Lesson 5.1. Conditional branching (if...else)

The if...else instruction executes a block of instructions depending on the result of a test:

If the weather is nice, I'll go for a run, otherwise I'll work on my C class.

Syntax

Here is the syntax of the if...else statement:

if (test) {
  ...
  // Instruction block 1
}
else {
  ...
  // Instruction block 2
}

The first block of instructions will only be executed if the test is true.

It should be noted that:

• do not put a semicolon at the end of the if;
• parentheses in the test are mandatory;
• the else{...} is optional;
• braces can be omitted if the block contains only one statement.

Example 1

This example displays two different messages depending on whether A is larger than B or not:

if (A>B) {
  printf ("A is greater than B\n");
}
else {
  printf ("A is smaller than or equal to B\n");
}

Example 2

In the previous example, each instruction block contains only one statement. The code can be simplified by omitting the braces:

if (A>B) 
  printf ("A is greater than B\n");
else     
  printf ("A is smaller than or equal to B\n");

Example 3

This example illustrates the optional nature of the else {} :

// Compare A and B
if (A>B) printf ("A is greater than B\n");
if (A<B) printf ("est plus petit que\n");
if (A==B) printf ("A is equal to B\n");

Exercises

Exercise 1

Write a program that asks the user to enter his or her grade point average.

• If the average is greater than or equal to 10, the program displays "Congratulations! You graduated.";
• otherwise, it displays "Maybe next time!".

Enter your grade point average: 10.0
Congratulations! You graduated.

#include <stdio.h> int main(void) { float gpa; // Ask the GPA printf("Enter your grade point average: "); scanf("%f", &gpa); // Display if graduated or not // COMPLETE HERE return 0; }

Exercise 2

Write a program that asks the user to enter a number between 0 and 9 included. The program then displays the number in words. We will assume that the user enters a valid number (there is no need to check the input):

Enter a number [0;9]: 4
4 is written four

#include <stdio.h> int main(void) { unsigned int number; // Ask for a number between 0 and 9 included printf("Enter a number [0;9]: "); scanf("%u", &number); // Displays the number in words // COMPLETE HERE return 0; }

Exercise 3

Write a program that asks the user to enter two integers (x1 and x2). If x1 is not smaller than x2, the contents of the variables are swapped. At the time of display, x1 must always be smaller than x2 :

Enter the first integer: 5
Enter the second integer: 3
x1=3    x2=5

Enter the first integer: 3
Enter the second integer: 5
x1=3    x2=5

#include <stdio.h> int main(void) { int x1, x2, tmp; // Ask the user two integers printf ("Enter the first integer: "); scanf ("%d", &x1); printf ("Enter the second integer: "); scanf ("%d", &x2); // Swap x1 and x2 is x2 is greater than x1 // COMPLETE HERE // Affiche les entiers : printf ("x1=%d\tx2=%d\n", x1, x2); return 0; }

Quiz

What does the following code display?

if (4<2)
  printf ("Crac");
else
  printf ("Boum");

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When can you omit the curly braces?

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What does the following code display?

if (0)
  printf ("Crac");
  printf ("Boum");

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What does the following code display?

if (0)
  printf ("Crac"); printf ("Boum");

---

What does the following code display?

if (0);
  printf ("Crac"); 
printf ("Boum");

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When can the test parentheses be omitted?

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See also

• Learn C programming
• Lesson 1.1. History of C programming language
• Lesson 1.2. First program
• Lesson 1.3. Compilation
• Lesson 1.4. Preprocessor directives
• Lesson 1.5. Which compiler to choose?
• Lesson 1.6. Flowchart
• Lesson 2.1. Types and variables
• Lesson 2.2. Integers
• Lesson 2.3. Floating point numbers
• Lesson 2.4. Characters
• Lesson 2.5. Variables initialization
• Lesson 2.6. Ariane flight 501
• Lesson 3.1. Arithmetic operators
• Lesson 3.2. modulo
• Lesson 3.3. Operators and types
• Lesson 3.4. Type casting
• Lesson 3.5. Bitwise operators
• Lesson 3.6. Bitwise operators details
• Lesson 3.7. Shifting operators
• Lesson 3.8. Assignment operators
• Lesson 3.9. Increment and decrement operators
• Lesson 3.10. Comparison operators
• Lesson 3.11. Logical operators
• Lesson 3.12. Operator precedence
• Lesson 4.1. printf
• Lesson 4.2. scanf
• Lesson 4.3. putchar
• Lesson 5.2. Nested if and indentation
• Lesson 5.3. Checking intervals
• Lesson 5.4. Ternary Operator ?:
• Lesson 5.5. switch..case statement
• Lesson 5.6. Break keyword in switch..case
• Lesson 6.1. do..while loop in C
• Lesson 6.2. While loop in C
• Lesson 6.3. For loop in C
• Lesson 6.4. How to choose the right loop in C ?
• Lesson 6.5. Exercices on loops in C
• Lesson 7.1. Bit masking
• Lesson 7.2. Bit masking, how to clear a bit in C?
• Lesson 7.3. Bit masking, how to set a bit in C?
• Lesson 7.4. Bit masking, how to toggle a bit in C?
• Lesson 7.5. Bit masking, how to check a single bit in C?
• Lesson 7.6. Overview of bit masking
• Lesson 8.1. Basic syntax of C functions
• Lesson 8.2. Calling C functions
• Lesson 8.3. Void keyword
• Lesson 8.4. Return keyword
• Lesson 8.5. Variable scope
• Lesson 8.6. Global variables
• Lesson 8.7. Static variables
• Lesson 8.8. Random numbers in C
• Lesson 8.9. Mathematical functions in C
• Lesson 9.1. Syntax of C arrays
• Lesson 9.2. Array initialization
• Lesson 9.3. Multidimensional arrays
• Lesson 9.4. How C arrays are stored in memory?
• Lesson 9.5. Arrays and functions
• Lesson 9.6. Exercises on C arrays
• Lesson 10.1. Strings in C
• Lesson 10.2. Null terminated string
• Lesson 10.3. The string.h library
• Lesson 10.4. String and functions
• Lesson 11.1. Introduction to pointers in C
• Lesson 11.2. Syntax of C pointers
• Lesson 11.3. Dynamic memory allocation
• Lesson 11.4. Incrementing pointers
• Lesson 11.5. Pointers as function argument in C
• Lesson 12.1. Introduction to structures in C
• Lesson 12.2. Properties of structures in C
• Lesson 12.3. Structures and pointers
• Lesson 12.4. Structures and functions
• Lesson 13.1. Recursive functions in C
• Lesson 13.2. Depth of recursive functions
• Lesson 13.2. Mutual recursion
• Lesson 14.1. Additional exercises

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