Function Declaration
In C programming, a function declaration, also known as a function prototype, is a statement that gives the function's name, return type, and parameters to the compiler, but does not include the function body. It tells the compiler what the function looks like without detailing what it does. This is particularly useful in scenarios where multiple files are used in a project, allowing the compiler to handle calls to functions defined in different files.
Here is how a typical function declaration is formatted:
return_type function_name(parameter_type1 parameter1, parameter_type2 parameter2, ...);
The key components of a function declaration include:
-
return_type: Specifies the type of value the function returns. If the function does not return a value, void is used as the return type.
-
function_name: The name of the function, which is used for calling it in other parts of the program.
- parameters: A comma-separated list of input parameters with their data types, enclosed in parentheses. If the function takes no parameters, you can use void or leave it empty.
For example, a function that calculates the maximum of two numbers might be declared as follows:
int max(int num1, int num2);
This declaration tells the compiler that:
-
max is a function returning an integer.
- It expects two integer parameters.
The actual implementation of the max function would be provided elsewhere in the program or in another file, but with this declaration, any part of your program can call max knowing what it expects and what it returns, without needing to see the implementation.
Function declarations are crucial for:
-
Organizing code into more manageable pieces.
-
Allowing teams to work on different functions in separate files simultaneously.
- Enabling the use of libraries where the actual implementations of functions are hidden, but their functionalities are accessible through their declarations.
Function Definition
The function definition in C programming is where the actual body of the function is provided. Unlike the function declaration, which only introduces the function and its signature to the compiler, the function definition includes the complete set of instructions that define what the function does. This is where you write the code that executes when the function is called.
Here's the basic format of a function definition:
return_type function_name(parameter_type1 parameter1, parameter_type2 parameter2, ...) {
// Body of the function
// Statements that execute tasks
return value; // Return a value that matches the return type
}
The components of a function definition are:
-
return_type: This defines the type of data that the function will return to the part of the program that calls it. If the function does not return a value, this is specified as void.
-
function_name: This is the identifier by which the function can be invoked.
-
parameters: These are variables used by the function to receive values or references from the calling part of the program. They are optional; a function might not need any input from the caller.
-
function body: Enclosed in curly braces {}, this section contains all the commands that the function will execute. It ends with a return statement if the function returns a value.
For example, if we define the previously declared max function, it would look like this:
int max(int num1, int num2) {
if (num1 > num2)
return num1;
else
return num2;
}
In this example:
-
The function max takes two integers as input.
-
It checks which of the two is greater.
-
It returns the greater integer.
Function definitions are essential because they are the actual implementations of the behaviors you want to execute. A correctly defined function can be called from anywhere within its scope or from other files if it's declared appropriately in a header file. Writing clear and effective function definitions is an important part of achieving desired behaviors and reusing code efficiently in larger projects.
Function Calls in C
A function call in C programming is how you execute a function that you've either defined or included from a library. When you call a function, you instruct the program to interrupt its current sequence of execution and start executing the code inside the function definition. After the function has been executed, the control returns to the point in the program where the function was called.
Here’s how you generally call a function:
function_name(argument1, argument2, ...);
-
function_name: This is the identifier for the function you want to execute.
- arguments: These are the values or references you pass to the function, corresponding to its parameters. These are used within the function to perform its operations.
For example, using the max function we defined earlier, you would call it like this:
int result = max(10, 20);
Here, max is called with 10 and 20 as arguments. The function will process these inputs and return the greater number, which is 20 in this case. The returned value is then stored in the variable result.
Function calls are powerful because they allow you to:
-
Execute the same code from multiple places in your program without rewriting it.
-
Organize your code into logical blocks, making it easier to read and maintain.
- Reuse code, which reduces errors and saves time.
Effectively using function calls is crucial for creating efficient and manageable code, especially in larger projects where tasks are often repetitive and separated into different functions for clarity and reusability.
Example of a C Function
Let's take a look at a simple example where we'll use the max function we previously discussed. This function will determine the larger of two numbers and return that value.
