C Dynamic Memory Allocation

In this post we will discuss how we can use dynamic Arrays in C. We will use the stdlib.h functions for memory management called malloc() and realloc(). This 2 functions return a pointer to a memory location of a requested size. You have to set up a pointer and save the return value to this pointer.

For example: Suppose we want an dynamic integer array. We will create a pointer to an integer as follow **int *A; **

Then using malloc() we can setup the first size of the array with **A = (int*)malloc(N*sizeof(int)); where you have to put an typecasting to int* because it returns void* by default and put the size you want as N * sizeof(int), with N being the number of items you want it to have. You can change it afterwards using *A = (int)realloc(A, M*sizeof(int)); ** making it smaller or bigger setting a new M size value. This is how you can allocate memory at runtime and by the needs of your programm.

Code:

We want to get informations infinitely until we stop it (ID will equal 00000) about taxpayer’s for their tax payment.

Informations:

ID (it has to be of length 5 and has only numbers),
name (string),
surname (string),
birthday as string (we will “extract” the age and he will need to be atleast 18),
category (self_employed, wage_earner),
income as float
taxes to pay as float.

Taxes go as follows:

self_employed always 30% of it’s income

wage_earner with Staggered charge:

0% until 10.000
20% for 10.000 to 30.000
30% for 30.000 and upwards

When the infinite loop stops we print all the informations as a table. Because the number of taxpayers is not known we will have to use dynamic arrays for all of them.

#include <stdio.h>

#include <stdlib.h>

#include <string.h>

#define Y 2017

int main(){

    int *ids; //dynamic array of ids

    char **names; //dynamic array of names

    char **surnames; //dynamice array of surnames

    int *ages; //dynamic array of ages

    char *category; //dynamic array of categories of the taxpayers

    float *incomes; //dynamic array of incomes

    float *taxes; //dynamic array of taxes

    char id[6]; //reading as string to have it be of length 5

    char name[30], surname[30]; //temporary name, surname variables

    char date[11]; //for reading birthday date in format DD/MM/YYYY

    char *p; //pointer for spliting the birthday

    int day, month, year; //birthday seperated and in integer form

    float income, tax; //for reading income and calculating tax temporary 

    char cat; //temporary category

    int i; //loop variable

    int flag; //flag for checking

    int count = 0; //count of taxpayer's we inserted

    printf("TAX PROGRAMM:\n"); 

	while(1){

		//insert id (5 integers)

		while(1){

			flag = 1; //if it gets 0 it will reask the id

   	 		printf("id: "); 

			scanf("%5s", id); 

			fflush(stdin);

			if( strlen(id) < 5) flag = 0; //check length

			else{

				for(i = 0; i < 5; i++){	//check it has only numbers

   	    			if( (id[i]<'0') || (id[i]>'9') ){

   	    				flag = 0; 

   	    				break; 

					}

			   	}

			}			

			if(flag == 1) break;

    	}

    	if(atoi(id) == 0){ //stop looping when its 00000

    		break;  

		}

    	if(count == 0){ //first one

    		ids = (int*)malloc(1*sizeof(int)); 

		}

		else{ //every other one

			ids = (int*)realloc(ids, (count+1)*sizeof(int));

		}

		ids[count] = atoi(id);

		//name and surname input		

   		printf("name: "); 

   		scanf("%29s", name); 

   		fflush(stdin); 

   		printf("surname: "); 

   		scanf("%29s", surname); 

   		fflush(stdin); 

  		if(count == 0){ //first one

    		names = (char**)malloc(1*sizeof(char*)); 

    		surnames = (char**)malloc(1*sizeof(char*)); 

		}

		else{ //every other one

			names = (char**)realloc(names, (count+1)*sizeof(char*));

			surnames = (char**)realloc(surnames, (count+1)*sizeof(char*));

		}

  		names[count] = (char*)malloc((strlen(name) + 1) * sizeof(char)); //allocate memory for name

  		strcpy(names[count], name); 

  		surnames[count] = (char*)malloc((strlen(surname) + 1) * sizeof(char)); //allocate memory for surname

  		strcpy(surnames[count], surname); 

  		//birthday input

		while(1){

   			flag = 1; 	//if it gets 0 it will reask the date

			printf("DD/MM/YYYY: ");

			scanf("%10s",date);

			fflush(stdin);

			day = atoi(date);

			p=strchr(date,'/');

			p=p+1;

			month=atoi(p);

			p=strchr(p,'/');

			p=p+1;

			year=atoi(p);			

			if( (day<=0) || (day>31 ) ) flag = 0; //check reality of day

			if( (month<=0) || (month>12) ) flag = 0; //check reality of month

			if (year > Y - 18) flag = 0; //check year for age (easy way)

			if(flag == 1) break;

  		}

  		if(count == 0){ //first one

    		ages = (int*)malloc(1*sizeof(int)); 

		}

		else{ //every other one

			ages = (int*)realloc(ages, (count+1)*sizeof(int));

		}

  		ages[count] = Y - year; //save the age into the array

  		//income input

  		while (1){ //need to be positive

       		printf("yearly income: "); 

       		scanf("%f" , &income); 

       		fflush(stdin); 

      		if(income > 0) break; 

   		}

   		if(count == 0){ //first one

    		incomes = (float*)malloc(1*sizeof(float)); 

		}

		else{ //every other one

			incomes = (float*)realloc(incomes, (count+1)*sizeof(float));

		}

		incomes[count] = income;

		//category

		while(1){

			printf("Are you self employer(s) or wage earner(w): ");

			scanf("%c", &cat);

			fflush(stdin);

			if(cat == 's' || cat == 'w') break;

		}

		if(count == 0){ //first one

    		category = (char*)malloc(1*sizeof(char)); 

		}

		else{ //every other one

			category = (char*)realloc(category, (count+1)*sizeof(char));

		}

		category[count] = cat;

		//calculate tax

		if(cat == 's'){ //self employed

			tax = income * 0.3;

		}

		else{

			if(income <= 10000){

				tax = 0;

			}

			else if(income > 10000 && income <= 30000){

				tax = 0 + (income - 10000) * 0.2;

			}

			else{ //if > 30000

				tax = 0 + 20000*0.2 + (income - 30000) * 0.3;

			}

		}

		printf("You have to pay %.2f tax!\n", tax);

		if(count == 0){ //first one

    		taxes = (float*)malloc(1*sizeof(float)); 

		}

		else{ //every other one

			taxes = (float*)realloc(taxes, (count+1)*sizeof(float));

		}

		taxes[count] = tax;

		count++; //increment count  		

	}

	//print informations

	printf("TAXPAYERS:\n");

	printf("ID\tNAME\tSURNAME\t\tAGE\tCATEGORY\tINCOME\t\tTAXES\n");

	for(i = 0; i < count; i++){

		printf("%d\t%s\t%s\t%d\t", ids[i], names[i], surnames[i], ages[i]);

		if(category[i] == 's') printf("Self employed\t");

		else printf("Wage earner\t");

		printf("%.2f\t%.2f\n", incomes[i], taxes[i]);				

	}

	//its difficult to keep them all in one line when names are too big

       //so you should split the information array into two or three parts

}