Greatest Common Divisor in C

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#include <stdio.h>

* Recursive implementation to find the gcd (greatest common divisor) of two integers using the euclidean algorithm.
* For more than two numbers, e.g. three, you can box it like this: gcd(a,gcd(b,greatest_common_divisor.c)) etc.
* This runs in O(log(n)) where n is the maximum of a and b.
* @param a the first integer
* @param b the second integer
* @return the greatest common divisor (gcd) of the two integers.
int gcd(int a, int b) {
    if (b == 0) return a;
    return gcd(b, a % b);

About the algorithm and language used in this code snippet:

Euclidean Greatest Common Divisor (GCD) Algorithm

The greatest common divisor of two numbers (in this case a and b) is the biggest number which both numbers can be divided by without a rest. This greatest common divisor algorithm, called the euclidean algorithm, determines this number. The greatest common divisor is also often abbreviated as gcd.

Description of the Algorithm

The basic principle behind thus gcd algorithm is to recursively determine the gcd of a and b by determining the gcd of b and a % b This hinges on the fact that the gcd of two numbers also divides their difference, e.g. the greatest common divisor of 16 and 24 (which is 8) is also the greatest common divisor of 24-16=8. This is therefore also true for 16 and 40 - in fact, rather than taking the difference, the remainder can also be used (repeatedly recursing on the difference will inevitable “pass” the remainder). In summary, the euclidean gcd algorithm uses these 2 steps:

  1. if a or b is 0, return the other one.
  2. Repeat with the new a as b and the new b as a % b.

Using the remainder has a much faster runtime compared to using the difference.

Example of the Algorithm

When computing the gcd of 1071 and 462, the following steps will be taken:

  1. a is 1071, new b is 462
  2. Recursing with new a = b and new b = a % b…
  3. New a is 462, new b is 147
  4. Recursing with new a = b and new b = a % b…
  5. New a is 147, new b is 21
  6. Recursing with new a = b and new b = a % b…
  7. New a is 21, new b is 0
  8. b is 0, stopping recursion, a is the gcd: 21

Runtime Complexity of the Algorithm

The runtime complexity of the Euclidean greatest common divisor algorithm is O(log(max(a,b))) (the logarithm of the maximum of the two numbers). Using the remainder rather than the difference is considerably faster - if the difference would’ve been used this greatest common divisor algorithm would’ve had a runtime of O(max(a,b))

Space Complexity of the Algorithm

The space complexity of the Euclidean greatest common divisor algorithm is equal to the runtime, since every recursive call is saved in the stack and everything else is constant.


C is a compiled language used for many purposes, although it can be primarily found in systems where importance is important. This is because C offers a lot of low-level support for optimization, at the cost of not having some of the convenient abstractions that other languages offer. C is therefore primarily found in situations where available computation power is low such as embedded systems, or situations where required computation power is high, such as simulation or deep learning.

Getting to “Hello World” in C

The most important things first - here’s how you can run your first line of code in C.

  1. If you’re on Linux or Mac, download and install the latest version of GCC, a C compiler, from You can also download an earlier version if your use case requires it.
  2. If you’re on Windows, you can also install GCC, even though it might cause problems. You also have other options outlined e.g. here.
  3. Open a terminal, make sure the gcc command is working (or the according command for whichever compiler you’re using), and that the command your’re going to be using is referring to the version you just installed by running gcc --version. If you’re getting a “command not found” error (or similar), try restarting your command line, and, if that doesn’t help, your computer. If the issue persists, here are some helpful forum questions for each platform:

  4. As soon as that’s working, copy the following snippet into a file named HelloWorld.c:

    int main() {
    printf("Hello World\n");
    return 0;
  5. Change directory by typing cd path/to/HelloWorld, then run gcc HelloWorld.c to compile the file (which creates the bytecode), then run ./a.out. This should print “Hello World” to your Terminal.

That’s it! People who know multiple programming languages will notice that the entry barrier in C is a little lower than Java even though it is lower-level, while the entry barrier to Python is lower even though it is higher-level. My personal observation is that low-level and high-level languages tend to have low barriers of entry, whereas mid-level languages have higher barriers.

Fundamentals in C

To understand algorithms and technologies implemented in C, one first needs to understand what basic programming concepts look like in this particular language. Each of the following snippets should be compiled and run using the commands mentioned above.

Variables and Arithmetic

Variables in C are statically typed, meaning the content of a variable needs to be specified when writing the code. The datatype for whole numbers, for example is int. Numbers with decimal places are typed float or double depending on the required precision. The type for text ist String.


int main() {
    int number = 5;
    double decimalNumber = 3.25;
    double result = number * decimalNumber;
    char callout [] = "The number is ";
    // In this instance, the values are concatenated rather than added because one of them is a String.
    printf("%s", callout);
    printf("%f", result);
    return 0;


Arrays in C are real arrays (as opposed to e.g. Python where they’re implemented as lists). The implications of that are that the size needs to be set when they are created and cannot be changed, but also that they are more efficient in C than they are in Python. Also, contrary to Java, C does not check array bounds. If you access an index that doesn’t exist, the program will read whatever is in the memory at that location (which will probably be gibberish).

int integers[5];
integers[3] = 12; // Assigning values to positions in the array
printf("%d\n", integers[0]); // will be 0
printf("%d\n", integers[3]); // will be 12
printf("%d\n", integers[6]); // will print something random that happened to be at that location in memory
return 0;


Just like most programming languages, C can do if-else statements. Additionally, C can also do switch-case statements.

int value = 5;
    if(value == 5){
        printf("%s\n", "Value is 5");
    } else if(value < 5){
        printf("%s\n", "Value is less than 5");
    } else {
        printf("%s\n", "Value is something else");
    switch (value){
        case 1:
            printf("%s\n", "Value is 1");
            break; // Don't go further down the cases
        case 2:
            printf("%s\n", "Value is 2");
            break; // Don't go further down the cases
        case 3:
            printf("%s\n", "Value is 3");
            break; // Don't go further down the cases
        case 4:
            printf("%s\n", "Value is 4");
            break; // Don't go further down the cases
        case 5:
            printf("%s\n", "Value is 5");
            break; // Don't go further down the cases
            printf("%s\n", "Value is something else");

The above C code will print “Value is 5” twice.


C supports for, while as well as do while loops. break and continue statements are also supported. The below example illustrates the differences:

int value = 2;
for (int i = 0; i < value; i++) {
    printf("%d\n", i);
while (value > 0) {
    printf("%d\n", value);
do {
    printf("%d\n", value);
} while (value > 0);

This will print the following to the terminal:


Note the last 0: it is printed because in the do-while-loop, compared to the while-loop. the code block is executed at least once before the condition is checked.


Functions in C can be declared similar to Java, but require less boilerplate since they don’t need to be part of classes or objects. Here is a minimal example of a function:


int addNumbers(int numberOne, int numberTwo) {
    return numberOne + numberTwo;

int main() {
    printf("%d\n", addNumbers(3, 4));


C requires the use of curly brackets ({}) to surround code blocks in conditions, loops, functions etc.; It also requires semicolons at then end of statements. While this can lead to some annoying syntax errors, it also means the use of whitespace for preferred formatting (e.g. indentation of code pieces) does not affect the code. Note how the Syntax of C is very similar to Java. The Syntax of Java, and many other languages that came after and/or were derived from C copy many aspects of its Syntax.

Advanced Knowledge of C

C was first released in 1972, is statically typed and was ported to many platforms with various implementations (one of which is GCC which was presented in this article). For more information, C has a great Wikipedia) article.