Here is the complete code for the max function, along with a small program that calls this function:
C
#include <stdio.h> // Standard input/output header
// Function declaration
int max(int num1, int num2);
// Main function where the program execution begins
int main() {
int a = 15;
int b = 20;
int result;
// Call the max function
result = max(a, b);
// Print the result
printf("The maximum of %d and %d is %d\n", a, b, result);
return 0; // Return statement of the main function
}
// Function definition
int max(int num1, int num2) {
if (num1 > num2)
return num1;
else
return num2;
}
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Output
The maximum of 15 and 20 is 20
In this code:
-
We start with including the stdio.h library, which is used for input and output operations. This is necessary for using the printf function.
-
The max function is declared at the beginning. This tells the compiler about the function’s existence before it is actually defined or used.
-
The main function is where the program execution starts. Inside main, we define two integer variables a and b and initialize them with values 15 and 20.
-
The max function is called with a and b as arguments. The returned value from max is stored in the variable result.
-
Finally, we print the result using printf, which shows the maximum of the two numbers on the screen.
This example clearly shows how functions are called in C and how they can be used to perform specific tasks, like comparing two numbers to find the maximum
Function Return Types in C
In C programming, every function is designed to return a value, although it can also return void, which means it does not return anything. The return type of a function is defined at the beginning of the function declaration and definition, indicating the type of data the function will send back to the part of the program that called it.
Here are some common return types in C:
-
int: Returns an integer value.
-
float: Returns a floating-point number.
-
char: Returns a character.
-
double: Returns a double-precision floating-point number.
-
void: Specifies that no value will be returned.
For example, consider a function that calculates the area of a circle. Since the area of a circle is typically a decimal value, you might want to use float or double as the return type:
double circleArea(double radius) {
return 3.14159 * radius * radius; // Pi * r^2
}
In this function:
-
double circleArea(double radius) declares that the function will return a double and takes one double argument.
-
The operation within the function calculates the area by squaring the radius and multiplying by Pi.
-
The result of this calculation is returned to wherever the function was called.
Returning the correct type is crucial for the function's usability in other parts of your program. If a function is expected to return a numerical result for further calculations, returning a void would not be appropriate and would cause a compilation error.
Function Arguments in C
Function arguments, also referred to as parameters, play a crucial role in C programming. They are the means by which information is passed from one part of your program to a function. This allows functions to operate on different data without needing to change the function's code itself.
Here’s a detailed look at how function arguments work:
Types of Arguments
-
Formal Arguments: These are the variables used within the function definition. They act as placeholders that receive values when the function is called. For example, in the function definition int add(int x, int y), x and y are formal arguments.
- Actual Arguments: These are the real values or variables that you pass to the function when you call it. For instance, if you have int sum = add(a, b);, a and b are actual arguments.
How Arguments are Passed
Pass by Value: This is the default method of passing arguments in C. When you pass arguments by value, a copy of the value is made and used inside the function. Changes made to this copy do not affect the original variable outside the function.
void increment(int num) {
num += 1; // Only the copy is incremented
}
Pass by Reference: In this method, instead of passing a copy of the variable, you pass its address. This means that any changes made to the parameter will affect the original variable. This is achieved using pointers in C.
void increment(int *num) {
*num += 1; // The original variable is incremented
}
Example Using Arguments
Let’s see an example that demonstrates the use of arguments in a function to calculate the power of a number:
C
#include <stdio.h>
// Function to calculate the power of a number
double power(double base, int exp) {
double result = 1.0;
for (int i = 0; i < exp; i++) {
result *= base;
}
return result;
}
int main() {
double base = 2.0;
int exponent = 3;
double result = power(base, exponent); // Calling the function with arguments
printf("The result of %.1f raised to the power of %d is %.1f\n", base, exponent, result);
return 0;
}
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Output
The result of 2.0 raised to the power of 3 is 8.0
In this program:
-
power function takes two arguments, a double for the base and an int for the exponent.
-
It calculates the base raised to the exponent using a loop.
- The function is called with actual values 2.0 for the base and 3 for the exponent, demonstrating how arguments are passed and used within the function.
