Dionysia Lemonaki
In the C programming language, you have the ability to control the flow of a program.
In particular, the program is able to make decisions on what it should do next. And those decisions are based on the state of certain pre-defined conditions you set.
The program will decide what the next steps should be based on whether the conditions are met or not.
The act of doing one thing if a particular condition is met and a different thing if that particular condition is not met is called control flow.
For example, you may want to perform an action under only a specific condition. And you may want to perform another action under an entirely different condition. Or, you may want to perform another, completely different action when that specific condition you set is not met.
To be able to do all of the above, and control the flow of a program, you will need to use an if statement.
In this article, you will learn all about the if statement – its syntax and examples of how to use it so you can understand how it works.
You will also learn about the if else statement – that is the else statement that is added to the if statement for additional program flexibility.
In addition, you will learn about the else if statement for when you want to add more choices to your conditions.
Here is what we will cover:
An if statement is also known as a conditional statement and is used for decision-making. It acts as a fork in the road or a branch.
A conditional statement takes a specific action based on the result of a check or comparison that takes place.
So, all in all, the if statement makes a decision based on a condition.
The condition is a Boolean expression. A Boolean expression can only be one of two values – true or false.
If the given condition evaluates to true only then is the code inside the if block executed.
If the given condition evaluates to false , the code inside the if block is ignored and skipped.
The general syntax for an if statement in C is the following:
if (condition) < // run this code if condition is true > Let's break it down:
Next, let’s see a practical example of an if statement.
I will create a variable named age that will hold an integer value.
I will then prompt the user to enter their age and store the answer in the variable age .
Then, I will create a condition that checks whether the value contained in the variable age is less than 18.
If so, I want a message printed to the console letting the user know that to proceed, the user should be at least 18 years of age.
#include int main(void) < // variable age int age; // prompt user to enter their age printf("Please enter your age: "); // store user's answer in the variable scanf("%i", &age); // check if age is less than 18 // if it is, then and only then, print a message to the console if (age < 18) < printf("You need to be over 18 years old to continue\n"); > > I compile the code using gcc conditionals.c , where gcc is the name of the C compiler and conditionals.c is the name of the file containing the C source code.
Then, to run the code I type ./a.out .
When asked for my age I enter 16 and get the following output:
#output Please enter your age: 16 You need to be over 18 years old to continue Then, I re-compile and re-run the program.
This time, when asked for my age, I enter 28 and get the following output:
#output Please enter your age: 28 Well. There is no output.
This is because the condition evaluates to false and therefore the body of the if block is skipped.
I have also not specified what should happen in the case that the user's age is greater than 18.
I could write another if statement that will print a message to the console if the user's age is greater than 18 so the code is a bit clearer:
#include int main(void) < // variable age int age; // prompt user to enter their age printf("Please enter your age: "); // store user's answer in the variable scanf("%i", &age); // check if age is less than 18 // if it is, print a message to the console if (age < 18) < printf("You need to be over 18 years old to continue\n"); > // check if age is greater than 18 // if it is, print a message to the console if (age > 18) < printf("You are over 18 so you can continue \n"); > > I compile and run the code, and when prompted for my age I enter again 28:
#output Please enter your age: 28 You are over 18 so you can continue This code works. That said, there is a better way to write it and you will see how to do that in the following section.
Multiple if statements on their own are not helpful – especially as the programs grow larger and larger.
So, for that reason, an if statement is accompanied by an else statement.
The if else statement essentially means that " if this condition is true do the following thing, else do this thing instead".
If the condition inside the parentheses evaluates to true , the code inside the if block will execute. However, if that condition evaluates to false , the code inside the else block will execute.
The else keyword is the solution for when the if condition is false and the code inside the if block doesn't run. It provides an alternative.
The general syntax looks something like the following:
if (condition) < // run this code if condition is true > else < // if the condition above is false run this code > Now, let's revisit the example with the two separate if statements from earlier on:
#include int main(void) < int age; printf("Please enter your age: "); scanf("%i", &age); if (age < 18) < printf("You need to be over 18 years old to continue\n"); > if (age > 18) < printf("You are over 18 so you can continue \n"); > > Let's re-write it using an if else statement instead:
#include int main(void) < int age; printf("Please enter your age: "); scanf("%i", &age); // if the condition in the parentheses is true the code inside the curly braces will execute // otherwise it is skipped // and the code in the else block will execute if (age < 18) < printf("You need to be over 18 years old to continue\n"); > else < printf("You are over 18 so you can continue \n"); > > If the condition is true the code in the if block runs:
#output Please enter your age: 14 You need to be over 18 years old to continue If the condition is false the code in the if block is skipped and the code in the else block runs instead:
#output Please enter your age: 45 You are over 18 so you can continue But what happens when you want to have more than one condition to choose from?
If you wish to chose between more than one option and want to have a greater variety in actions, then you can introduce an else if statement.
An else if statement essentially means that "If this condition is true, do the following. If it isn't, do this instead. However, if none of the above is true and all else fails, finally do this."
The general syntax looks something like the following:
if (condition) < // if condition is true run this code > else if(another_condition) < // if the above condition was false and this condition is true, // run the code in this block > else < // if the two above conditions are false run this code > Let's see how an else if statement works.
Say you have the following example:
#include int main(void) < int age; printf("Please enter your age: "); scanf("%i", &age); if (age < 18) < printf("You need to be over 18 years old to continue\n"); > else if (age < 21) < printf("You need to be over 21\n"); > else < printf("You are over 18 and older than 21 so you can continue \n"); > > If the first if statement is true, the rest of the block will not run:
#output Please enter your age: 17 You need to be over 18 years old to continue If the first if statement is false, then the program moves on to the next condition.
If that is true the code inside the else if block executes and the rest of the block doesn't run:
#output Please enter your age: 20 You are need to be over 21 If both of the previous conditions are all false, then the last resort is the else block which is the one to execute:
#output Please enter your age: 22 You are over 18 and older than 21 so you can continue And there you have it – you now know the basics of if , if else , and else if statements in C!
I hope you found this article helpful.
To learn more about the C programming language, check out the following free resources:
Thank you so much for reading and happy coding :)
