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 c++-if-statement

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C++ if Statement

  • The if statement evaluates the test expression inside parenthesis.
  • If test expression is evaluated to true, statements inside the body of if is executed.
 if statement
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Syntax

if (testExpression) 
{
   // statements
}
  • The expression is called a conditional expression.
  • The most basic kind of conditional branch in C++ is the if statement. An if statement takes the form:
if (expression)
    statement
or
if (expression)
    statement
else
    statement2
  • Here is a simple program that uses an if statement:
#include <iostream>
 
int main()
{
    std::cout << "Enter a number: ";
    int x;
    std::cin >> x;
 
    if (x > 10)
        std::cout << x << "is greater than 10\n";
    else
        std::cout << x << "is not greater than 10\n";
 
    return 0;
}

Using if with multiple statements - If else statement

  • Note that the if statement only executes a single statement if the expression is true, and the else only executes a single statement if the expression is false.
  • learn c++ tutorials - if else while loop

    learn c++ tutorials - if else loop Example

  • In order to execute multiple statements, we can use a block:
#include <iostream>
 
int main()
{
    std::cout << "Enter a number: ";
    int x;
    std::cin >> x;
 
    if (x > 10)
        {
        // both statements will be executed if x > 10
        std::cout << "You entered " << x << "\n";
        std::cout << x << "is greater than 10\n";
        }
    else
        {
        // both statements will be executed if x <= 10
        std::cout << "You entered " << x << "\n";
        std::cout << x << "is not greater than 10\n";
        }
 
    return 0;
}

Implicit blocks

  • If the programmer does not declare a block in the statement portion of an if statement or else statement, the compiler will implicitly declare one. Thus:
if (expression)
    statement
else
    statement2
is actually the equivalent of:
if (expression)
{
    statement
}
else
{
    statement2
}
  • Most of the time, this doesn’t matter. However, new programmers sometimes try to do something like this:
#include <iostream>
 
void main()
{
    if (1)
        int x = 5;
    else
        int x = 6;
 
    std::cout << x;
 
    return 0;
}
  • This won’t compile, with the compiler generating an error that identifier x isn’t defined. This is because the above example is the equivalent of:
#include <iostream>
 
void main()
{
    if (1)
    {
        int x = 5;
    } // x destroyed here
    else
    {
        int x = 6;
    } // x destroyed here
 
    std::cout << x; // x isn't defined here
 
    return 0;
}
  • In this context, it’s clearer that variable x has block scope and is destroyed at the end of the block.
  • By the time we get to the std::cout line, x doesn’t exist
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Chaining if statements

  • It is possible to chain if-else statements together:
#include <iostream>
 
int main()
{
    std::cout << "Enter a number: ";
    int x;
    std::cin >> x;
 
    if (x > 10)
        std::cout << x << "is greater than 10\n";
    else if (x < 10) 
        std::cout << x << "is less than 10\n";
    else
        std::cout << x << "is exactly 10\n";
 
    return 0;
}

Nesting if statements

  • It is also possible to nest if statements within other if statements :
#include <iostream>
 
int main()
{
    std::cout << "Enter a number: ";
    int x;
    std::cin >> x;
 
    if (x > 10) // outer if statement
        // it is bad coding style to nest if statements </> this way
        if (x < 20) // inner if statement
            std::cout << x << "is between 10 and 20\n";
 
        // which if statement does this else belong to?
        else
            std::cout << x << "is greater than 20\n";
 
    return 0;
}
  • The above program introduces a source of potential ambiguity called a dangling else problem.
  • Is the else statement in the above program matched up with the outer or inner if statement?
  • The answer is that an else statement is paired up with the last unmatched if statement in the same block.
  • Thus, in the program above, the else is matched up with the inner if statement.
  • To avoid such ambiguities when nesting complex statements, it is generally a good idea to enclose the statement within a block.
  • Here is the above program written without ambiguity:
#include <iostream>
 
int main()
{
    std::cout << "Enter a number: ";
    int x;
    std::cin >> x;
 