Conditions of Return Types and Arguments in C
In C programming, the behavior of functions can be tailored using various conditions that affect both the return types and the arguments passed. Understanding how to apply these conditions properly ensures that your functions are not only versatile but also robust, catering to the needs of diverse programming scenarios.
Conditional Return Types
A function in C can return different types of data based on certain conditions, although this practice is generally not straightforward. Typically, a function is declared to return a specific type, and trying to return a different type may lead to errors or warnings. However, you can design functions to handle different types of data processing through clever use of structures or pointers, as shown in the following example:
C
#include <stdio.h>
typedef union {
int i;
float f;
} Number;
// Function to return either an integer or a float based on the input flag
Number getNumber(int type) {
Number number;
if (type == 0) {
number.i = 5; // Return an integer
} else {
number.f = 5.5; // Return a float
}
return number;
}
int main() {
Number n = getNumber(0); // Request an integer
printf("Integer: %d\n", n.i);
n = getNumber(1); // Request a float
printf("Float: %.1f\n", n.f);
return 0;
}
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Output
Integer: 5
Float: 5.5
In this program:
-
A union named Number can hold either an integer or a float.
-
The getNumber function uses a type argument to decide whether to return an integer or a float.
- This allows the function to adapt its return type based on input conditions, demonstrating a way to circumvent the fixed return type limitation.
Conditional Arguments
Arguments can also be made conditional by using pointers and optional arguments. A function can inspect the arguments it receives to decide how to process them, which is particularly useful in functions that must handle variable input scenarios:
C
#include <stdio.h>
// Function to display numbers; if count is not provided, it defaults to 1
void displayNumbers(int num, int *count) {
int loopCount = (count == NULL) ? 1 : *count;
for (int i = 0; i < loopCount; i++) {
printf("%d ", num);
}
printf("\n");
}
int main() {
int num = 7;
displayNumbers(num, NULL); // Displays the number once
int count = 3;
displayNumbers(num, &count); // Displays the number three times
return 0;
}
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Output
7
7 7 7
In this example:
-
The displayNumbers function takes an integer and a pointer to another integer.
-
If the pointer is NULL, the number is displayed once.
- If the pointer points to an integer, it displays the number as many times as specified by the integer to which the pointer points.
How Does a C Function Work?
Function in C operates from the moment it is called to when it completes its execution. This process helps in designing functions that are efficient and fit well within the overall structure of your program.
Function Execution Flow
-
Calling the Function: The execution of a program enters a function when it is called from another part of the program. The call is made by specifying the function name followed by parentheses, which may include arguments.
-
Passing Arguments: If the function requires arguments, these are evaluated and passed to the function at the time of the call. In C, arguments are generally passed by value, which means a copy of the value is made and used inside the function.
-
Executing Function Body: Once the function is called and arguments are passed, the control of the program moves to the first line inside the function body. The code inside the function is then executed sequentially.
-
Returning a Value: After the function body has been executed, the function may return a value to the calling location using a return statement. This value must match the return type specified in the function declaration.
- Return to Calling Location: After the return statement is executed, or when the end of the function is reached, the control of the program returns to the point immediately following where the function was called.
Example: Function Workflow
Here’s a simple example to show the workflow of a function in C that calculates the square of a number:
C
#include <stdio.h>
// Function declaration
int square(int x);
// Main function
int main() {
int number = 4;
int result = square(number); // Function call
printf("The square of %d is %d\n", number, result);
return 0;
}
// Function definition
int square(int x) {
return x * x; // Returns the square of the passed number
}
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Output
The square of 4 is 16
In this example:
-
The square function is defined to take an integer as input and return its square.
-
The function is called in the main function with the number 4.
-
Inside square, the calculation is performed, and the result is returned back to main, where it is printed.
The example clearly demonstrates the function call, argument passing, execution, and return process, which are fundamental aspects of how functions operate in C.
Library Functions in C
Library functions in C are pre-written code stored in libraries that you can include and use in your programs. These functions cover a wide range of operations, from input/output processing to complex mathematical calculations, allowing programmers to implement functionalities without having to code them from scratch.
Common Library Functions
Here’s a look at some widely used library functions in C and the headers they are found in:
-
printf() and scanf() from <stdio.h>: These are used for output and input operations.
