Problem decreasing the size of a stack in c

Question

This program is an exercise to work with stacks, it’s all right, but when I decrease the stack size to exclude the last term it appears in the penultimate element of the vector.

#include <stdio.h>
#include <windows.h>

typedef struct viagem {
  char nome[50];
  char uf[2];
}v;

typedef struct pilha{
  int topo;
  int distancia;
  int visitadas;
  v cidades[10];
}p;

void empilhar (p *pilha) {
  int dist;
  system("cls");
  pilha->topo++;
  printf("Cidade destino: " );
  scanf("%[^\n]s",&pilha->cidades[pilha->topo].nome );
  printf("UF: " );
  scanf("%s",&pilha->cidades[pilha->topo].uf );
  printf("Distancia: " );
  scanf("%i",&dist);
  pilha->visitadas++;
  pilha->distancia=pilha->distancia+dist;
}
void desempilhar (p *pilha){
  pilha->topo--;
}
int main (){
  p pilha;
  pilha.topo=-1;
  pilha.visitadas = 0;
  pilha.distancia = 0;
  int flag=1;
  int opc;
  do {
    system ("cls");
    if (pilha.topo == -1) {
      printf("Voce esta em Maringa-Pr !");
    }
    else {
      printf("Voce esta em %s-%s",pilha.cidades[pilha.topo].nome,pilha.cidades[pilha.topo].uf);
    }
    printf("\nVoce visitou %i cidades", pilha.visitadas );
    printf("\nVoce percoreu %ikm\n", pilha.distancia );
    printf("\n1-Avancar");
    printf("\n2-Voltar");
    printf("\n3-Sair");
    printf("\nopcao: " );
    scanf("%i%*c",&opc );
    switch (opc) {
      case 1:
        empilhar (&pilha);
      break;
      case 2:
        desempilhar (&pilha);
      break;
      case 3:
        flag=0;
      break;
    }
  } while(flag);
  printf("Programa finalizado ! " );
  scanf("%*i",&opc);
}

Answer 1

Good afternoon Wingeds,

I believe there is a conceptual problem in your stack, which, by definition, are data structures LIFO (Last in, first out). They can be implemented, among other forms, through Dynamic vectors and of Chained lists.

In the case that you presented what I could notice is that you are using the instance of a TAD to store data in it, but this conceptually is not a stack.

Illustration of a stack below

Getting back to your problem:

The operations of Insert (Push) and Remove (Pop) on the stack will vary according to the model you are using. In the case of vectors the Pop is through a "Logical Erase", which is to return a position on the "Top" of the stack without necessarily removing the value, and the Push overwriting these previous values or inserting values into empty spaces. In the case of chained lists the Push is through dynamic allocation, famous malloc(), where you will allocate the space of a cell (TAD) and the link at the top of the list, and the Pop occurs through the function free(), where you will free the space allocated to the cell you want to delete, taking care, of course, not to lose the reference of the previous value.

Abstract illustration of Push and Pop

If my remark is incorrect and your activity is really to use the instance of your struct to store your data please signal so that I can delete the answer!

Hugs and good studies!

Answer 2

Problem 1:

typedef struct viagem {
  char nome[50];
  // char uf[2]; // <-------------- ERRO
  char uf[3];
} v;

Problem 2:

void desempilhar (p *pilha){
  if (pilha->topo >= 0)  // <------- FALTA
    pilha->topo--;
}

And the @v. Santos remark is correct: distance and "visited" should be part of each stack element, not just a value for the whole stack.