Everything Basics of C

Question 1

If you don’t want others (or yourself) to change existing variable values, you can use the keyword.

This will declare the variable as “constant”, which means unchangeable and read-only

Answer 1

const

Question 2

When you declare a constant variable, it must be assigned with a value

Answer 2

const int myNum = 15; // myNum will always be 15
myNum = 10; // error: assignment of read-only variable ‘myNum’

Question 3

In C, the bool type is not a built-in data type, like int or char.

It was introduced in C99, and you must import the following header file to use it:

Answer 3

include <stdbool.h></stdbool.h>

Question 4

Use the if statement to specify a block of code to be executed if a condition is true

Answer 4

if (condition1) {
// block of code to be executed if condition1 is true
} else if (condition2) {
// block of code to be executed if the condition1 is false and condition2 is true
} else {
// block of code to be executed if the condition1 is false and condition2 is false
}

Question 5

Short hand if else

Answer 5

variable = (condition) ? expressionTrue : expressionFalse
int time = 20;
(time < 18) ? printf(“Good day.”) : printf(“Good evening.”);

Question 6

Instead of writing many if..else statements, you can use the .

The switch statement selects one of many code blocks to be executed

Answer 6

Switch Statements
switch (expression) {
case x:
// code block
break;
case y:
// code block
break;
default:
// code block
}

Question 7

The switch expression is evaluated once
The value of the expression is compared with the values of each case
If there is a match, the associated block of code is executed
The … statement breaks out of the switch block and stops the execution
The … statement is optional, and specifies some code to run if there is no case match

Answer 7

Break
default

Question 8

The … loop loops through a block of code as long as a specified condition is true

Answer 8

while
while (condition) {
// code block to be executed
}
int i = 0;

while (i < 5) {
printf(“%d\n”, i);
i++;
}

Question 9

The … loop is a variant of the while loop. This loop will execute the code block once, before checking if the condition is true, then it will repeat the loop as long as the condition is true

Answer 9

do/while
do {
// code block to be executed
}
while (condition);
int i = 0;

do {
printf(“%d\n”, i);
i++;
}
while (i < 5);

Question 10

for (expression 1; expression 2; expression 3) {
// code block to be executed
}

Answer 10

Expression 1 is executed (one time) before the execution of the code block.

Expression 2 defines the condition for executing the code block.

Expression 3 is executed (every time) after the code block has been executed

Question 11

It is also possible to place a loop inside another loop. This is called a nested loop.

The “inner loop” will be executed one time for each iteration of the “outer loop”

Answer 11

int i, j;

// Outer loop
for (i = 1; i <= 2; ++i) {
printf(“Outer: %d\n”, i); // Executes 2 times

// Inner loop
for (j = 1; j <= 3; ++j) {
printf(“ Inner: %d\n”, j); // Executes 6 times (2 * 3)
}
}

Question 12

The continue statement breaks one iteration (in the loop), if a specified condition occurs, and continues with the next iteration in the loop

Answer 12

int i;

for (i = 0; i < 10; i++) {
if (i == 4) {
continue;
}
printf(“%d\n”, i);
}

Question 13

You can also use break and continue in while loops

Answer 13

int i = 0;

while (i < 10) {
if (i == 4) {
break;
}
printf(“%d\n”, i);
i++;
}
Continue Example
int i = 0;

while (i < 10) {
if (i == 4) {
i++;
continue;
}
printf(“%d\n”, i);
i++;
}

Question 14

are used to store multiple values in a single variable, instead of declaring separate variables for each value.

To create an array, define the data type (like int) and specify the name of the array followed by square brackets []

Answer 14

Arrays

Question 15

int myNumbers[] = {25, 50, 75, 100};
myNumbers[0] = 33;

printf(“%d”, myNumbers[0]);

Answer 15

Change an array element

Question 16

int myNumbers[] = {25, 50, 75, 100};
int i;

for (i = 0; i < 4; i++) {
printf(“%d\n”, myNumbers[i]);
}

Answer 16

You can loop through the array elements with the for loop.