-
sqrt(), pow(), and abs() from <math.h>: These perform mathematical calculations such as square root, power, and absolute value.
- strcpy(), strcat(), and strlen() from <string.h>: These are used for string manipulation tasks.
Advantages of Using Library Functions
-
Efficiency: These functions are optimized for performance, ensuring that your program runs efficiently.
-
Reliability: Since they are thoroughly tested, you can rely on these functions to perform their tasks without errors.
-
Save Time: Using these functions saves time as you don’t need to develop and test code for common tasks.
-
Standardization: Library functions help standardize your code, making it easier to understand and maintain by other programmers familiar with the standard C libraries.
-
How to Use a Library Function
To use a library function, you must include the appropriate library header at the beginning of your C program with an #include statement. Here’s how you might use some of the functions:
C
#include <stdio.h>
#include <math.h>
#include <string.h>
int main() {
// Using printf and scanf
int number;
printf("Enter a number: ");
scanf("%d", &number);
// Using sqrt from math.h
double squareRoot = sqrt((double)number);
printf("Square root: %.2f\n", squareRoot);
// Using strcpy and strcat from string.h
char text[100] = "Hello";
char moreText[] = " World";
strcat(text, moreText);
printf("Concatenated Text: %s\n", text);
return 0;
}
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Output
Enter a number: 5
Square root: 2.24
Concatenated Text: Hello World
In this example:
-
scanf() and printf() are used for reading an integer from the user and printing values.
-
sqrt() is used to calculate the square root of the number.
- strcat() concatenates two strings together.
User-Defined Functions in C
User-defined functions are custom functions that programmers create to perform specific tasks within their C programs. These functions are tailored to solve particular problems, enhance readability, and simplify complex code by breaking it down into smaller, more manageable pieces.
Creating User-Defined Functions
To create a user-defined function, you need to define it with a specific syntax that includes a return type, a function name, and any necessary parameters. Here’s the basic structure:
return_type function_name(parameter_type1 parameter1, parameter_type2 parameter2, ...) {
// Body of the function
return value; // This must match the return type
}
-
return_type: Specifies what type of data the function will return. It can be any data type like int, float, char, or void if no value is returned.
-
function_name: The name of the function, which is used to call it within other parts of the program.
- parameters: Optional inputs the function uses to perform its operations. Each parameter is listed with its type.
Example of a User-Defined Function
Let's consider a simple example where we create a function to calculate the average of three numbers:
C
#include <stdio.h>
// Function declaration
double average(int num1, int num2, int num3);
// Main function
int main() {
int x = 10, y = 20, z = 30;
double result = average(x, y, z); // Calling the user-defined function
printf("Average is: %.2f\n", result);
return 0;
}
// Function definition
double average(int num1, int num2, int num3) {
return (num1 + num2 + num3) / 3.0; // Calculating the average
}
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Output
Average is: 20.00
In this code:
-
average is a function that takes three integers and returns their average as a double.
-
The function is called in the main function where the integers x, y, and z are passed as arguments.
- The result is then printed to the console.
Advantages of User-Defined Functions
-
Modularity: They make the program modular, making it easier to test, maintain, and debug.
-
Reusability: Functions can be reused across different parts of a program or even in different programs, which saves time and effort.
- Simplification of Complex Tasks: By dividing a large, complex task into smaller sections, user-defined functions make the program easier to understand and manage.
Recursive Functions in C
Recursive functions are a type of user-defined function in C that call themselves from within their own code. They are particularly useful for solving problems that can be broken down into smaller, similar problems. This method of programming is commonly used in tasks involving iterative processes such as sorting, searching, and traversing complex data structures like trees and graphs.
How Recursive Functions Work
A recursive function continues to call itself until it reaches a base case or a condition that stops further recursion. Without this stopping condition, the function would continue to call itself indefinitely, leading to a stack overflow error.
Basic Structure of a Recursive Function
Here's the general structure of a recursive function:
return_type function_name(parameters) {
if (base_case_condition) {
return base_case_value; // Stop recursion
} else {
function_name(modified_parameters); // Recursive call
}
}
-
base_case_condition: Determines when the recursion should stop.
