diff --git a/helloworld/H15_E1.c b/helloworld/H15_E1.c
new file mode 100644
index 0000000..ff15b87
--- /dev/null
+++ b/helloworld/H15_E1.c
@@ -0,0 +1,32 @@
+//1. Rewrite the program in Listing 15.2. This time use the format specifier %c, instead
+//of %s, to print out the character string of the local time on your computer.
+
+#include <stdio.h>
+#include <time.h>
+
+void GetDateTime(void);
+
+main() {
+	printf("Before the GetDateTime() function is called.\n");
+	GetDateTime();
+	printf("After the GetDateTime() function is called.\n");
+	return 0;
+}
+/* GetDateTime() definition */
+void GetDateTime(void) {
+	time_t now;
+	int i;
+	char *str;
+	printf("Within GetDateTime().\n");
+	time(&now);
+	str = asctime (localtime (&now));
+	printf("Current date and time is: ");
+	for (i=0; str[i]; i++) printf("%c", str[i]);
+}
+
+
+
+
+
+
+
diff --git a/helloworld/H15_E2.c b/helloworld/H15_E2.c
new file mode 100644
index 0000000..9a3a225
--- /dev/null
+++ b/helloworld/H15_E2.c
@@ -0,0 +1,20 @@
+//2. Declare and define a function, called MultiTwo(), that can perform multiplication
+//on two integer variables. Call the MultiTwo() function from the main() function
+//and pass two integers to MultiTwo(). Then print out the result returned by the
+//MultiTwo() function on the screen.
+
+#include <stdio.h>
+
+MultiTwo(int x, int y);
+
+main () {
+	 int i = 24;
+	 int j = 13;
+	 printf("%d multiplied by %d equal %d.\n", i, j, MultiTwo(i,j));
+	 return 0;
+}
+
+MultiTwo(int x, int y) {
+	return x * y;
+}
+
diff --git a/helloworld/H15_E3.c b/helloworld/H15_E3.c
new file mode 100644
index 0000000..a443c7a
--- /dev/null
+++ b/helloworld/H15_E3.c
@@ -0,0 +1,38 @@
+//3. Rewrite the program in Listing 15.3. This time, make a function that takes 
+//a variable number of int arguments and performs the operation of multiplication on
+//these arguments.
+
+#include <stdio.h>
+#include <stdarg.h>
+
+double AddDouble(int x, ...);
+main () {
+	double d1 = 1.5;
+	double d2 = 2.5;
+	double d3 = 3.5;
+	double d4 = 4.5;
+	
+	printf("Given an argument: %2.1f\n", d1);
+	printf("The result returned by AddDouble() is: %2.1f\n\n", AddDouble(1, d1));
+	printf("Given arguments: %2.1f and %2.1f\n", d1, d2);
+	printf("The result returned by AddDouble() is: %2.1f\n\n", AddDouble(2, d1, d2));
+	printf("Given arguments: %2.1f, %2.1f and %2.1f\n", d1, d2, d3);
+	printf("The result returned by AddDouble() is: %2.1f\n\n", AddDouble(3, d1, d2, d3));
+	printf("Given arguments: %2.1f, %2.1f, %2.1f, and %2.1f\n", d1, d2, d3, d4);
+	printf("The result returned by AddDouble() is: %2.1f\n", AddDouble(4, d1, d2, d3, d4));
+	return 0;
+}
+	/* definition of AddDouble() */
+double AddDouble(int x, ...) {
+	va_list arglist;
+	int i;
+	double result = 0.0;
+	printf("The number of arguments is: %d\n", x);
+	va_start (arglist, x);
+	for (i=0; i<x; i++) result += va_arg(arglist, double);
+	va_end (arglist);
+return result;
+
+}
+
+
diff --git a/helloworld/H15_E4.c b/helloworld/H15_E4.c
new file mode 100644
index 0000000..fd3e5be
--- /dev/null
+++ b/helloworld/H15_E4.c
@@ -0,0 +1,38 @@
+//4. Rewrite the program in Listing 15.3 again. This time, print out all arguments
+//passed to the AddDouble() function. Does va_arg() fetch each argument in the
+//same order (that is, from left to right) of the argument list passed to AddDouble()?
