How to use Binary Search on array of string - c

Question

I have to write a program only using things that i've learned in class, where I register information from a car, sort it and use the Binary Search to search for a cars' license plate. If the license plate is found, print all the info about that especific car, If don't pritnt "Not found" and returns -1. All works fine until the Binary Search, the problem is that it won't find the license plate, It always returns -1.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAX_CARRO 5 // DEFINE TAMANHO DA STRUCT
typedef struct // STRUCT
{
char placa[50];
char marca[50];
char modelo[50];
char ano[50];
char valordiaria[50];
} carro;
carro car[MAX_CARRO];
int compare (const void * a, const void * b)
{
carro *carroA = (carro *)a;
carro *carroB = (carro *)b;
return strcmp(carroA, carroB);
}
int main()
{
int x=0; //COUNTER
for(x=0; x<MAX_CARRO; x++) // CAR REGISTER
{
printf("\nCarro: %d", (x+1));
printf("\nPlaca: ");
scanf("%s",car[x].placa);
printf("Marca: ");
scanf("%s",car[x].marca);
printf("Modelo: ");
scanf("%s",car[x].modelo);
printf("Ano: ");
scanf("%s",car[x].ano);
printf("Valor da diaria: ");
scanf("%s",car[x].valordiaria);
}
qsort (car, MAX_CARRO, sizeof(carro), compare); // USO DO QSORT
printf("\n\nSTRUCT ORDENADA: \n");
for(x=0; x<MAX_CARRO; x++) // MOSTRA NA TELA A STRUCT ORDENADA
{
printf("\n\n\nCarro: %d", (x+1));
printf("\nPlaca: %s", car[x].placa);
printf("\nMarca: %s", car[x].marca);
printf("\nModelo: %s", car[x].modelo);
printf("\nAno: %s", car[x].ano);
printf("\nValor da diaria: %s", car[x].valordiaria);
}
char k[10];
// *****BINARY SEARCH******
printf("\n\n\n*****BUSCA DE PLACAS*****\n\n\n\n");
printf("Digite a placa que deseja procurar: \n");
scanf("%s", &k);
// ***PROBLEM***
int low, high, mid;
low=0;
high = MAX_CARRO-1;
while(low<=high)
{
mid = (low+high)/2;
if (strcmp(k, car[mid].placa)<0)
high=mid-1;
else if (strcmp(k, car[mid].placa)>0)
low=mid+1;
else
{
printf("\nPlaca: %s", car[mid].placa);
printf("\nMarca: %s", car[mid].marca);
printf("\nModelo: %s", car[mid].modelo);
printf("\nAno: %s", car[mid].ano);
printf("\nValor da diaria: %s", car[mid].valordiaria);
}
printf("\n\n****Not found****\n\n\n");
return -1; //
}
}

Answer 1

You have few bugs.
First one is in your compare() function. You can not write this line
return strcmp(carroA, carroB);
Because strcmp is working just with char* types, and carroA and carroB are of type carro*. You should place
return strcmp(carroA->placo, carroB->placo);
Now you will sort your structures by the value of placo.
Now lets take a look at your main function.
First bug in main() is in your line of code
scanf("%s", &k);
You don't need &, you should make it like this
scanf("%s",k);
Second bug is in your while() loop in the last line
return -1;
This is a huge thing because main() will return -1. If you want main() to exit like an error occurred you should write return 1;
Maybe you should use one variable to determine weather you have found what you was looking for in your binary search
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAX_CARRO 5 // DEFINE TAMANHO DA STRUCT
typedef struct // STRUCT
{
char placa[50];
char marca[50];
char modelo[50];
char ano[50];
char valordiaria[50];
} carro;
carro car[MAX_CARRO];
int compare (const void * a, const void * b)
{
carro *carroA = (carro *)a;
carro *carroB = (carro *)b;
return strcmp(carroA->placa, carroB->placa);
}
int main()
{
int x=0; //COUNTER
for(x=0; x<MAX_CARRO; x++) // CAR REGISTER
{
printf("\nCarro: %d", (x+1));
printf("\nPlaca: ");
scanf("%s",car[x].placa);
printf("Marca: ");
scanf("%s",car[x].marca);
printf("Modelo: ");
scanf("%s",car[x].modelo);
printf("Ano: ");
scanf("%s",car[x].ano);
printf("Valor da diaria: ");
scanf("%s",car[x].valordiaria);
}
qsort (car, MAX_CARRO, sizeof(carro), compare); // USO DO QSORT
printf("\n\nSTRUCT ORDENADA: \n");
for(x=0; x<MAX_CARRO; x++) // MOSTRA NA TELA A STRUCT ORDENADA
{
printf("\n\n\nCarro: %d", (x+1));
printf("\nPlaca: %s", car[x].placa);
printf("\nMarca: %s", car[x].marca);
printf("\nModelo: %s", car[x].modelo);
printf("\nAno: %s", car[x].ano);
printf("\nValor da diaria: %s", car[x].valordiaria);
}
char k[10];
// *****BINARY SEARCH******
printf("\n\n\n*****BUSCA DE PLACAS*****\n\n\n\n");
printf("Digite a placa que deseja procurar: \n");
scanf("%s", k);
// ***PROBLEM***
int low, high, mid;
low=0;
high = MAX_CARRO-1;
int found=0;
while(low<=high && !found)
{
mid = (low+high)/2;
if (strcmp(k, car[mid].placa)<0)
high=mid-1;
else if (strcmp(k, car[mid].placa)>0)
low=mid+1;
else
{
found = 1;
printf("\nPlaca: %s", car[mid].placa);
printf("\nMarca: %s", car[mid].marca);
printf("\nModelo: %s", car[mid].modelo);
printf("\nAno: %s", car[mid].ano);
printf("\nValor da diaria: %s", car[mid].valordiaria);
}
}
if(!found)
printf("\n\n****Not found****\n\n\n");
return 0; //
}