🚀 C Advanced

This page provides an introduction to C typedef, enum, struct, pointers, functions & memory allocation.

Typedef

// typedef existing_type new_name;
typedef char String[50];
String name = "This is first line";
printf("%s", name);

Enum

typedef enum {
  SUNDAY = 1, MONDAY = 2, TUESDAY = 3, WEDNESDAY = 4, 
  THURSDAY = 5, FRIDAY = 6, SATURDAY = 7
}Day;

Day today = SUNDAY;
printf("%d", today); // this will print 1

Struct

#include <string.h>
#include <stdio.h>
#include <stdbool.h>

typedef struct {
  char name[50];
  int age;
  float gpa;
  bool isFullTime; 
} Student;

Student student = {"SpongeBob", 30, 2.5, true};
Student emptyStudent = {0};

printf("%s \n", student.name);
printf("%d \n", student.age);
printf("%.2f \n", student.float);
printf("%d\n", student.isFullTime);
printf("%s\n", (student.isFullTime) ? "YES" : "NO");

Pointers

Pointers helps avoid wasting memory by allowing you to pass the address of a large data structure instead of copying the entire data.

int age = 25;
printf("%p", &age); //%p stands for pointer: Memory address of variable

// create a pointer
int *pAge = &age;
printf("\n%p", pAge);

Functions

#include <stdio.h>

// By default parameters in function are pass by value: 
// i.e. a copy of variable is made and passed to the function
void happyBirthday(char name[]) {
  printf("\nHappy Birthday to you!");
  printf("\nHappy Birthday to you!");
  printf("\nHappy Birthday dear %s!", name);
  printf("\nHappy Birthday to you!");
}

// passing variable as a pointer
void happyBirthdayByPointer(char *name[]) {
  printf("\nHappy Birthday to you!");
  printf("\nHappy Birthday to you!");
  printf("\nHappy Birthday dear %s!", *name);
  printf("\nHappy Birthday to you!");
}

int main() {
  char name[] = "unnamed1";
  
  // by default pass by value
  happyBirthday(name);

  // pass by reference
  happyBirthdayByPointer(&name);
  return 0;
}

File

Write a file

#include <stdio.h>

int main() {
  // open a file in write mode
  FILE *pFile = fopen("output.txt", "w");

  char text[] = "This is first line.\n This is second line.";

  if(pFile == NULL) {
    printf("Error opening a file");
    return 1;
  }

  fprintf(pFile, "%s", text);

  printf("File was written successfully");

  // close the file
  fclose(pFile);

  return 0;
}

Read a file

#include <stdio.h>

int main() {
  // open a file in read mode
  FILE *pFile = fopen("input.txt", "r");
  char buffer[1024] = {0};
  
  if(pFile == NULL) {
    printf("Couldn't open a file");
    return 1;
  }

  while(fgets(buffer, sizeof(buffer), pFile) != NULL) {
    printf("%s", buffer);
  } 

  // close the file
  fclose(pFile);

  return 0;
}

Malloc

A function in C that dynamically allocates a specified number of bytes in memory

#include <stdio.h>
#include <stdlib.h>

int main() {
  int number = 0;

  printf("Enter the number of grades");
  scanf("%d", &number);

  char *grades  = malloc(number * sizeof(char));

  if(grades == NULL) {
    printf("Memory allocation failed!");
    return 1;
  }

  for(int i = 0; i < number; i++) {
    printf("\nEnter grade number %d: ", i + 1);
    scanf(" %c", &grades[i]);
  }

  for(int i = 0; i < number; i++) {
    printf("%c ", grades[i]);
  }

  // returning rented space back to operating system.
  free(grades);
  // avoids dangling pointers
  grades = NULL; 

  return 0;
}

Calloc

Contiguous allocation. Allocates memory dynamically and sets all allocated bytes to 0. malloc() is faster, but calloc() leads to less bugs.

#include <stdio.h>

int main() {
  
  int number = 0;
  printf("Enter the number of players: ");
  scanf("%d", &number);

  int *scores = calloc(number, sizeof(int));

  if(scores == NULL) {
    printf("Memory allocation failed!");
    return 1;
  }

  for(int i = 0; i < numbers; i++) {
    printf("%d", scores[i]);  // prints everything as 0
  }

  free(scores);
  scores = NULL;

  return 0;
}

Realloc

Reallocates previously allocated memory

#include <stdio.h>
#include <stdlib.h>

int main() {
  int number = 0;
  printf("Enter the total number of prices: ");
  scanf("%d", &number);

  float *prices[] = malloc(number * sizeof(float));

  if(prices == NULL) {
    printf("Memory allocation failed");
    return 1;
  }

  for(int i = 0; i < number; i++) {
    printf("\nEnter price number %d: ", i + 1);
    scanf("%f", &prices[i]);
  }

  int newNumber = 0;
  printf("\nEnter a new number of prices: ");
  scanf("%d", &newNumber);

  // use realloc
  float *temp = realloc(prices, newNumber * sizeof(float));

  if(temp == NULL) {
    printf("\nCould not reallocate memory");
  }
  else {
    prices = temp;
  }

  for(int i = 0; i < number; i++) {
    printf("$%.2f: ", prices[i]);
  }

  free(prices);
  prices = NULL;
}