Getting integer for a switch without using scanf-CodePudding

So I have written some code that has a switch, and I need to give it an integer to select a case within the switch. I can't use scanf() because I have multiple fgets() further down the line and the '\n' from the scanf() input breaks the code.

Here is my code:

main.c

#include "functions.h"
#include <stdlib.h>

int main() {
    int choice;
    char temp[10];
    
    do {
        printf("Menu\n\n");
        printf("1. Read student information from file\n");
        printf("2. Write student information to file\n");
        printf("3. Exit\n");
        fgets(choice, 10, stdin);
    
        switch (choice) {
          case 1:
            fileRead();
            break;
          case 2:
            fileWrite();
            break;
          default:
            printf("Program stopped!\n");
            break;
        }
    } while (choice != 3);
    return 0;
}

functions.h

#ifndef UNTITLED17_FUNCTIONS_H
#define UNTITLED17_FUNCTIONS_H

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

struct student {
    char studentID[100];
    char studentName[100];
    char age[100];
} student_t;

void fileRead() {
    FILE *f = fopen("student_read.txt", "r");
    if (f == NULL) {
        printf("Failed to open file(s)!\n");
    }
    printf("Type your student ID:");
    fgets(student_t.studentID, 100, stdin);

    printf("Type your name:");
    fgets(student_t.studentName, 100, stdin);

    printf("Type your age:");
    fgets(student_t.age, 100, stdin);

    printf("Student id: %s\n", student_t.studentID);
    printf("Name: %s\n", student_t.studentName);
    printf("Age: %s\n", student_t.age);
}

void fileWrite() {
    FILE *f = fopen("student_write.txt", "w");
    if (f == NULL) {
        printf("Failed to open file(s)!\n");
    }
    printf("Type your student ID:");
    fgets(student_t.studentID, 100, stdin);
    printf("Type your name:");
    fgets(student_t.studentName, 100, stdin);
    printf("Type your age:");
    fgets(student_t.age, 100, stdin);

    printf("Student id: %s\n", student_t.studentID);
    printf("Name: %s\n", student_t.studentName);
    printf("Age: %s\n", student_t.age);
}

#endif //UNTITLED17_FUNCTIONS_H

Any ideas?

Thanks :)

CodePudding user response：

As OP mentions the qualified nature of input, I'd go with a simple fgets(), atoi(). Read into a buffer and convert to an int.

//fgets(choice, 10, stdin);
//switch(choice){

// Initialize just in case no input.
// Not so big to prevent atoi() overflow - which is UB. 
char buf[5] = ""; 

fgets(buf, sizeof buf, stdin);
choice = atoi(buf);
switch(choice) {

If input was not qualified, I'd use a larger buffer, strtol() and more error checking.

In that case, makes sense to make a helper function to get an int.

Some untested code:

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

// On EOF, return EOF
// Else return 1 for success, 0 failure.
int getint(const char *prompt, int *dest) {
  // Some buffer generously sized to the type.
  char buf[sizeof *dest * CHAR_BIT];

  if (prompt) {
    fputs(prompt, stdout);
  }
  if (fgets(buf, sizeof buf, stdin) == NULL) {
    return EOF;
  }
  char *endptr;
  errno = 0;
  long val = strtol(buf, &endptr, 0);
  if (buf == endptr) {
    return 0;  // No conversion
  }
  if (errno == ERANGE) {
    return 0;  // out of long range
  }
  #if LONG_MIN < INT_MIN || LONG_MAX > INT_MAX
  if (val > INT_MAX || val < INT_MIN) {
    errno = ERANGE;
    return 0; // Out of int range
  }
  #endif
  *dest = (int) val;
  return 1;
}

CodePudding user response：

fgets, as the s suggests, is meant for reading a string from a file, which is why the compiler complains about the first argument: it should be a pointer to where that string will go. Once there, as @Cheetah suggests, parse that to get the integer value it represents.

CodePudding user response：

This line from your code reads a line into the choice buffer:

fgets(choice, 10, stdin);

You can add this directly afterwards:

int n;
if (sscanf(choice, "%d", &n) != 1)) {
    fprintf(stderr, "Invalid input\n");
    exit(1);
}

Most notably here is the sscanf that reads the choice buffer that should contain a string, since it was returned by fgets. The value is stored in n. You can check that in your switch statement.

I added an if around the sscanf, to check that 1 is returned, that is the number of arguments that were read successfully.

CodePudding user response：

fgets(choice, 10, stdin); is definitely incorrect as choice is an int, not a pointer to char nor a char array.

You should read the string and convert its contents as an integer this way:

    if (!fgets(temp, sizeof tmp, stdin))
        return 1;
    choice = atoi(temp);

Also note these remarks:

if the file cannot be open in fileRead() and fileWrite(), you should return after you output the error message.
functions should not be defined in header files, only declarations belong there
you should test the return value of fgets() to detect end of file.
fileRead and fileWrite should return an success/failure indicator.
you should strip the newline left by fgets() at the end of the array. Even better: you should write a function that reads a line into the destination array, truncating to available space and stripping initial and trailing whitespace.