Question 17

int myNumbers[] = {10, 25, 50, 75, 100};
printf(“%lu”, sizeof(myNumbers)); // Prints 20

Answer 17

Get Array Size or Length

Question 17

It is because the sizeof operator returns the size of a type in bytes

Question 18

A 2D array is also known as a … (a table of rows and columns)

Answer 18

matrix

Question 18

int matrix[2][3] = { {1, 4, 2}, {3, 6, 8} };

Answer 18

A 2D array

Question 19

int matrix[2][3] = { {1, 4, 2}, {3, 6, 8} };

printf(“%d”, matrix[0][2]); // Outputs 2

Answer 19

Access the Elements of a 2D Array

Question 20

int matrix[2][3] = { {1, 4, 2}, {3, 6, 8} };

int i, j;
for (i = 0; i < 2; i++) {
for (j = 0; j < 3; j++) {
printf(“%d\n”, matrix[i][j]);
}
}

Answer 20

int matrix[2][3] = { {1, 4, 2}, {3, 6, 8} };

int i, j;
for (i = 0; i < 2; i++) {
for (j = 0; j < 3; j++) {
printf(“%d\n”, matrix[i][j]);
}
}

Question 21

int matrix[2][3] = { {1, 4, 2}, {3, 6, 8} };
matrix[0][0] = 9;

printf(“%d”, matrix[0][0]);

Answer 21

Change Elements in a 2D Array

Question 22

char greetings[] = “Hello World

Answer 22

Strings

Question 23

char greetings[] = “Hello World!”;
printf(“%c”, greetings[0])

Answer 23

Access Strings
Since strings are actually arrays in C, you can access a string by referring to its index number inside square brackets []

Question 24

char greetings[] = “Hello World!”;
greetings[0] = ‘J’;
printf(“%s”, greetings);

Answer 24

Modify Strings

Question 25

char carName[] = “Volvo”;
int i;

for (i = 0; i < 5; ++i) {
printf(“%c\n”, carName[i]);
}

Answer 25

Loop Through a String

Question 26

char carName[] = “Volvo”;
int length = sizeof(carName) / sizeof(carName[0]);
int i;

for (i = 0; i < length; ++i) {
printf(“%c\n”, carName[i]);
}

Answer 26

Loop Through a String

Question 27

char greetings[] = {‘H’, ‘e’, ‘l’, ‘l’, ‘o’, ‘ ‘, ‘W’, ‘o’, ‘r’, ‘l’, ‘d’, ‘!’, ‘\0’};
char greetings2[] = “Hello World!”;

printf(“%lu\n”, sizeof(greetings)); // Outputs 13
printf(“%lu\n”, sizeof(greetings2));

Question 28

Escape character Result Description
' ‘ Single quote
" “ Double quote
\ \ Backslash

Question 29

The sequence … inserts a single quote in a string

Answer 29

'
char txt[] = “It's alright.”

Question 30

The sequence … inserts a single backslash in a string

Answer 30

char txt[] = “The character \ is called backslash.”;

Question 31

Escape Character Result
\n New Line
\t Tab
\0 Null

Question 32

C also has many useful string functions, which can be used to perform certain operations on strings.

To use them, you must include the … header file in your program:

Answer 32

<string.h>
</string.h>

Question 33

For example, to get the length of a string, you can use the … function

Answer 33

strlen()
char alphabet[] = “ABCDEFGHIJKLMNOPQRSTUVWXYZ”;
printf(“%d”, strlen(alphabet));

Question 34

char alphabet[] = “ABCDEFGHIJKLMNOPQRSTUVWXYZ”;
printf(“%d”, strlen(alphabet)); // 26
printf(“%d”, sizeof(alphabet)); // 27

Answer 34

In the Strings chapter, we used sizeof to get the size of a string/array. Note that sizeof and strlen behaves differently, as sizeof also includes the \0 character when counting:

