
cout << i << " is negative.\n";
else
cout << i << " is not negative.\n";
}

Nested if-then-else

main () {
/* Determine whether a given integer is positive, negative, or zero.*/
int i;
cout << "Give value.\n";
cin >> i;
if (i < 0)
    cout << i << " is negative.\n";
else
    if (i == 0)
      cout << i << "is zero.\n";
    else
      cout << i << " is positive.\n";
}

More than one statement in then or else

main () {
int i;
cout << "Give value.\n";
cin >> i;
if (i < 0) {
    cout << i << " is negative.\n";
    i = -1 * i;
}
else {
    cout << i << " is not negative.\n";
    i = 2 * i;
}
}

Use of other types of expressions as condition

There are no Boolean data types in C++; BUT any zero value used in the place of a condition is considered false and any non-zero value is considered to be true. Therefore the result of:

Example 1

if (37)
cout << "in then portion\n";
else
cout << "in else portion\n";

will be:

in then portion

and the result of

Example 2

i = 0;
if (i)
cout << "in then portion\n";
else
cout << "in else portion\n";

will be:

in else portion

The return value of an assignment is its final lvalue. Therefore, the result of

Example 3

if (a = 2)
cout << "a = 2\n";
else
cout << "a != 2\n";

is:

a = 2

because the value of a = 2 is 2.

And the result of

Example 4

if (a = 0)
cout << "a = 0\n";
else
cout << "a != 0\n";

is:

a != 0

because the value of a = 0 is 0 (false).

REMINDER: If the condition is true, the then portion is executed. If the condition is false, the else portion is executed.

Input statements as conditions

- Input statements also have value. The value of cin.get(ch) and cin >> ch is zero if eof was encountered and a reference to the istream otherwise, i.e. not zero.
Therefore, the result of

Example 5

if (cin >> b)
cout << "b\n";
else
cout << "eof encountered\n";

will result in the value of b being printed if there was a value to read and the eof message if the last value HAS ALREADY BEEN READ.

Conditional operator (?:)

	Format <arithmeticif> ::= <condition> ? <expression1> : <expression2>
	Examples i ? cout << "i != 0\n" : cout << "i == 0\n"; cout << (i ? "i != 0\n" : "i == 0\n"); a = (i>=0 ? i : -i);

Switch statement

Use instead of multiply nested if statement

Format
<switchstatement> ::= switch (<expression>) {<cases>}
<cases> ::= case <valx>: <body> break;
| <cases> case <valy>: <body> break;
| <cases> default: <body>
for <valx> and <valy> discrete values

Example

switch (ch) {
case '\n':
    cout << "eoln\n";
    break;
case '\t':
    cout << "tab\n";
    break;
case '\\':
    cout << "backslash\n";
    break;
case '\r':
    cout << "carriage return\n";
    break;
case '\a':
    cout << "alert\n";
    break;
default:
    cout << "none of these\n";
}

While statement

while
- Pretest loop
- Format
whilecondition
whilestatement
- Examples

while (x<y)
cout << x++ << y << '\n';

while (x>0 || y!=28) {
cout << x << y++;
x-=5;
}

numneg = 0;
sum = 0;
while (cin >> val)
if (val>=0)
sum +=val;
else
numneg++;

char ch;
while (cin.get(ch) && ch != ' ' && ch != '\n')
cout << ch;

sum = 0;
while (cin>>val && val>=0) {
sum += val;
}

char ch;
while (cin.get(ch) && (ch == ' ' || ch == '\n'));

char ch;
while (cin >> ch);

For Statement

for
- Pretest loop
- Usually used for counted looping
- Format
for (init_cntrl; cont_cond; mod_cntrl)
forstatement
- Examples

for (int i=0; i<5; i++)
cout << i;
cout << '\n';

Results in the following being sent to output: 01234

NOTE: This loop is equivalent to:

int i = 0;
while (i<5) {
cout << i;
i++;
}

Note that the loop control variable is incremented at the END of the body of the loop.

for (int cnt=10; cnt>1; cnt--) {
cin >> next;
cout<<"next is "<<next<<'\n';
}

- Scope: A loop control id which is declared in the for line is in scope only in the for loop (per ANSI committee).

Do While Statement

do
- Posttest loop
- Format
do
statement
while (condition);

- Example

do {
cout << '*';
x++; }
while (x < y);

Example 2

#include <iostream.h>

main()
{
/* Find the sums of several pairs of integers and determine whether each sum is positive or negative.*/

float f1, f2, avg;

while (cin >> f1 ) {
    cin >> f2;
    avg = (f1 + f2) / 2;
    cout <<"The average of " << f1 << " and " << f2 << is << avg << '/n';
    if (avg < 0)
      cout << avg << " is negative.\n";
    else
      cout << avg << " is positive.\n";
}
}

Writing loops
- Indent properly
- In as far as is possible, arrange statements within loop to avoid repetition of checking same condition twice.
- ONE ENTRANCE, ONE EXIT (no break or exit)