+
+#include <stdio.h>
+#include <stdarg.h>
+
+double AddDouble(int x, ...);
+main () {
+	double d1 = 1.5;
+	double d2 = 2.5;
+	double d3 = 3.5;
+	double d4 = 4.5;
+	
+	printf("Given an argument: %2.1f\n", d1);
+	printf("The result returned by AddDouble() is: %2.1f\n\n", 	AddDouble(1, d1));
+	printf("Given arguments: %2.1f and %2.1f\n", d1, d2);
+	printf("The result returned by AddDouble() is: %2.1f\n\n", 	AddDouble(2, d1, d2));
+	printf("Given arguments: %2.1f, %2.1f and %2.1f\n", d1, d2, d3);
+	printf("The result returned by AddDouble() is: %2.1f\n\n", 	AddDouble(3, d1, d2, d3));
+	printf("Given arguments: %2.1f, %2.1f, %2.1f, and %2.1f\n", d1, d2, d3, d4);
+	printf("The result returned by AddDouble() is: %2.1f\n", AddDouble(4, d1, d2, d3, d4));
+	return 0;
+}
+	/* definition of AddDouble() */
+double AddDouble(int x, ...) {
+	va_list arglist;
+	int i;
+	double result = 0.0;
+	printf("The number of arguments is: %d\n", x);
+	va_start (arglist, x);
+	for (i=0; i<x; i++) result += va_arg(arglist, double);
+	va_end (arglist);
+return result;
+//na ezzel ugyanaz a problémám, mint az előző feladattal. :-(
+	}
+	
+	
\ No newline at end of file
diff --git a/helloworld/H16_E1.c b/helloworld/H16_E1.c
new file mode 100644
index 0000000..b5bd7bb
--- /dev/null
+++ b/helloworld/H16_E1.c
@@ -0,0 +1,21 @@
+//1. Given a character string, I like C!, write a program to pass the string to 
+//a function that displays the string on the screen.
+
+#include <stdio.h>
+
+void Strprint(char *ch);
+
+main() {
+	char str[] = "I like C!";
+	char *ptr_str;
+	ptr_str = str;
+	Strprint(ptr_str);
+	return 0;
+}
+
+//function defaul
+
+void Strprint(char *ch) {
+	printf("%s\n", ch);
+}
+
diff --git a/helloworld/H16_E2.c b/helloworld/H16_E2.c
new file mode 100644
index 0000000..712176e
--- /dev/null
+++ b/helloworld/H16_E2.c
@@ -0,0 +1,25 @@
+//2. Rewrite the program in exercise 1. This time, change the string of I like C! to I
+//love C! by moving a pointer that is initialized with the start address of the string
+//and updating the string with new characters. Then, pass the updated string to the
+//function to display the content of the string on the screen.#include <stdio.h>
+#include <stdio.h>
+void Strprint(char *ch);
+
+main() {
+	char str[] = "I like C!";
+	char *ptr_str;
+	ptr_str = str;
+	*(ptr_str + 3) = 'o';
+	*(ptr_str + 4) = 'v';
+	Strprint(ptr_str);
+	return 0;
+}
+
+//function definition
+
+void Strprint(char *ch) {
+	printf("%s\n", ch);
+}
+
+
+
diff --git a/helloworld/H16_E3.c b/helloworld/H16_E3.c
new file mode 100644
index 0000000..7a45d90
--- /dev/null
+++ b/helloworld/H16_E3.c
@@ -0,0 +1,25 @@
+//3. Given a two-dimensional character array, str, that is initialized as
+//char str[2][15] = { "You know what,", "C is powerful." };
+//write a program to pass the start address of str to a function that prints out the
+//content of the character array.