Question 35

It is also important that you know that sizeof will always return the … , and not the actual string length

Answer 35

memory size (in bytes)

Question 36

char str1[20] = “Hello “;
char str2[] = “World!”;

// Concatenate str2 to str1 (result is stored in str1)
strcat(str1, str2);

// Print str1
printf(“%s”, str1);

Answer 36

Concatenate Strings

Question 37

To concatenate (combine) two strings, you can use the … function

Answer 37

strcat()

Question 38

To copy the value of one string to another, you can use the .. function

Answer 38

strcpy()

Question 39

char str1[20] = “Hello World!”;
char str2[20];

// Copy str1 to str2
strcpy(str2, str1);

// Print str2
printf(“%s”, str2);

Answer 39

o copy the value of one string to another, you can use the strcpy() function

Question 40

To compare two strings, you can use the function.

It returns 0 if the two strings are equal, otherwise a value that is not 0

Answer 40

strcmp()

Question 41

char str1[] = “Hello”;
char str2[] = “Hello”;
char str3[] = “Hi”;

// Compare str1 and str2, and print the result
printf(“%d\n”, strcmp(str1, str2)); // Returns 0 (the strings are equal)

// Compare str1 and str3, and print the result
printf(“%d\n”, strcmp(str1, str3)); // Returns -4 (the strings are not equal)

Answer 41

Compare Strings

Question 42

When a variable is created in C, a … is assigned to the variable.

The … is the location of where the variable is stored on the computer.

When we assign a value to the variable, it is stored in this …. .

To access it, use the reference operator (&), and the result represents where the variable is stored

Answer 42

memory address

Question 43

… are important in C, because they allow us to manipulate the data in the computer’s memory

Answer 43

Pointers

Question 44

that we can get the memory address of a variable with the reference operator

Answer 44

&

Question 45

int myAge = 43; // an int variable

printf(“%d”, myAge);
printf(“%p”, &myAge);

Answer 45

Creating Pointers

Question 46

A … is a variable that stores the memory address of another variable as its value.

A … variable points to a data type (like int) of the same type, and is created with the * operator

Answer 46

pointer

Question 47

int myAge = 43;
int* ptr = &myAge; address of myAge

printf(“%d\n”, myAge);

printf(“%p\n”, &myAge);

printf(“%p\n”, ptr);

Answer 47

pointer

Question 48

we use the pointer variable to get the memory address of a variable (used together with the & reference operator).

You can also get the value of the variable the pointer points to, by using the * operator ….

Answer 48

(the dereference operator)

Question 49

When used in declaration (int* ptr), it creates a pointer variable.
When not used in declaration, it act as a dereference operator.

Question 50

int myNumbers[4] = {25, 50, 75, 100};
int i;

for (i = 0; i < 4; i++) {
printf(“%d\n”, myNumbers[i]);
}

Answer 50

Result:

25
50
75
100

Question 51

int myNumbers[4] = {25, 50, 75, 100};
int i;

for (i = 0; i < 4; i++) {
printf(“%p\n”, &myNumbers[i]);
}

Answer 51

Result:

0x7ffe70f9d8f0
0x7ffe70f9d8f4
0x7ffe70f9d8f8
0x7ffe70f9d8fc

the last number of each of the elements’ memory address is different, with an addition of 4.

It is because the size of an int type is typically 4 bytes

Question 52

void myFunction(char name[]) {
printf(“Hello %s\n”, name);
}

int main() {
myFunction(“Liam”);
myFunction(“Jenny”);
myFunction(“Anja”);
return 0;
}

Answer 52

// Hello Liam
// Hello Jenny
// Hello Anja

Question 53

void myFunction(char name[], int age) {
printf(“Hello %s. You are %d years old.\n”, name, age);
}

int main() {
myFunction(“Liam”, 3);
myFunction(“Jenny”, 14);
myFunction(“Anja”, 30);
return 0;
}

