If/else
So far, everything that is written in the main() function is executed directly and nothing is skipped in our programs.
Oftentimes, though, programmers want to modify the control flow of the program, for example to execute some code only when a certain condition is met.
C and C++ (as it is in almost all the other programming languages) provide us with some syntactic constructs for making conditionals1, the most basic and fundamental of which is the "if statement".
The syntax is as follows:
if (condition) {
// code that is executed only
// if the above condition is met
}
Let's see a simple example.
If​
We'll make a program that prints on the console if a number inserted by the user is positive (just positive), otherwise we'll do nothing:
#include <iostream>
using namespace std;
int main() {
int num;
// get the number from the user
cout << "Insert a number:" << endl;
cin >> num;
// if statement and condition
if (num > 0) {
cout << "The number is positive" << endl;
}
return 0;
}
The condition num > 0 (always inside the round parentheses) is expressed with the > ("greater than") symbol, which as we've seen in a previous chapter, is one of the comparison operators which indicates whether the value of the left operand (num) is greater than the value of the right operand (0).
num > 0 is a boolean expression, meaning that it evaluates to a value that is either true or false. Only if the condition is true the block of code inside the if statement is executed by the compiler. If not, it is completely skipped.
Else​
What if we want to print in our program "The number is negative or null" if the the number inserted is not greater than 0?
What we actually want to do is to execute code when the condition we've written in the if statement returns false. For that, we need the else keyword.
#include <iostream>
using namespace std;
int main() {
int num;
// get the number from the user
cout << "Insert a number:" << endl;
cin >> num;
// if statement and condition
if (num > 0) {
cout << "The number is positive" << endl;
} else { // otherwise
cout << "The number is negative or null" << endl;
}
return 0;
}
Else if and nested if​
A more complex version of else is the else if statement, where we specify and additional condition if the first condition is false. In our program, let's specify the condition where the number is equal to 0:
#include <iostream>
using namespace std;
int main() {
int num;
// get the number from the user
cout << "Insert a number:" << endl;
cin >> num;
// if statement and condition
if (num > 0) {
cout << "The number is positive" << endl;
} else if (num == 0) {
cout << "The number is 0" << endl;
} else { // if not greater or equal to 0 ...
cout << "The number is negative" << endl;
}
return 0;
}
Here, the == operator is used in the conditional expression to check if the value of num is 0.
There can be more multiple else if statements, but only one if and else statements.
Alternatively, it's possible to replicate the same logic by nesting multiple if statements one inside the other, without using the else if statement. This is sometimes useful for readability, but it's important not to exaggerate with conditions and nesting as it can become almost impossible to understand the code.
#include <iostream>
using namespace std;
int main() {
int num;
// get the number from the user
cout << "Insert a number:" << endl;
cin >> num;
// if statement and condition
if (num >= 0) {
if (num == 0) {
cout << "The number is 0" << endl;
} else {
cout << "The number is positive" << endl;
}
} else {
cout << "The number is negative" << endl;
}
return 0;
}
When the block inside the statements is only one code expression (the line of code that ends with ;) like in all the cases above except for the main() function, curly brackets are optional. However, I recommend using them always because they make the block less error-prone to future modifications by you but especially by other programmers. Block of more than one line of code, in fact, always require the use of curly brackets. There's also a problem of ambiguity in nested if-else statements called "dandling else" and by using braces you'll most likely never encounter it.
Multiple conditions​
Inside the round parentheses if the if and else if statements, multiple conditions can also be specified. Each individual condition is connected and put in relationship with the others using the logical operators that we have previously learned.
A useful application of multiple conditions is for intervals/ranges. If we want to check if the value of a variable val is between a certain range we write:
if (n1 < val && val < n2) {
// code that executes if val is between n1 and (&&) n2
}
On the other hand, if we want to check if the variable val is outside the interval we use the || (or) operator in this way:
if (val < n1 || val > n2) {
// code that executes if val is outside the interval n1-n2
}