-
base_case_value: The value returned when the recursion stops.
- modified_parameters: Adjusted parameters for the recursive call, ensuring progress towards the base case.
Example of a Recursive Function
Let's look at a simple example of a recursive function that calculates the factorial of a number:
C
#include <stdio.h>
// Recursive function to calculate factorial
int factorial(int n) {
if (n == 0) { // Base case: factorial of 0 is 1
return 1;
} else { // Recursive case
return n * factorial(n - 1);
}
}
int main() {
int num = 5;
printf("Factorial of %d is %d\n", num, factorial(num));
return 0;
}
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Outputs:
Factorial of 5 is 120
In this code:
-
The factorial function takes an integer n.
-
If n is 0 (the base case), it returns 1.
- If n is not 0, the function calls itself with n-1, multiplying the result by n until it reaches the base case.
Advantages of Recursive Functions
-
Simplicity: Recursive functions can make the code simpler and clearer, especially for algorithms that naturally fit recursion, such as tree traversals.
-
Reduction of Code: Often, recursive solutions require fewer lines of code than their iterative counterparts.
- Direct Mapping to Problem Statement: Many mathematical problems are defined recursively, making recursive functions a direct translation of the problem statement into code.
Passing Parameters to Functions in C
Passing parameters to functions is a fundamental concept in C programming, allowing you to provide inputs that the functions use to perform their tasks. There are two primary ways to pass parameters in C: pass by value and pass by reference. Understanding both methods is crucial for effective programming, as it affects how functions interact with data.
Pass by Value
In the pass by value method, a copy of the actual parameter's value is passed to the function. Changes made to this parameter inside the function do not affect the original variable outside the function. This method is used when you want to ensure that the original data is not altered.
Example of Pass by Value:
C
#include <stdio.h>
// Function to modify the value
void modifyValue(int a) {
a = 10; // This change will not affect the original value
printf("Value inside function: %d\n", a);
}
int main() {
int x = 5;
modifyValue(x);
printf("Original value after function call: %d\n", x); // Outputs: 5
return 0;
}
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Output
Value inside function: 10
Original value after function call: 5
In this example:
-
x is passed to modifyValue by value.
- Changes inside modifyValue do not impact the value of x in main.
Pass by Reference
Pass by reference involves passing the address of the variable (a pointer) rather than a copy of the variable itself. Any changes made to the parameter inside the function will affect the original variable, as both the parameter and the original variable refer to the same memory location.
Example of Pass by Reference:
C
#include <stdio.h>
// Function to modify the value
void modifyReference(int *a) {
*a = 10; // This change will affect the original value
printf("Value inside function: %d\n", *a);
}
int main() {
int x = 5;
modifyReference(&x);
printf("Original value after function call: %d\n", x); // Outputs: 10
return 0;
}
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Output
Value inside function: 10
Original value after function call: 10
In this example:
-
The address of x is passed to modifyReference.
- Changes inside modifyReference directly affect x because a points to x.
Advantages of Each Method
Pass by Value
-
Safeguards the original data from unintended changes.
- Simple and straightforward for basic data types.
Pass by Reference
-
Efficient for passing large data structures like arrays or complex objects since it avoids copying large amounts of data.
- Allows functions to modify the original data, which is useful for updating values and working with data structures.
Advantages of Functions in C
Code Reusability
One of the primary advantages of using functions is code reusability. Functions allow you to encapsulate a piece of code that performs a specific task and reuse it throughout your program. This reduces duplication, which not only saves time and effort but also decreases the chance of errors since the function's code is tested and debugged once.
Example
C
#include <stdio.h>
// Function to calculate the square of a number
int square(int num) {
return num * num;
}
int main() {
printf("Square of 4 is %d\n", square(4));
printf("Square of 5 is %d\n", square(5));
return 0;
}
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Output
Square of 4 is 16
Square of 5 is 25
In this example, the square function is defined once and used multiple times to calculate the square of different numbers.
Improved Code Organization
Functions help organize code into logical blocks. Each function is designed to perform a specific task, making the program easier to understand and manage. Well-organized code is also easier to test and debug, as each function can be independently verified.