Answer 53

// Hello Liam. You are 3 years old.
// Hello Jenny. You are 14 years old.
// Hello Anja. Yo

Question 54

void calculateSum(int x, int y) {
int sum = x + y;
printf(“The sum of %d + %d is: %d\n”, x, y, sum);
}

int main() {
calculateSum(5, 3);
calculateSum(8, 2);
calculateSum(15, 15);
return 0;
}

Question 55

void myFunction(int myNumbers[5]) {
for (int i = 0; i < 5; i++) {
printf(“%d\n”, myNumbers[i]);
}
}

int main() {
int myNumbers[5] = {10, 20, 30, 40, 50};
myFunction(myNumbers);
return 0;
}

Question 56

Return Values
The void keyword, used in the previous examples, indicates that the function should not return a value. If you want the function to return a value, you can use a data type (such as int or float, etc.) instead of void, and use the return keyword inside the function:

Example
int myFunction(int x) {
return 5 + x;
}

int main() {
printf(“Result is: %d”, myFunction(3));
return 0;
}

Answer 56

Outputs 8 (5 + 3)

Question 57

int myFunction(int x, int y) {
return x + y;
}

int main() {
printf(“Result is: %d”, myFunction(5, 3));
return 0;
}

Answer 57

Outputs 8 (5 + 3)

Question 58

int calculateSum(int x, int y) {
return x + y;
}

int main() {
int result1 = calculateSum(5, 3);
int result2 = calculateSum(8, 2);
int result3 = calculateSum(15, 15);

printf(“Result1 is: %d\n”, result1);
printf(“Result2 is: %d\n”, result2);
printf(“Result3 is: %d\n”, result3);

return 0;

Answer 58

If we consider the “Calculate the Sum of Numbers” example one more time, we can use return instead and store the results in different variables. This will make the program even more flexible and easier to control:

Question 59

int calculateSum(int x, int y) {
return x + y;
}

int main() {
// Create an array
int resultArr[6];

// Call the function with different arguments and store the results in the array
resultArr[0] = calculateSum(5, 3);
resultArr[1] = calculateSum(8, 2);
resultArr[2] = calculateSum(15, 15);
resultArr[3] = calculateSum(9, 1);
resultArr[4] = calculateSum(7, 7);
resultArr[5] = calculateSum(1, 1);

for (int i = 0; i < 6; i++) {
printf(“Result%d is = %d\n”, i + 1, resultArr[i]);
}

return 0;
}

Question 60

// Function to convert Fahrenheit to Celsius
float toCelsius(float fahrenheit) {
return (5.0 / 9.0) * (fahrenheit - 32.0);
}

int main() {
// Set a fahrenheit value
float f_value = 98.8;

// Call the function with the fahrenheit value
float result = toCelsius(f_value);

// Print the fahrenheit value
printf(“Fahrenheit: %.2f\n”, f_value);

// Print the result
printf(“Convert Fahrenheit to Celsius: %.2f\n”, result);

return 0;
}

Question 61

// Global variable
int x = 5;

void myFunction() {
printf(“%d\n”, ++x); // Increment the value of x by 1 and print it
}

int main() {
myFunction();

printf(“%d\n”, x); // Print the global variable x
return 0;
}

Answer 61

The value of x is now 6 (no longer 5)

Question 62

// Function declaration
int myFunction(int x, int y);

// The main method
int main() {
int result = myFunction(5, 3); // call the function
printf(“Result is = %d”, result);
return 0;
}

// Function definition
int myFunction(int x, int y) {
return x + y;
}

Question 63

nt sum(int k);

int main() {
int result = sum(10);
printf(“%d”, result);
return 0;
}

int sum(int k) {
if (k > 0) {
return k + sum(k - 1);
} else {
return 0;
}
}

Answer 63

Recursion

Question 64

There is also a list of math functions available, that allows you to perform mathematical tasks on numbers.

