Storing user input into linked list - C

Question

I have an assignment for C beginner class: I'm supposed to create a structure that has one variable (named value) and one pointer to the list. I'm supposed to prompt the user for 5 values as input and store them in the linked list. Then print out the list. I can't figure out how to store input into the list. I did manage to write a program in which I initialize the 5 values. But I don't know how to accept input and store it into the list.

Here's the working program where I initialize the values:

#include <stdio.h>

struct list
{
    double value;
    struct list *nextVal;
};

int main()
{
    struct list v1 = {10};
    struct list v2 = {17.97};
    struct list v3 = {166};
    struct list v4 = {2};
    struct list v5 = {387.55};
    struct list *first;

    first = &v1;
    v1.nextVal = &v2;
    v2.nextVal = &v3;
    v3.nextVal = &v4;
    v4.nextVal = &v5;
    v5.nextVal = NULL;

    printf("All the values are: %.2f, %.2f, %.2f, %.2f, %.2f.",
    first->value,v1.nextVal->value,v2.nextVal->value,
    v3.nextVal->value,v4.nextVal->value);

    return 0;
}

Here's a program where I tried getting user input and storing that into the list. (I only have 2 values instead of 5, cuz it's easier to work with that when trying to make it work.) When I compile this program, I get no errors. When I run it, I am properly prompted for the two input values; however, when the output that the program prints just writes 0 for both values. My assumption is that I'm storing the value into the list wrong - because I don't know how it's actually done. (Can't find any info in my textbook or online.) Last night someone commented on a different question I had to try using breakpoint to find out exactly where the problem is - I've been trying to figure it out since then but don't know how to do that yet. (I'm assuming it's something very simple - but everywhere I looked online, people explain it in a way that seems like I'm supposed to know what they're talking about - which I don't - And honestly, even if I could figure it out, I still wouldn't know what to do with that information, since I just can't find any information on how to store user input into a linked list.) Anyways, so here's my trial program that complies and runs but gives me 0 for both final values:

#include <stdio.h>

struct list
{
    double value;
    struct list *nextVal;
};

int main()
{
    double val1,val2;

    printf("Please enter a number: ");
    scanf("%f",&val1);
    printf("Please enter a number: ");
    scanf("%f",&val2);

    struct list v1 = {val1};
    struct list v2 = {val2};
    struct list *first;

    first = &v1;
    v1.nextVal = &v2;
    v2.nextVal = NULL;
    printf("The two values entered are %.2f and %.2f.",
    first->value,v1.nextVal->value);

    return 0;
}

Thank you in advance! And I'm reaaaally a beginner - so even if you suggest something super easy - I might have no clue what it is..so please explain! Thank you!

Do you know about function malloc?

Vlad from Moscow
– Vlad from Moscow

2016-04-04 23:30:25 +00:00
Commented Apr 4, 2016 at 23:30 — Vlad from Moscow
– Vlad from Moscow, Commented Apr 4, 2016 at 23:30

kaylum · Accepted Answer · 2016-04-04 23:16:55Z

1

scanf("%f",&val1);

%f requires a float variable but the given variable is a double. Use %lf for double.

scanf("%lf",&val1);

Your compiler should have given you a warning for that and you should always heed those and resolve them:

warning: format ‘%f’ expects argument of type ‘float *’, but argument 2 has type ‘double *’ [-Wformat=]
 scanf("%f",&val1);

A few extra words of advice though they are not directly addressing your question:

Always check the function return values. Specifically, check scanf to ensure that the expected input was parsed. The code as it is will fail if the user enters something unexpected.
Break up your code into functions. Specifically, make a seperate insert function to add items to the linked list.
Use loops where you need to repeat code. Such as for the scanf calls. That way it can be easily extended when you want to take more input.

edited Apr 4, 2016 at 23:16

answered Apr 4, 2016 at 23:11

kaylum

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2 Comments

Taylor Ash Over a year ago

Thank you so much. I corrected that and it works. My compiler did not give me any warning - which was why I was so confused as to what the issue was.

kaylum Over a year ago

Which compiler are you using? Most compilers will have an option to turn up warning levels. Suggest you find out how to do that and enable it for your builds.

Harry · Accepted Answer · 2016-04-04 23:16:02Z

Try compiling with warnings on etc. At a minimum I use this...

gcc -Wall -pedantic -std=c11

Working version

include <stdio.h>

struct list
{
  double value;
  struct list *nextVal;
};

int main()
{
  double val1,val2;

  printf("Please enter a number: ");
  scanf("%lf",&val1);
  printf("Please enter a number: ");
  scanf("%lf",&val2);

  struct list v1 = {val1};
  struct list v2 = {val2};
  struct list *first;

  first = &v1;
  v1.nextVal = &v2;
  v2.nextVal = NULL;
  printf("The two values entered are %.2f and %.2f.",
      first->value,v1.nextVal->value);

  return 0;
}

You were scanning quotes and trying to assign it to a double. A good compiler would give you a warning..

scanf.c:14:14: warning: format specifies type 'float *' but the argument has type 'double *' [-Wformat]
  scanf("%f",&val1);
         ~~  ^~~~~
         %lf
scanf.c:16:14: warning: format specifies type 'float *' but the argument has type 'double *' [-Wformat]
  scanf("%f",&val2);
         ~~  ^~~~~
         %lf

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