(Extreme Noob here) Why wont this C code work?

Question

Why won't this work, I'm very new to programming but I can't seem to figure out why this wont work correctly.

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

int main(){
    int num1;
    printf("Enter 1, 2, 3.");
    scanf("%d", &num1);
    if(num1 = 1)
        printf("You entered one");
    else if(num1 = 2)
        printf("You entered two");
    else if(num1 = 3)
        printf("You entered three");
    else
        printf("Invalid");
}

If you use `gcc` to compile, I would recommend that you include flags `-Wall` and `-Wextra` to avoid issues like this in the future. — Christian Mann, Jun 02 '13 at 22:34
Won't work **how**? Always include *what you expect*, and *what you're seeing instead*, and *why you think that's surprising*. — T.J. Crowder, Jun 02 '13 at 22:39
@effeffe: Or turn on warnings, and formulate conditions in a reasonable, non-Yoda way. — T.J. Crowder, Jun 02 '13 at 22:42

score 9 · Answer 1 · edited May 23 '17 at 11:57

9

In C it is valid to use assignment (int x = 5) within a conditional (if statement).

For example:

int x = 0;

if (x = 5)
{

}

This will evaluate to true (it returns 5 to the "if" and all non zero terms are true by convention) if the assignment could be done and the value != 0. Which, in this case, it can be done and returns 5.

You were likely looking for this:

int x = 0;

if (x == 5)
{

}

This will evaluate to false (0).

Remember: You use a single equal sign "=" to mean "assignment". Use a double equal sign "==" to mean "comparison".

edited May 23 '17 at 11:57

Community

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answered Jun 02 '13 at 22:33

MrHappyAsthma

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Use a double equal sign "==" to mean **comparison** – mf_ Jun 02 '13 at 22:35
Better way to put it. Updated :) – MrHappyAsthma Jun 02 '13 at 22:35
Note that the assignment will return 5, not 1 -- that still evaluates as true as far as the if statement is concerned, but it isn't "true". – walkytalky Jun 02 '13 at 22:38
That was just mindless error. Hahah. Fixed. I meant to say "evaluate to" again. I was just in a rush and careless. – MrHappyAsthma Jun 02 '13 at 22:39

score 6 · Answer 2 · edited Jun 02 '13 at 22:34

6

Replace all the = with == and you should be fine (because = is used for assignment, while == is used to test for equality, which seems to be what you want to do)

edited Jun 02 '13 at 22:34

Scott Hunter

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answered Jun 02 '13 at 22:31

Tomer Arazy

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I'm surprised that the OP compiler does not warn for this. – Martin James Jun 02 '13 at 22:32
@MartinJames OPs tend to use compilers without warnings :) – Elazar Jun 02 '13 at 22:33
Ohh yeah, wow can't believe I forgot about that, Thank you. :) – user2446045 Jun 02 '13 at 22:33
1

That's probably against SO rules but...Hi @Elazar :) – Tomer Arazy Jun 02 '13 at 22:35
@user2446045 add `-Wall -Wextra` to turn on compiler warnings – FDinoff Jun 02 '13 at 22:35
@user2446045 - try to compile with `-Wall` flag – Tomer Arazy Jun 02 '13 at 22:35

score 2 · Answer 3 · answered Jun 02 '13 at 22:36

In C, as in other many programming languages, the = operator means "assignment". When you do a = 3, that means "assign a with 3", which of course it's something that succeeds and returns true, that's why your program will always enter the first branch.

What you have to do is use the "equality testing" operator, ==, so that a == 3 returns true if and only if the value held by variable a is 3.

EAGLE · Answer 4 · 2013-06-02T23:31:49.203

0

Your code having one mistake you have taken = instead of ==, in C = operator means assignment operator while== operator is used for comparision.

To clear about your doubts regarding operators read this link http://www.tutorialspoint.com/cprogramming/c_operators.htm

edited Jun 02 '13 at 23:31

answered Jun 02 '13 at 23:07

EAGLE

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score 0 · Answer 5 · answered Jun 03 '13 at 00:27

0

And because you started with int main() just for compiler reasons put return 0; at the end of your program to be more correct.

answered Jun 03 '13 at 00:27

Spyros Chiotakis

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From C99 the `return 0;` is implicit, and if we want to be correct, the signature should be `int main(void)`. – effeffe Jun 03 '13 at 09:28

score 0 · Answer 6 · edited Feb 15 '14 at 01:20

0

It doesn't work because you need to change the = sign to ==. You use the equal sign sometimes when you declare a int or char. == is meaning equal to and you want to use that when your not declaring ints and chars.While != means not equal.You should also put a return 0; at the end of your program.

edited Feb 15 '14 at 01:20

falsetru

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answered Feb 15 '14 at 01:02

LFM

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(Extreme Noob here) Why wont this C code work?

6 Answers6