if statement can conditionally execute code, and is a frequently used statement in programming. The execution depends on Boolean expressions. Such as in the following example, When the value of num is less than 5, the statement will be executed and print "The number is less than 5."
// if.cpp
int num = 2;
// some operations may change num's value
if (num < 5)
cout << "The number is less than 5." << endl;If it is a compound statement to be executed, the compound statement should be brace-enclosed. Even if it is a one-line statement, it can also be brace-enclosed.
if (num == 5 )
{
cout << "The number is 5." << endl;
//some other lines
}else statement will be executed when the condition follows if is not true.
if (num == 5 )
{
cout << "The number is 5." << endl;
}
else
{
cout << "The number is not 5." << endl;
}If there are multiple conditions to be evaluated, you can use else if as follows. The first condition num < 5 will be evaluated first, and then the second condition num > 10. If no matched conditions, the else statement will be executed. It should be noted that if the first condition is true, the second condition in else if will not be evaluated.
if (num < 5)
cout << "The number is less than 5." << endl;
else if (num > 10)
cout << "The number is greater than 10." << endl;
else
cout << "The number is in range [5, 10]." << endl;else will be associated with the closest if. In the following code, else is associated with if(num < 5), not if(num < 10). Even no error is in the following code, some compilers, which are smart enough, will also give some warning messages since it may confuse readers.
if(num < 10)
if(num < 5)
cout << "The number is less than 5" << endl;
else
cout << "Where I'm?" << endl;To make the source code easier to understand, I suggest using braces and indents to format the source code as follows.
if(num < 20)
{
if(num < 5)
cout << "The number is less than 5" << endl;
else
cout << "Where I'm?" << endl;
}The ternary conditional operator ? : is also widely used to simplify some if else statements. Let us look at the following code first. The variable factor will be assigned different values depending on the condition of isPositive.
//ternary.cpp
bool isPositive = true;
int factor = 0;
//some operations may change isPositive's value
if(isPositive)
factor = 1;
else
factor = -1;There are 4 lines of source code in the previous program for if else. The 4 lines of if else statement can be replaced by one line using a ternary condition operator ? :.
factor = isPositive ? 1 : -1;If the condition before ? is true, the value will be the expression between ? and :. Otherwise, the value will be the expression after :.
The previous example can even be simplified to an expression without a jump statement. In the instructions compiled from if, ? :, for, while and some others, there are jump statements. Different from them, the following code does not contain jumps. Since isPositive is Boolean and can be converted to int implicitly, so the conditional assignment by if else is converted a single assignment. It may be more efficient, at least not less efficient than if else. Sometimes our C/C++ compilers may optimize the generated binary executive code by converting if else to the following style statement. Anyway, not all ? : statements can be converted to the following style statement.
factor = (isPositive) * 2 - 1;There is a conditional expression following if or else if, such as num < 5 in the following example. It should be an expression that is convertible to bool. It can be a relational expression, a logical expression or a combination of them.
if (num < 5)
cout << "The number is less than 5. " << endl;There are 6 relational (comparison) operators as listed as follows. The value of a relational expression will be true or false. You can also regard them as 1 or 0. Even in some programming languages, such as in Java, true is different from 1``, here in C or C++, you can think trueis equivalent to1andfalseis0`.
| Operation name | Example |
|---|---|
| equal to | a == b |
| not equal to | a != b |
| less than | a < b |
| greater than | a > b |
| less than or equal to | a <= b |
| greater than or equal to | a >= b |
There are 3 logical operators as listed in the following table. In logical expressions, the operands are expected to be Boolean. If they are not, they will be converted to bool implicitly.
| Operator name | Symbol-like operator | keyword-like operator | Example |
|---|---|---|---|
| negation | ! |
not | !a, not a |
| AND | && |
and | a && b, a and b |
| Inclusive OR | ` | ` |
It should be noted that the precedences of the 3 logical operators are different. not operator has the highest precedence, then and, and or has the lowest precedence among them.
If the conditional expressions in if statements are not Boolean, they will be converted to bool implicitly. It is the same in the conditions of for, while and switch statements. If the value of the conditional expression is char, int, long, float or double, non-zero values will be converted to true, and the rest values will be false.
The following example is correct in grammar. But it is not recommended. A condition of count == 0 or (count < FLT_EPSILON) && (count > -FLT_EPSILON) will be better.
float count = 0.2f;
if (count) // not recommend to use a float-point number
cout << "There are some." << endl;A pointer, which is an unsigned integer actually, can also be used in a conditional expression. (!p) in the following example is equivalent to (p == NULL).
int * p = new int[1024];
if (!p) // if(p == NULL)
cout << "Memory allocation failed." << endl;Whereas in a if statement, a while loop or a for loop, you have to be careful enough not to misspell == as = in conditional expressions. It is a common error among beginners. The following loop is an endless loop.
bool flag = false;
// Do something ...
if (flag = true) // error!
