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 arequested 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 2017int 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}This task actually had statistics like some previous posts of myself, but because this one is only showing of dynamic arrays I took them off.
Next time we will do something similar to store information, but with a dynamic struct and save and load from file.