`. Without this library, the compiler won't identify the `bool` variable type.
(declare-vs-initialization)=
## Declaring Vs. Initializing Variables
In your code, if you need to declare a variable, you do it as follows
```
int var;
```
The compiler will understand that you *declared* a variable with `int` type and identifier `var`. When running your code, the computer will reserve a space for it in the memory. The question is, what is the value of this declared variable? The answer is not `0`.
The variable is **uninitialized**. It is not holding any value. It is just a variable that is declared, but not initialized. To be more specific, if you try printing the value of the declared but uninitialized variable, you print the value that is there in the memory location of the variable. Probably this value was there before the program started running. It is a random value. Some people call it "garbage" value.
For example, when I ran the code below on my computer, the value in `var` variable was `174739296`, because I never initialized `var`. However, when you run the code, you may get a different value. The value will be different for each run too. You can download from {download}`declare-vs-initialize.c <../../code/chapter2/declare-vs-initialize/declare-vs-initialize.c>` to play with the code.
**Code**
#include <stdio.h>
int main(void) {
int var;
printf("Value of uninitialized variable \"var\": %d\n", var);
int var2 = 0;
printf("Value of initialized variable \"var\": %d\n", var2);
return 0;
}
```{note}
If you compile the code above, you will get a warning stating: `variable ‘var’ is uninitialized when used here [-Wuninitialized]`. The compiler will also ask you to initialize `var` to silence the warning.
```
This becomes a problem, if you are unaware of it. If you use an uninitialized variable later, your program's behavior will be undefined. Therefore, it is best practice to declare a variable **AND** initialize it, e.g. `int var = 0;`. `int var` declares the `var` variable and `= 0;` initializes the `var` variable to `0`.
## Taking in input from the user using `scanf`
Given that we now know the format specifiers of `int`, `double`, `char` and `bool` data types, there are a few tricks you need to know as you use these to take input from the user using `scanf`.
1. **Take multiple numbers in multiple variables.**
You can take multiple numbers from the user in one single `scanf`. The `scanf` should separate the format specifiers by a space. The user should separate the numbers by **delimiters**. Delimiters can be a space, return or tab and are used to separate two different inputs. An example code that takes multiple numbers as input from user is shown below.
**Code**
#include <stdio.h>
int main(void) {
int num1 = 0, num2 = 0;
double dnum1 = 0, dnum2 = 0;
printf("Enter a number: ");
scanf("%d %lf %d %lf", &num1, &dnum1, &num2, &dnum2);
printf("Numbers entered: %d %lf %d %lf\n", num1, dnum1, num2, dnum2);
return 0;
}
2. **Take numbers and characters.**
You can take numbers and characters in the same `scanf` line. The example code shown below takes in an ID from the user that begins with a character and is followed by a number. It does not require a delimiter between the character entered and the numbers. This is because the `%c` format specifier will take one character, and stop taking more input. The rest will be taken by `%d`. If the user enters a space between the character and the numbers it will be ignored.
**Code**
#include <stdio.h>
int main(void) {
char idChar;
int idNum;
printf("Enter your ID: ");
scanf("%c %d", &idChar, &idNum);
printf("ID entered: %c%d\n", idChar, idNum);
return 0;
}
You can also write the code above with no spaces between `%c` and `%d` in `scanf` as follows.
**Code**
#include <stdio.h>
int main(void) {
char idChar;
int idNum;
printf("Enter your ID: ");
scanf("%c%d", &idChar, &idNum);
printf("ID entered: %c%d\n", idChar, idNum);
return 0;
}
**Code**
#include <stdio.h>
int main(void) {
char idChar;
int idNum;
printf("Enter your ID: ");
scanf("%d%c", &idNum, &idChar);
printf("ID entered: %c%d\n", idChar, idNum);
return 0;
}
3. **Take in characters and ignoring leading spaces.**
If you want to take in character by character, but you are entering spaces or returns between them, what should you do? To ignore spaces between characters entered, you need to add a space between the format specifier `%c`. For example, the following code takes in the 4 letters and three numbers of a license plate in Ontario. If you do not add a space between the `%c`, any delimiter entered will be considered a character, and taken into the character variable.
**Code**
#include <stdio.h>
int main(void) {
char c1, c2, c3, c4, c5, c6, c7;
printf("Enter license plate letters and numbers: ");
scanf("%c %c %c %c %c %c %c", &c1, &c2, &c3, &c4, &c5, &c6, &c7);
printf("Licence plate entered: %c%c%c%c-%c%c%c\n", c1, c2, c3, c4, c5, c6,
c7);
return 0;
}
4. **Common mistake: Spaces after format specifiers**
Do not include a space after a format specifier, if there is no format specifier after it in `scanf`, like `scanf("%d ", &num);`. This is because `scanf` will wait for a delimiter after you enter your number and another input too. Although it won't put that second input into another variable. For example, the following code will not proceed with executing other statements unless you enter another input after your number.
**Code**
#include <stdio.h>
int main(void) {
double dnum1 = 0;
printf("Enter a number: ");
scanf(" %lf ", &dnum1);
printf("Number entered: %.2lf\n", dnum1);
return 0;
}
{{quiz_embed | replace("%%FILENAME%%", "chapter-2/sec-2") }}