Why does my program enter an infinite loop when I use float values in a While loop?

Question

I was tasked to do a program to read the name of one student, 4 of their subjects and their respective grades. I have worked with for and while loops, also with if statements and here is my code so far:

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

main() {
    printf("Este programa captura el nombre de un alumno \n");
    printf  ("y cuatro de sus materias y sus respectivas notas\n");
    printf      ("Nota: El programa solo toma en cuenta los dos primeros\n");
    printf          ("decimales de la notas expresada.\n\n");
    
    char alumno[40] = {'\0'};
    char mat[4][20] = {'\0', '\0', '\0', '\0'};
    float calif[4] = {-1, -1, -1, -1};
    int i;
    
    while (alumno[0] == '\0') {
        printf("Ingresa el nombre del alumno: ");
        gets(alumno);
        if (alumno[0] == '\0') {
            printf("\nError. El alumno debe de llevar un nombre.");
            printf  ("\nTrata nuevamente\n\n");
        };
    };
    for (i = 0; i < 4; ++i) {
        while (mat[i][0] == '\0') {
            printf("Ingresa el nombre de la materia %d: ", i+1);
            gets(mat[i]);
            if (mat[i][0] == '\0') {
                printf("\nError. Las materias deben ser declaradas.");
                printf  ("\nTrata nuevamente.\n\n");
            };
        };
        while (calif[i] < 0 || calif[i] > 10) {
            printf("Ingrese la nota correspondiente a esta materia (0-10): ");
            scanf("%2.2f", calif[i]);
            if (calif[i] < 0 || calif[i] > 10) {
                printf("\nError. Debe ingresar una nota válidad entre 0 y 10.");
                printf  ("\nTrata nuevamente.\n\n");
            };
        };
    };
    
    return 0;
};

The program seems to be working fine until it gets to the point to ask the grade of the first subject. Any grade I put it causes an infinite loop. I have searched about this problem to no avail. So please, let me know what I am doing wrong here.

Answer 1

There are multiple problems in your code:

• [major] you read the mark with scanf("%2.2f", calif[i]): the format string is incorrect, the first 2 means read at most 2 bytes and the .2 is an error as scanf() does not support the precision syntax at all. It should just be "%f" and you should pass the address of the destination variable &calif[i] instead of its value. Furthermore, you should test the return value to check for invalid or missing input.

• the prototype for main is int main(), int main(void) or int main(int argc, char *argv[]), the missing return type is an obsolete syntax.

• reading input with gets() is risky as there is no way to prevent buffer overflow with sufficiently long input strings. You should instead use scanf("%39s", alumno) or fgets(alumno, sizeof alumno, stdin) and test the return value to detect premature end of input stream. Same remark for the second instance of gets().