Easier Maintenance
When a program is divided into functions, making changes or updates can be easier and less risky. If a change is required, you can modify a specific function without affecting other parts of the program. This modular approach significantly simplifies maintenance and updates.
Parameter Passing
Functions enable the passing of parameters, which means you can execute the same code with different values. This flexibility makes functions powerful tools for performing repetitive tasks with varying inputs.
Example
C
#include <stdio.h>
// Function to add two numbers
int add(int a, int b) {
return a + b;
}
int main() {
printf("Sum of 10 and 20 is %d\n", add(10, 20));
printf("Sum of 30 and 40 is %d\n", add(30, 40));
return 0;
}
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Output
Sum of 10 and 20 is 30
Sum of 30 and 40 is 70
Here, the add function is used to add different pairs of numbers, demonstrating how parameters are passed and used within functions.
Data Hiding
Functions promote data hiding as they can be designed to manipulate data internally without exposing it to the rest of the program. This encapsulation safeguards the data, making the functions and the program more robust and secure.
Reduced Complexity
By breaking down a problem into smaller chunks handled by individual functions, the overall complexity of the program is reduced. This makes complex problems more manageable and solutions more straightforward to implement.
Scalability
Programs that use functions are generally more scalable, as additional functionality can be added more smoothly. New functions can be created to handle new features without altering the existing code structure significantly.
Disadvantages of Functions in C
Overhead
Functions can introduce additional overhead, especially in terms of execution time and memory usage. Every time a function is called, the program must save the current state of execution (like local variables and the program counter), set up the environment for the function (passing arguments and allocating space for local variables), and then restore the original state upon completion. This process can slow down the program, particularly if the function is called repeatedly in a loop.
Complexity in Debugging
Although functions are useful for organizing code, they can sometimes make debugging more challenging. If a function is buggy or produces unexpected results, identifying the source of the problem can be difficult, especially if the function interacts with external variables or other functions in complex ways.
Improper Use
Misusing functions, such as creating functions that are too long, too complex, or poorly designed, can lead to code that is just as difficult to manage as not using functions at all. Functions should be concise and focused on a single task to maintain clarity and effectiveness.
Stack Overflow
Recursive functions, if not carefully managed with proper base cases, can lead to stack overflow errors. Since each recursive call consumes stack space, excessive recursion can exhaust the available stack, leading to program crashes. This is particularly critical in environments with limited stack size.
Dependency Issues
Functions that rely heavily on external or global variables can lead to code that is hard to understand and maintain. Such dependencies can also make functions less reusable in different contexts, as the external dependencies need to be replicated.
Testing Challenges
While individual functions can be easier to test, ensuring that all possible interactions between multiple functions are tested can be challenging. Functions with side effects (functions that modify some state outside their scope) can particularly complicate testing and debugging.
Increased Development Time
While writing functions can save time in the long run due to reusability, the initial development time can be higher. Designing a good function interface, implementing the function, and testing it thoroughly requires time and effort.
Frequently Asked Questions
What is the difference between passing parameters by value and by reference?
When passing by value, a copy of the variable is sent to the function, so the original variable is not modified. When passing by reference, a reference (pointer) to the original variable is passed, allowing the function to modify the original variable.
How can I prevent memory leaks in C functions?
To prevent memory leaks, always ensure that for every allocation of memory with malloc or calloc, there is a corresponding free call. Also, keep track of all memory allocations and deallocations meticulously.
Why use recursive functions, and what are the risks?
Recursive functions are used for solving problems that can be divided into smaller, similar problems, like tree traversals or sorting algorithms. The main risk is stack overflow if the recursion goes too deep or if there is no proper termination condition.
Conclusion
In this article, we have learned about the fundamentals of functions in C programming. We covered the syntax of functions, including the return type, function name, parameters, & function body. We also discussed how to declare functions using function prototypes & how to define functions with their actual implementation.
Furthermore, we explored how to call functions from other parts of your code, passing arguments as needed. We looked at examples of functions with & without return values, as well as how to use function calls in various contexts like conditionals & loops.
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