I'm trying to write a program to add two numbers with a graphical interface written in GTK.
My problem is that it fails to recognize the values ​​entered in the text-box, as numbers.

I searched a long time through google, but I have not found an answer. I have found similar problems, but without an appropriate response.

I hope my query is not outside the margins of the forum.

Can anybody help me to solve the problem that I have in the "action_button function"?

Here my program:


#include <gtk/gtk.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
 
#define Length1 3
#define Length2 5
 
GtkWidget *n1, *n2;
GtkWidget *label_sum;
 
//The Callback funktion. Here is the problem:
void action_button(GtkButton *button, gpointer data){
 
    char result_sum;
    float float_n1, float_n2, sum;
 
    float_n1    = (float)(gtk_entry_get_text(GTK_ENTRY(n1)));
    float_n2    = (float)(gtk_entry_get_text(GTK_ENTRY(n2)));
 
    sum = float_n1 + float_n2;
 
    printf(result_sum, "HA - %d", sum);
    gtk_label_set_label((GtkLabel *)label_sum, result_sum);
 
}


//The Interface part. The problem is not here. 
int main(int argc, char *argv[]){
 
     GtkWidget *window, *descr[15], *layout, *button;
 
     gtk_init_check(&argc, &argv);
 
    //Build window.
     window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
     g_signal_connect(window, "destroy", G_CALLBACK(gtk_main_quit), NULL);
     gtk_window_set_title(GTK_WINDOW(window), "Calculo de Nota");
     gtk_container_set_border_width(GTK_CONTAINER(window), 0);
     gtk_window_set_default_size(GTK_WINDOW(window), 300, 300);
 
    //Build layout.
     layout = gtk_layout_new(NULL, NULL);
 
    //Build labels, one title and two indicator.
     descr[0] = gtk_label_new("Add two numbers");
     descr[1] = gtk_label_new("Number 1: ");
     descr[2] = gtk_label_new("Number 2: ");
     label_sum  = gtk_label_new("");
 
    //Build text fields.
     n1     = gtk_entry_new();
     n2     = gtk_entry_new();
 
     gtk_entry_set_max_length(GTK_ENTRY(n1), Length1);
     gtk_entry_set_max_length(GTK_ENTRY(n2), Length1);
 
     gtk_entry_set_width_chars(GTK_ENTRY(n1), Length2);
     gtk_entry_set_width_chars(GTK_ENTRY(n2), Length2);
 
    //Build the button.
     button = gtk_button_new_with_label("Calcular");
     //g_signal_connect(button, "clicked", G_CALLBACK(action_button), NULL);
 
    //Insert the widgets into the layout.
     gtk_layout_put(GTK_LAYOUT(layout), descr[0], 10, 10);
     gtk_layout_put(GTK_LAYOUT(layout), descr[1], 10, 50);
     gtk_layout_put(GTK_LAYOUT(layout), descr[2], 10, 90);
     gtk_layout_put(GTK_LAYOUT(layout), n1, 180, 50);
     gtk_layout_put(GTK_LAYOUT(layout), n2, 180, 90);
     gtk_layout_put(GTK_LAYOUT(layout), button, 10, 200);
     gtk_layout_put(GTK_LAYOUT(layout), label_sum, 90, 205);//Widget label for printing result.
 
    //Put the layout into the window.
     gtk_container_add(GTK_CONTAINER(window), layout);
 
     gtk_widget_show_all(window);
     gtk_main();
 
return 0;
 }

I get the following error:
"prog.c: In function ‘action_button’:
prog.c:17:5: error: pointer value used where a floating point value was expected
prog.c:18:5: error: pointer value used where a floating point value was expected
prog.c:22:5: warning: passing argument 1 of ‘printf’ makes pointer from integer without a cast [enabled by default]"


What I'm doing wrong?
Thanks for any help.
(Sorry for my bad English. I'm not native speaker.).
Gisi

(Answers could be in Spanish, English or German)

Hi GisiNa, the problem with your code is that gtk_entry_get_text() returns a string and the compiler warns you that the conversion: string(which is actually a const gchar *) to float is not possible. The solution is to convert the number in the string to float using the sscanf function, so replace this:

float_n1    = (float)(gtk_entry_get_text(GTK_ENTRY(n1)));
float_n2    = (float)(gtk_entry_get_text(GTK_ENTRY(n2)));

By this:

sscanf(gtk_entry_get_text(GTK_ENTRY(n1)),"%f", &float_n1);
sscanf(gtk_entry_get_text(GTK_ENTRY(n2)),"%f", &float_n2);

Gracias a 0xDani por la ayuda con el inglés  
Thanks to 0xDani for the help with the English  

Si eres española nativa, por que no posteas en español?

Ups, sorry for the mistake I forgot two parenthesis:
Ups, perdón por el error, olvidé dos paréntesis:

sscanf(gtk_entry_get_text(GTK_ENTRY(n1)),"%f", &float_n1);
sscanf(gtk_entry_get_text(GTK_ENTRY(n2)),"%f", &float_n2);

This not have much sense:
Esto no tiene mucho sentido:

    //"%d"? o %f ...
   // Que esta haciendo result_sum aqui?
   // What is doing result_sum here?
    printf(result_sum, "HA - %d", sum);

The printf function is defined so:
La función printf está definida así:

int printf ( const char * format, ... );

So this should be run(the mistakes are in the comments):
Así que esto debería funcionar (los errores están en los comentarios):

void action_button(GtkButton *button, gpointer data)
{
    //You have to declare an array for show this then... not only a char.
    //Tienes que declarar un array para mostrarlo después, no solamente un caracter.
    char result_sum[100]; 
    float float_n1, float_n2, sum;
 
    sscanf(gtk_entry_get_text(GTK_ENTRY(n1)),"%f", &float_n1);
    sscanf(gtk_entry_get_text(GTK_ENTRY(n2)),"%f", &float_n2);
 
    sum = float_n1 + float_n2;
    //You had confused printf with sprintf
    // Habías confundido printf con sprintf
    sprintf(result_sum,"HA - %f", sum);
    gtk_label_set_label((GtkLabel *)label_sum, result_sum);
}