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Branches

By default, a program is executed following the order of code. Sometimes when certain statements need to be selectively executed, we can use the the branch feature to achieve such flow control. Choosing the appropriate branch statement can improve the efficiency of the program.

if statement

Basic if statement

The following code shows the structure of a basic if statement.

if (condition) {
  main body;
}

The if statement calculates the condition to check: if the result is true (nonzero), the statement is executed, otherwise it is not executed.

If there is only a single statement in the body, the curly braces can be omitted.

if...else statement

if (condition) {
  statement1;
} else {
  statement2;
}

The if...else statement is similar to the if statement, and the else does not need to write more conditions. When the condition of the if statement is satisfied, the statement in the if will be executed. Otherwise, the statement in else will be executed. Similarly, the curly braces can be omitted when the main part has only one statement.

else if statement

if (condition1) {
  statement1;
} else if (condition2) {
  statement2;
} else if (condition3) {
  statement3;
} else {
  statement4;
}

The else if statement is a combination of if and else, which checks multiple conditions and selects different statement branches. There is no need to write conditions in the last else statement. For example, if condition 1 is true, execute the statement 1; if condition 3 is true and both conditions 1 and 2 are false, statement 3 is executed, and statement 4 is executed when all conditions are false.

In fact, this statement is equivalent to the else clause of the first if with only one if statement, so the curly braces are omitted and placed together. If the conditions are parallel to each other, writing this way can make the code logic clearer.

Logically, it is roughly equivalent to this passage:

When solving a quadratic equation in one variable, the relationship between the roots of the equation and the discriminant:

• If ( ) The equation has no solution;
• Otherwise, if ( ) The equation has two identical real number solutions;
• Otherwise The equation has two different real number solutions;

switch statement

switch (condition) {
  case label1:
    statement1;
  case label2:
    statement2;
  default:
    statement3;
}

When the switch statement is executed, the value is selected first, and then the corresponding label is selected according to the value of the choice sentence. And the execution starts from that label. Among them, the choice expression must be an integer type expression, and the label must be an integer type constant. E.g:

int i = 1;  // the data type of i here is an integer, which satisfies the requirements of an expression of integer type
switch (i) {
  case 1:
    cout << "OI WIKI" << endl;
}

char i = 'A';
// the data type of i here is character, but it is a kind of integer in
// nature and satisfies the requirement of an integer-typed expression
switch (i) {
  case 'A':
    cout << "OI WIKI" << endl;
}

The switch statement should be interrupted by adding the break according to the requirements, otherwise all the statements in the following case and the statement in the default will be executed after the corresponding case is selected. Please refer to the code for specific examples.

char i = 'B';
switch (i) {
  case 'A':
    cout << "OI" << endl;
    break;

  case 'B':
    cout << "WIKI" << endl;

  default:
    cout << "Hello World" << endl;
}

If you don't want the following branch statements to be executed, the break is required. For specific examples, please refer to the following code.

char i = 'B';
switch (i) {
  case 'A':
    cout << "OI" << endl;
    break;

  case 'B':
    cout << "WIKI" << endl;
    break;

  default:
    cout << "Hello World" << endl;
}

The output of above code is "WIKI", because the following statements will not continue to be executed for the existence of break statement. The default statement does not need a break, because there is no statement below.

It is optional to add curly braces in the case clause of switch. But note that the curly braces must be added if you need to define a variable in the switch statement. E.g:

char i = 'B';
switch (i) {
  case 'A': {
    int i = 1, j = 2;
    cout << "OI" << endl;
    ans = i + j;
    break;
  }

  case 'B': {
    int qwq = 3;
    cout << "WIKI" << endl;
    ans = qwq * qwq;
    break;
  }

  default: { cout << "Hello World" << endl; }
}

How to understand switch

In the article above, a lot of terms such as "case clause" and "case sub-clause" are used. In fact, in the underlying implementation, switch is equivalent to a set of jump statements. For this reason, clever techniques like Duff's Device exists. You can dig deeper about that if you are interested.

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