Instructions

Create the If Else Statement

1

Begin with the basics. The basic structure of every If Else statement in C takes this form:

if (condition) action;
2

Bracket your action correctly. The action can be a single command, or more often a series of commands, inside curly brackets.

Create the Condition

1

Realize that a condition is anything that returns 0 (false) or anything else (true).
2

Know that the most common kind is a comparison between variables or values:
== equality
!= inequality
> greater than
< less than
>= greater than or equal to
<= less than or equal to
3

Compare numeric and character types, if you so choose, but be careful: characters compare based on the character set in use on that system, and that may be different some day on some other operating system.
4

Do not try to compare arrays or structures directly. Instead, write a function that will carry out the comparison.
5

Understand that C standard libraries include such functions for strings. See strcmp(), stricmp(), and strncmp().
6

Use assignment to return the value that was assigned.

if ((x = malloc(100)) == NULL) abort();The value returned by malloc() is both saved to x and compared to NULL, making your code more concise.
7

Use a tried and true technique: It's common to have functions return 1 on success and 0 on failure, and use them as a condition.
8

Realize that in C, everything has a value, so anything can be the condition. For instance, a comparison to 0 is usually redundant.

if (x) printf("x is not zero\n");

Combine and Modify Conditions

1

Use the ! (not) operator to negate, or reverse, a condition.

if (!x) printf("x is zero\n");
2

Use && (and) and || (or) to combine conditions logically. && requires both conditions true to be true; || is true if either one is.
3

Use parentheses to control how conditions are evaluated.

if (x == 5 and y == 6 or z == 7)is ambiguous;

if ((x == 5 and y == 6) or z == 7)is clear.
4

Take advantage of C's limitations. C will always evaluate the parts of a && or || from left to right, and stop evaluating once it knows the answer. Use this to your advantage. For example,

if (x and *x)prevents runtime errors in case x is NULL, since C will have stopped evaluating before the dereference.

Add Else Clauses

1

Add an alternative action if the condition is false, simply by adding the word else after the action, then another action, like so:

if (condition) action; else action;
2

Know that both actions can be, and typically are, whole groups of commands included in curly brackets, like this:

if (condition) {
do stuff;
} else {
do other stuff;
}
3

Realize that it's common for the Else statement to include another If, to select the right one from a whole set of cases. Consider this example:

if (x <= 80) {
grade = 'C';
} else if (x <= 90){
grade = 'B';
} else {
grade = 'A';
}