    if (x > 10)
    {
        if (x < 20)
            std::cout << x << "is between 10 and 20 (inclusive)\n";
        else // attached to inner if statement
            std::cout << x << "is greater than 20\n";
    }
 
    return 0;
}
  • Now it is much clearer that the else statement belongs to the inner if statement.
  • Encasing the inner if statement in a block also allows us to explicitly attach an else to the outer if statement:
#include <iostream>
 
int main()
{
    std::cout << "Enter a number: ";
    int x;
    std::cin >> x;
 
    if (x > 10)
    {
        if (x < 20)
            std::cout << x << "is between 10 and 20 (inclusive)\n";
    }
    else // attached to outer if statement
        std::cout << x << "is less than 10\n";
 
    return 0;
}
  • The use of a block tells the compiler that the else statement should attach to the if statement before the block.
  • Without the block, the else statement would attach to the nearest unmatched if statement, which would be the inner if statement.
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Using logical operators with if statements

  • You can also have if statements check multiple conditions together by using the logical operators :
include <iostream>
 
int main()
{
    std::cout << "Enter an integer: ";
    int x;
    std::cin >> x;
 
    std::cout << "Enter another integer: ";
    int y;
    std::cin >> y;
 
    if (x > 0 && y > 0) // && is logical and -- checks if both conditions are true
        std::cout << "both numbers are positive\n";
    else if (x > 0 || y > 0) // || is logical or -- checks if either condition is true
        std::cout << "One of the numbers is positive\n";
    else
        std::cout << "Neither number is positive\n";
 
    return 0;
}

Common uses for if statements

  • If statements > are commonly used to do error checking. For example, to calculate a square root.
  • the value passed to the square root function should be a non-negative number:
#include <iostream>
#include <cmath> // for sqrt()
 
void printSqrt(double value)
{
    if (value >= 0.0)
        std::cout << "The square root of " << value << " is " << sqrt(value) << "\n";
    else
        std::cout << "Error: " << value << " is negative\n";
}
  • If statements can also be used to do early returns, where a function returns control to the caller before the end of the function.
  • In the following program, if the parameter value is negative, the function returns a symbolic constant or enumerated value error code to the caller right away.
include <iostream>
 
enum class ErrorCode
{
    ERROR_SUCCESS = 0,
    ERROR_NEGATIVE_NUMBER = -1
};
 
ErrorCode doSomething(int value)
{
    // if value is a negative number
    if (value < 0)
       // early return an error code
        return ErrorCode::ERROR_NEGATIVE_NUMBER;
 
    // Do whatever here
 
    return ErrorCode::ERROR_SUCCESS;
}
 
int main()
{
    std::cout << "Enter a positive number: ";
    int x;
    std::cin >> x;
 
    if (doSomething(x) == ErrorCode::ERROR_NEGATIVE_NUMBER)
    {
        std::cout << "You entered a negative number!\n";
    }
    else
    {
        std::cout << "It worked!\n";
    }
 
    return 0;
}
  • If statements are also commonly used to do simple math functionality, such as a min() or max() function that returns the minimum or maximum of its parameters:
int min(int x, int y)
{
    if (x > y)
        return y;
    else
        return x;
}
  • Note that this last function is so simple, it can also be written using the conditional operator (?:):
int min(int x, int y)
{
    return (x > y) ? y : x;
}

Null statements

  • It is possible to omit the statement part of an if statement.
  • A statement with no body is called a null statement, and it is declared by using a single semicolon in place of the statement. For readability purposes, the semicolon of a null statement is typically placed on its own line.
  • This indicates that the use of a null statement was intentional, and makes it harder to overlook the use of the null statement.
if (x > 10)
    ; // this is a null statement
  • While null statements are rarely used in conjunction with if statements intentionally, they sometimes unintentionally cause problems.
  • the following snippet:
if (x == 0);
    x = 1;
  • In the above snippet, the user accidentally put a semicolon on the end of the if statement.
  • This unassuming error actually causes the above snippet to execute like this:
if (x == 0)
    ; // the semicolon acts as a null statement
x = 1; // and this line always gets executed!

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