+
+#include <stdio.h>
+
+void Strprint(char * ch [][15]);
+
+main() {
+	char str[2][15] = {"You know what,", "C is powerful."};
+	char *ptr_str;
+	ptr_str = str;
+	Strprint(ptr_str);
+	Strprint(ptr_str + 15);
+	return 0;
+}
+
+//function defaul
+
+void Strprint(char *ch [][15]) {
+	printf("%s\n", ch);
+//fordításkor nyom egy csomó warningot, amit nem tudok értelmezni, viszont működik.
+}
+
diff --git a/helloworld/H16_E4.c b/helloworld/H16_E4.c
new file mode 100644
index 0000000..b7ead34
--- /dev/null
+++ b/helloworld/H16_E4.c
@@ -0,0 +1,35 @@
+//4. Rewrite the program in Listing 16.7. This time, the array of pointers is initialized
+//with the following strings:
+//"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", and
+//"Saturday".
+
+#include <stdio.h>
+/* function declarations */
+void StrPrint1(char **str1, int size);
+void StrPrint2(char *str2);
+
+/* main() function */
+main() {
+	char *str[7] = {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", 
+"Saturday\n"};
+	int i, size = 7;
+	StrPrint1(str, size);
+	for (i=0; i<size; i++)
+	StrPrint2(str[i]);
+	
+	return 0;
+}
+	/* function definition */
+void StrPrint1(char **str1, int size) {
+	int i;
+	/* Print all strings in an array of pointers to strings */
+	for (i=0; i<size; i++)
+		printf("%s\n", str1[i]);
+}
+/* function definition */
+void StrPrint2(char *str2) {
+/* Prints one string at a time */
+	printf("%s\n", str2);
+}
+
+
diff --git a/helloworld/H17_E1.c b/helloworld/H17_E1.c
new file mode 100644
index 0000000..150680d
--- /dev/null
+++ b/helloworld/H17_E1.c
@@ -0,0 +1,40 @@
+//1. Write a program to ask the user to enter the total number of bytes he or she wants
+//to allocate. Then, initialize the allocated memory with consecutive integers, starting
+//from 1. Add all the integers contained by the memory block and print out the final
+//result on the screen.
+
+#include <stdio.h>
+#include <stdlib.h>
+
+main() {
+	int SizeofAllmemory;
+	int *ptr_x;
+	int sum = 0;
+	int result;
+	setbuf(stdout, NULL);
+	printf("Enter the total number of bytes you want to allocate:");
+	scanf("%d", &SizeofAllmemory);
+	ptr_x = malloc (SizeofAllmemory/2 * sizeof(int));
+	//ptr_x = malloc (SizeofAllmemory); Ez vajon miért nem jó helyette? segmentation faultot ad vissza
+	if (ptr_x != NULL) {
+		for (int i = 0; i < SizeofAllmemory/2; i++) {
+			ptr_x[i] = i + 1;
+			printf("%d\n", *(ptr_x + i));
+		}
+		printf("Az összeg az elején: %d\n", sum);
+		for (int i = 0; i < SizeofAllmemory/2; i++) {
+			sum += ptr_x[i];
+			printf("Az összeg most: %d\n", sum);
+		}
+		printf("The sum is %d\n", sum);
+		result = 0;
+	}
+	else {
+		printf("malloc() function failed.\n");
+		result = 1;
+	}
+	printf("%d", result);
+	return result;
+}
+
+
diff --git a/helloworld/H17_E2.c b/helloworld/H17_E2.c
new file mode 100644
index 0000000..8ecb077
--- /dev/null
+++ b/helloworld/H17_E2.c
@@ -0,0 +1,31 @@
+//2. Write a program that allocates a block of memory space to hold 100 items of the
+//float data type by calling the calloc() function. Then, reallocate the block of
+//memory in order to hold 50 more items of the float data type.
