CSCI1190: Beginning C Programming for Engineers

1
CSCI-1190 Beginning C Programming for Engineers

• Lecture 3 Functions
• Gang Chen

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Control Structures

• if, if-else, switch
• while, do-while
• for

3
From 1 to 100 step 1

• int i1
• do
• i
• while (ilt100)

int i1 while(ilt100) i
for(i1ilt100i)
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From 1 to 100 step 2

• int i1
• do
• ii2
• while (ilt100)

int i1 while(ilt100) i2
for(i1ilt100i2)
Question What is the value of i in the last
iteration?
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From 100 to 1 step -1

• int i100
• do
• i--
• while (igt0)

int i100 while(igt0) i--
for(i100igt0i--)
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1,2,4,8,16,32,64,,1024

• int i1
• do
• i2
• while (ilt1024)

int i1 while(ilt1024) i2
for(i1ilt1024i2)

• Other sequences
• 1,-2,3,-4,5,-6,7,-8,
• 1,1/2,1/4,1/8,1/16,1/32,1/64,
• 1,1/2,1/3,1/4,1/5,1/6,1/7,
• 1,2,3,5,8,13,21,34,

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Functions

• Functions are modules with parameters (or
  arguments), statements, and a result
• Why functions?
• Decomposition
• Code reuse

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Prototypes and Definitions

• A prototype declares a function with a return
  type and a parameter list.
• float power (float a, float b)
• A definition implements the function
• float power (float a, float b)
• float s
• / calculate the power of a and b /
• / put the result into the variable s/
• return s
• The definition must match the prototype
• Either the prototype or the definition must
  appear before the function call

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Calling Functions

• Variable(s) must match the parameter(s)
• Same number, same order, compatible type,
• Names could be different
• How about printf, scanf?
• printf(hello)printf(ad,a)
• Variable-length parameter list
• int printf(char, ) /stdio.h/

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Type Conversion

• float power(float a, float b)
• float x,y,z
• int i,j
• power(x) / error /
• power(x,y,z) / error /
• power(i,j) / ok /

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void

• For functions taking no parameters
• int f(void)
• For functions returning no values
• void g(void)

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Math Functions

• Header file include ltmath.hgt
• Compiling gcc Wall o prog prog.c -lm

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Random Number Generator

• include ltstdio.hgt
• include ltstdlib.hgt / srand() and rand() /
• include lttime.hgt / time() /
• int main()
• float r
• srand(time(NULL))
• r1.0rand()/RAND_MAX
• printf("Random number f\n",r)
• return 0

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In-Class Exercise 3-1

• Write a program to generate 1000 floating point
  random numbers between 0 and 1.
• Find out the largest number, the smallest number
  and the average.

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Call by Value

• The execution of a function does not affect the
  value of variables passed as parameters

void increment(int a) a int main()
int i1 increment(i)
printf("id\n",i) / i1, not 2 /
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Variable Scopes

• A scope of a variable is the portion of the
  program where the variable can be recognized
• File scope
• global variables
• Function scope
• local variables, parameters
• Block scope
• local variables

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Variable Scopes An Example

• include ltstdio.hgt
• int x / global variable /
• int f(float x) / parameter /
• return 0
• int g()
• int x / local variable /
• return 0
• int main()
• float x / local variable /
• int x / local variable /
• return 0

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Recursion

• A function can call itself, either directly or
  indirectly through other functions
• Some conditional code is needed to stop the
  recursion
• Recursion is equivalent to iteration

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Factorial

• The factorial of an integer
• n!n(n-1)(n-2)21
• 1!10!1
• Recursive equation
• n!n(n-1)!, if n0
• n!1, if n0

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Factorial Recursion

• include ltstdio.hgt
• int factorial(int n)
• if(n0) return 1
• else return nfactorial(n-1)
• int main()
• int n
• printf("Enter n ")
• scanf("d",n)
• printf("d! d\n",n,factorial(n))
• return 0

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Factorial Iteration

• int factorial(int n)
• int i,f
• for(i1,f1iltni)
• fi
• return f

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In-Class Exercise 3-1

• Use recursion to implement the power function
  that takes two integers.
• Hint power(a,b)apower(a,b-1)