To use them, you must include the math.h header file in your program

Answer 64

include <math.h></math.h>

Question 65

To find the square root of a number, use the sqrt() function

Answer 65

printf(“%f”, sqrt(16));

Question 66

printf(“%f”, ceil(1.4));
printf(“%f”, floor(1.4));

Answer 66

Round a Number
The ceil() function rounds a number upwards to its nearest integer, and the floor() method rounds a number downwards to its nearest integer

Question 67

printf(“%f”, pow(4, 3));

Answer 67

Power
The pow() function returns the value of x to the power of y (xy)

Question 68

FILE *fptr;
fptr = fopen(filename, mode);

Answer 68

In C, you can create, open, read, and write to files by declaring a pointer of type FILE, and use the fopen() function:

Question 69

FILE *fptr;

// Open a file in writing mode
fptr = fopen(“filename.txt”, “w”);

// Write some text to the file
fprintf(fptr, “Some text”);

// Close the file
fclose(fptr);

Answer 69

Write To a File

Question 70

In order to read the content of filename.txt, we can use the … function

Answer 70

fgets()

Question 71

fgets(myString, 100, fptr);

Question 72

FILE *fptr;

// Open a file in read mode
fptr = fopen(“filename.txt”, “r”);

// Store the content of the file
char myString[100];

// Read the content and store it inside myString
fgets(myString, 100, fptr);

// Print the file content
printf(“%s”, myString);

// Close the file
fclose(fptr);

Question 73

FILE *fptr;

// Open a file in read mode
fptr = fopen(“filename.txt”, “r”);

// Store the content of the file
char myString[100];

// Read the content and print it
while(fgets(myString, 100, fptr)) {
printf(“%s”, myString);
}

// Close the file
fclose(fptr);

Answer 73

The fgets function only reads the first line of the file. If you remember, there were two lines of text in filename.txt

Question 74

struct Car {
char brand[50];
char model[50];
int year;
};

int main() {
struct Car car1 = {“BMW”, “X5”, 1999};
struct Car car2 = {“Ford”, “Mustang”, 1969};
struct Car car3 = {“Toyota”, “Corolla”, 2011};

printf(“%s %s %d\n”, car1.brand, car1.model, car1.year);
printf(“%s %s %d\n”, car2.brand, car2.model, car2.year);
printf(“%s %s %d\n”, car3.brand, car3.model, car3.year);

return 0;
}

Answer 74

Structures
Structures (also called structs) are a way to group several related variables into one place. Each variable in the structure is known as a member of the structure.

Unlike an array, a structure can contain many different data types (int, float, char, etc.)

Question 75

Access Structure Members
To access members of a structure, use the … syntax

Answer 75

dot

Question 76

An … is a special type that represents a group of constants (unchangeable values)

Answer 76

enum

Question 77

enum Level {
LOW,
MEDIUM,
HIGH
};

Answer 77

C Enums

Question 78

The process of reserving memory is called

Answer 78

allocation

Question 79

C has two types of memory:

Answer 79

Static memory and dynamic memory

Question 80

… is memory that is reserved for variables before the program runs. Allocation of static memory is also known as compile time memory allocation.

Answer 80

Static memory

Question 81

int students[20];
printf(“%lu”, sizeof(students)); // 80 bytes

Answer 81

Static memory

Question 82

… is memory that is allocated after the program starts running. Allocation of dynamic memory can also be referred to as runtime memory allocation.

Unlike with static memory, you have full control over how much memory is being used at any time. You can write code to determine how much memory you need and allocate it.

… does not belong to a variable, it can only be accessed with pointers.

To allocate … , you can use the malloc() or calloc() functions. It is necessary to include the <stdlib.h> header to use them. The malloc() and calloc() functions allocate some memory and return a pointer to its address.</stdlib.h>

Answer 82

Dynamic memory

Question 83

int *ptr1 = malloc(size);
int *ptr2 = calloc(amount, size);

Answer 83

Dynamic memory