+
+
+#include <stdio.h>
+#include <stdlib.h>
+
+main() {
+	float *ptr_x;
+	int result;
+	ptr_x = calloc(100, sizeof(float));
+	if (ptr_x != NULL) {
+		result = 0;
+		printf("First allocation is done\n");
+	}
+	else {
+		printf("The first allocation wasn't successful! \n");
+		result = 1;
+	}
+	ptr_x = realloc (ptr_x, 150);
+	if (ptr_x != NULL) {
+		result = 0;
+		printf("The reallocation is done\n");
+	}
+	else {
+		printf("The reallocation wasn't successful! \n");
+		result = 1;
+	}
+	return result;
+}
\ No newline at end of file
diff --git a/helloworld/H17_E3.c b/helloworld/H17_E3.c
new file mode 100644
index 0000000..2f161a7
--- /dev/null
+++ b/helloworld/H17_E3.c
@@ -0,0 +1,36 @@
+
+//3. Write a program to ask the user to enter the total number of float data. Then use
+//the calloc() and malloc() functions to allocate two memory blocks with the
+//same size specified by the number, and print out the initial values of the two memory blocks.
+
+#include <stdio.h>
+#include <stdlib.h>
+
+main() {
+	int Nodata;
+	float *ptr_x1;
+	float *ptr_x2;
+	int result;
+	setbuf(stdout, NULL);
+	printf("Enter the total number of float data you want to allocate:");
+	scanf("%d", &Nodata);
+	ptr_x1 = malloc (Nodata * sizeof(float));
+	if (ptr_x1 != NULL){
+		result = 0;
+		printf("First allocation is done\n, the first value of the block is: %f\n", *ptr_x1);
+	}
+	else {
+		printf("The first allocation wasn't successful! \n");
+		result = 1;
+	}
+	ptr_x2 = calloc (Nodata, sizeof(float));
+	if (ptr_x2 != NULL) {
+		result = 0;
+		printf("Second allocation is done\n, the first value of the block is: %f\n", *ptr_x2);
+	}
+	else {
+		printf("The 2nd allocation wasn't successful! \n");
+		result = 1;
+	}
+	return result;
+}
\ No newline at end of file
diff --git a/helloworld/H17_E4.c b/helloworld/H17_E4.c
new file mode 100644
index 0000000..db0228f
--- /dev/null
+++ b/helloworld/H17_E4.c
@@ -0,0 +1,50 @@
+//4. Rewrite the program in Listing 17.4. This time, use the two special cases of the
+//realloc() function to replace the malloc() and free() functions.
+
+ /* 17L04.c: Using the realloc() function */
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+/* function declaration */
+void StrCopy(char *str1, char *str2);
+/* main() function */
+main() {
+	char *str[4] = {"There's music in the sighing of a reed;",
+	"There's music in the gushing of a rill;",
+	"There's music in all things if men had ears;",
+	"There earth is but an echo of the spheres.\n"
+	};
+	char *ptr;
+	int i;
+	int termination = 0;
+	ptr = realloc(NULL, (strlen(str[0]) + 1) * sizeof(char)); //ptr = malloc((strlen(str[0]) + 1) * sizeof(char));
+	if (ptr == NULL) {
+		printf("malloc() failed.\n");
+		termination = 1;
+	}
+	else {
+		StrCopy(str[0], ptr);
+		printf("%s\n", ptr);
+		for (i=1; i<4; i++) {
+			ptr = realloc(ptr, (strlen(str[i]) + 1) * sizeof(char));
+			if (ptr == NULL) {
+				printf("realloc() failed.\n");
+				termination = 1;
+				i = 4; /* break the for loop */
+			}
+			else {
+				StrCopy(str[i], ptr);
+				printf("%s\n", ptr);
+			}
+		}
+	}
+	realloc(ptr, 0); //free(ptr)
+	return termination;
+}
+	/* funciton definition */
+	void StrCopy(char *str1, char *str2) {
+	int i;
+	for (i=0; str1[i]; i++)
+	str2[i] = str1[i];
+	str2[i] = '\0';
+	}
\ No newline at end of file
diff --git a/helloworld/H18_E1.c b/helloworld/H18_E1.c
new file mode 100644
index 0000000..33f2b53
--- /dev/null
+++ b/helloworld/H18_E1.c
@@ -0,0 +1,14 @@
+//1. Write a program to print out the values represented by the enumerated names
+//declared in Quiz question 2 in this hour.