{
// Do something
}There are many kinds of expressions, such as 3 + 4, a + b and a == b. They all have value. a = b is also an expression, and its value is a, which is assigned the value of b. So if(flag = true) is equivalent to if(true), the body will be executed whatever the value of flag is.
If you can understand that a = b is an expression and has a value, then it is also easy to understand int m = (b = 8); in the following code. It means m will be assigned as the value of `(b = 8)
int b = 0;
int m = (b = 8);The syntax of the while loop is as follows. If the conditional expression is true, the loop body will be executed.
while( expression )
{
//...
}In the following example, the variable num is initialized to 10. The condition num > 0 is true since num is 10. Then the loop body is executed, and num becomes 9 after executing num--. The loop body will be executed 10 times until num is 0.
// while.cpp
#include <iostream>
using namespace std;
int main()
{
int num = 10;
while(num > 0)
{
cout << "num = " << num << endl;
num--;
}
}The test takes place before each iteration in a while loop. There is a do-while loop, and the test takes place after each iteration in it. The following do-while loop behavior is the same as the previous while loop.
int num = 10;
do
{
cout << "num = " << num << endl;
num--;
}while (num > 0);But be careful that the loop body will be executed in the first iteration even the conditional expression is not true. Such as the following example will print a line.
int num = 0; // the condition is false
do
{ //but the body will be execuated in the 1st iteration
cout << "num = " << num << endl;
num--;
}while (num > 0);The output of the previous example.
num = 0The break statement will terminate a loop when it is executed. The remaining part of the loop body will not be executed also.
int num = 10;
while (num > 0)
{
if (num == 5)
break;
cout << "num = " << num << endl;
num--;
}
// jump to here after `break` is executedThe continue statement will also change the routine of the loop, but it will not terminate the loop. The program will skip the remaining part of the loop body and continue the next iteration. In the following example, num = 5 will not be printed, but num = 6 and num = 4 will.
int num = 10;
while (num > 0)
{
if (num == 5)
continue; // "num = 5" will not be printed
cout << "num = " << num << endl;
num--;
}
// jump to here after `break` is executedThe syntax of the for loop is as follows. There are an initialization clause, a conditional expression and an iteration expression following the keyword for.
for (init-clause; cond-expression; iteration-expression)
loop-statementThe following code demonstrates the usage of for. The variable i is initialized to 1. The value of i will increase 1 by i++ in each iteration. The iteration will continue until the condition i < 10 becomes false.
int sum = 0;
for(int i = 0; i < 10; i++)
{
sum += i;
cout << "Line " << i << endl;
}
cout << "sum = " << sum << endl;The previous for loop can be converted to a while loop as follows. The only difference between the two pieces of code is the scope of i. The scope of i is inside the for loop body in the previous example. But it is outside of the while loop body since it is declared outside of the loop body.
int sum = 0;
int i = 0;
while(i < 10)
{
sum += i;
cout << "Line " << i << endl;
i++
}
cout << "sum = " << sum << endl;A while loop can also be converted to a for loop as shown in the following example.
while(num > 0)
{
cout << "num = " << num << endl;
num--;
}for(; num > 0; )
{
cout << "num = " << num << endl;
num--;
}Some may be curious that the previous for loop has no initialization clause and iteration expression. The truth is that even the conditional expression can be omitted as follows.
for(; ; )
{
if (num > 0)
break;
cout << "num = " << num << endl;
num--;
}The behaviors of break and continue are the same as those in the while loop. The details will not be presented here again.
The goto statement can make the program jump to the desired location. Too many goto statements may break the flow of execution. It is an unrecommended statement. The most frequently used place is for error handling. In the function mysqure(), a label EXIT_ERROR is declared. If the argument cannot meet the requrement, the program will use goto to jump to location EXIT_ERROR where are the error handling source code.
The goto statement is widely used in Linux kernel functions, which are in the C language. Those functions normally exit from multiple locations, and some common work such as cleanup has to be done. But in C++ exceptions are more commonly used error handling which is introduced in a later chapter.
//goto.cpp
float mysquare(float value)
{
float result = 0.0f;
if(value >= 1.0f || value <= 0)
{
cerr << "The input is out of range." << endl;
goto EXIT_ERROR;
}
result = value * value;
return result;
EXIT_ERROR:
//do sth such as closing files here
return 0.0f;
}The switch statement can execute one of several statements depending on the value of an expression. The expression follows the keyword switch, and it is input_char in the following example. Some beginners may forget break, which prevents executing the following statements. If switch is understood in the if - else if - else manner, break is easy to omit. Actually, you can think there are many entrances defined by case, and switch works like goto. The execution will start from a matched entrance defined by case, continue until reaching break.
//switch.cpp
switch (input_char)
{
case 'a':
case 'A':
cout << "Move left." << endl;
break;
case 'd':
case 'D':
cout << "Move right." << endl;
break;
default:
cout << "Undefined key." << endl;
break;
}- Please use a Makefile to manage multiple source files. You can use
maketo compile and link, andmake cleanto remove all generated files.