Question

EDIT: I have fully fixed my code now. Here it is:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <math.h>

int main(){
    int n, i , j;
    char **mensagem;
    char leitura[1000];
    FILE *arquivo;

    arquivo = fopen("entrada.txt", "r");

    fgets(leitura, 1000, arquivo);

    fclose(arquivo);

    arquivo = fopen("entrada.txt", "r");

    n = sqrt(strlen(leitura));

    mensagem = malloc(n * sizeof(char *));

    for (i = 0; i < n; i++){
        mensagem[i] = malloc(n * sizeof(char));
    }

    for (j = 0; j < n; j++){
        for (i = 0; i < n; i++){
            fscanf(arquivo, "%c", &mensagem[i][j]);
        }
    }

    for (j = 0; j < n; j++){
        for (i = 0; i < n; i++){
            if(mensagem[j][i] == '*'){
                printf(" ");
                i = n - 1;
            }
            else {
                printf("%c", mensagem[j][i]);
            }
        }
    }

    fclose(arquivo);
    free(mensagem);

    return 0;
}

The idea is to decypher messages like Cesar did back in the days :D. I read a coded message from a .txt file. I read the entire line then I dynamically allocate an array of arrays.
That way I can print the columns and the message will be printed normally.

Here’s my code:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <math.h>
int main(){
    int n, i , j;
    char **mensagem;
    char leitura[1000];
    FILE *arquivo;
    arquivo = fopen("entrada.txt", "r");

    fgets(leitura, 1000, arquivo);

    n = sqrt(strlen(leitura));

    mensagem = (char **)malloc(n * n * sizeof(char));

    for (j = 0; j < n; j++){
        for (i = 0; i < n; i++){
            fscanf(arquivo, "%c", &mensagem[i][j]);
        }
    }

    for (j = 0; j < n; j++){
        for (i = 0; i < n; i++){
            if(mensagem[i][j] == '*'){
                printf(" ");
                i = n - 1;
            }
            else {
                printf("%c", mensagem[i][j]);
            }
        }
    }

    fclose(arquivo);
    free(mensagem);

    return 0;
}

Here’s the data in the file:

AEEUMOLSHMSCGT*AGU2A***L:****T*****A

And here’s the expected output:

ALG2: ESTA EH UMA MSG OCULTA

And here’s the logic behind it:

A E E U M O
L S H M S C
G T * A G U
2 A * * * L
: * * * * T
* * * * * A

I do know that the problem is exactly with the call of printf, but I’m unsure on how to fix it.

Answer 1

This is a classic example of “two stars does not a two-dimensional array make”.

mensagem = (char **)malloc(n * n * sizeof(char));

for (j = 0; j < n; j++){
    for (i = 0; i < n; i++){
        fscanf(arquivo, "%c", &mensagem[i][j]);
    }
}

mensagem is a pointer to a block of n * n bytes. However, when you do mensagem[i][j], the compiler will read the value of the pointer, add i, and then read the pointer at that address. What is that pointer set to? Whatever happens to be at i * sizeof(char *) into your allocated memory – probably zero.

You need to do a two-stage allocation, first allocating n pointers, then allocating storage for each string of n bytes.

Edit: I intentionally haven’t written the code to solve your problem, because you will leard much more from writing it yourself…