+//enum tag { name1,name2 = 10,name3,name4 };
+
+#include <stdio.h>
+main() {
+enum tag { name1, name2 = 10,name3,name4 };
+printf("The value represented by name1 is: %d\n", name1);
+printf("The value represented by name2 is: %d\n", name2);
+printf("The value represented by name3 is: %d\n", name3);
+printf("The value represented by name4 is: %d\n", name4); 
+
+return 0;
+}
diff --git a/helloworld/H18_E2.c b/helloworld/H18_E2.c
new file mode 100644
index 0000000..b200b7a
--- /dev/null
+++ b/helloworld/H18_E2.c
@@ -0,0 +1,25 @@
+//2. Given the following declarations:
+//typedef char WORD;
+//typedef int SHORT;
+//typedef long LONG;
+//typedef float FLOAT;
+//typedef double DFLOAT;
+//write a program to measure the sizes of the synonyms to the data types.
+
+#include <stdio.h>
+main() {
+typedef char WORD;
+typedef int SHORT;
+typedef long LONG;
+typedef float FLOAT;
+typedef double DFLOAT;
+
+printf("The size of WORD is: %d\n", sizeof(WORD));
+printf("The size of SHORT is: %d\n", sizeof(SHORT));
+printf("The size of LONG is: %d\n", sizeof(LONG)); 
+printf("The size of FLOAT is: %d\n", sizeof(FLOAT));
+printf("The size of DFLOAT is: %d\n", sizeof(DFLOAT));
+
+return 0;
+}
+
diff --git a/helloworld/H18_E3.c b/helloworld/H18_E3.c
new file mode 100644
index 0000000..3a045d8
--- /dev/null
+++ b/helloworld/H18_E3.c
@@ -0,0 +1,27 @@
+//3. Rewrite the program in Listing 18.4. This time, add integers starting at the value of
+//MIN_NUM instead of the value of MAX_NUM.
+
+#include <stdio.h>
+enum con {MIN_NUM = 0,
+	MAX_NUM = 100};
+int fRecur(int n);
+
+main() {
+	int i, sum1, sum2;
+	sum1 = sum2 = 0;
+	for (i=1; i<=MAX_NUM; i++) {
+		sum1 += i;
+	}
+	printf("The value of sum1 is %d.\n", sum1);
+	sum2 = fRecur(MIN_NUM);
+	printf("The value returned by fRecur() is %d.\n", sum2);
+	
+	return 0;
+}
+/* function definition */
+int fRecur(int n) {
+	if (n > MAX_NUM)
+	return 0;
+	return fRecur(n + 1) + n;
+}
+
diff --git a/helloworld/H18_E4.c b/helloworld/H18_E4.c
new file mode 100644
index 0000000..e8d7cab
--- /dev/null
+++ b/helloworld/H18_E4.c
@@ -0,0 +1,25 @@
+//4. Write a program that accepts command-line arguments. If the number of 
+//command-line arguments, not including the name of the executable itself, 
+//is less than two, print out the usage format of the program and 
+//ask the user to reenter the command-line arguments.
+//Otherwise, display all command-line arguments entered by
+//the user.
+
+#include <stdio.h>
+main (int argc, char *argv[]) {
+	int i;
+	if (argc < 2) {
+		printf("The usage of this program is:\n");
+		printf("a.EXE argument1 argument2 [...argumentN]\n");
+		printf("Please start the program again'");
+	}
+	else {
+		printf("The number of the command-line arguments are: %d\n", argc);
+		printf("The command-line arguments are:\n");
+		for (i=1; i<argc; i++){
+			printf("%s ", argv[i]);
+		}
+		printf("\n");
+	}
+	return 0;
+}
\ No newline at end of file