Programming | 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 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.
He he! Old fashion code. Honestly I have difficulties to understand it eve if I made almost two years of C/C++ programming. Congrats for the article!
Thank yoy djvidov. C is good for understanding the basics in Programming generally. "Newer" Languages feature the same and more features in already made functions, but you don't know how it's been done in background and sometimes the Code even looks scary. But with this thinking we should start off with Assembly that doesn't give you anything to start off. But Assembly is also just too difficult to understand if we don't know the basics beforehand, but too much help is also not good, so I think C has a good mix of both for starting off. Hope you enjoy my stuff.