The debugger Some programs may compile correctly, yet not produce the desirable results. These programs are valid and correct C programs, yet not the programs.

The debugger Some programs may compile

correctly, yet not produce the desirable results.

These programs are valid and correct C programs, yet not the programs we meant to write!

The debugger can be used to follow the program step by step and may help detecting bugs in an already compiled program.

The debugger’s common features

Setting breakpoints (a point where the execution stops): bring the cursor to desired line and press the palm icon or F9. A dark red dot appears near the line.

Executing a debugged run: Build->start debug->go or F5. The program will run and stop at the first breakpoint.

The debugger’s common features (cont.) Stopping at a specific line: Bringing the

cursor to the line and press ctrl+F10, or Build->start debug->go to cursor. The program will stop at that point.

Stepping to the next line – F10. Entering a function – F11. Seeing variable values – quickwatch

and/or debug window at the bottom. The yellow arrow indicates our

whereabouts at any given moment.

Examples

factorial_func.cgetchar.c

Buggy examples

pi_bad.cexp_bad.c

Random numbers

Some applications require using random numbers

Computer games - Card games, raffles, adventure and strategy games etc.

Some cryptography stuff… int rand(void);

Declared in stdlib.h Returns a pseudo-random number

from 0 to RAND_MAX, which is at least 32,767

Sowing the seed of love The numbers returns by rand are

determined by the random-number generator’s ‘seed’

To change the seed, use -void srand(unsigned int seed);

(defined in stdlib.h) To truly randomize things, call –

srand(time(NULL));(and don’t forget to #include <time.h>)

Telling time

time_t time(NULL); time_t is an arithmetic type for storing

time periods Essentially an int

The function returns the number of seconds elapsed since 00:00:00 GMT, Jan 1, 1970

The parameter should always be NULL, until we learn about pointers

Simulating an event with probability p

(double)rand()/RAND_MAX < p First scale rand’s return value to

between 0 and 1 If result is smaller than p (between

0 and p), the event has occurred Otherwise, it hasn’t

0 1p

Simulating a die throw

Problem – implement the following function – int RandInRange(int min, int max);

That returns a random number between min and max

First step Implement a function that returns

a random value between 0 and N-1 for some N

int BoundedRand(int N){

return rand()%N;}

Full Solution

/* Returns a random integer between bottom and top */

int RandInRange(int bottom, int top){

return bottom + rand()%(top-bottom+1);}

Example

die_throws.c

Exercise

Write a program that generates a number at random and waits for the user to guess the right number

At each guess, the program informs the user if the real number is greater or smaller than the number she has chosen

The program should terminate when the user guesses correctly or enters -1

Solution

guess_work.c

Exercise Write a program that simulates a

nuclear decay experiment Input –

N – the number of nuclei P – the probability of decay per time step

Output – The number of time steps until all nuclei

have decayed Note – simulating the number of steps

should take place in a separate function

Solution

nuclear_decay.c Note – it might be more interesting

calculating the average time until full decay for different N’s and P’s by repeating the same experiment a large number of times.

Arrays

A block of many variables of the same type

Array can be declared for any type E.g. int A[10] is an array of 10 integers.

Examples: list of students’ marks series of numbers entered by user vectors matrices

Arrays in Memory

Sequence of variables of specified type The array variable itself holds the

address in memory of beginning of sequence

Example: double S[10];

The k-th element of array A is specified by A[k-1] (0 based)

0 1 2 3 4 5 6 7 8 9

S

……

Example - reverse#include <stdio.h>

int main(void){ int i, A[10];

printf("please enter 10 numbers:\n"); for(i=0; i<10; i++) scanf("%d",&A[i]);

printf("numbers in reversed order:\n"); for(i=9; i>=0; i--) printf("%d\n",A[i]);}

Define

Magic Numbers (like 10 in the last example) in the program convey little information to the reader

Hard to change in a systematic way #define defines a symbolic name During preprocessing phase,

symbolic names are replaced by the replacement text

Reverse with #define/* get 10 integers from the user and printing them in reversed order*/#include <stdio.h>

#define NUM 10

int main(void){ int i; int A[NUM];

printf(“Please enter %d numbers:\n",NUM); for(i=0; i<NUM; i++) scanf("%d",&A[i]);

printf("numbers in reversed order:\n"); for(i=NUM-1; i>=0; i--) printf("%d\n",A[i]);}

Initialization Like in the case of regular variables, we can initialize the

array during declaration. the number of initializers cannot be more than the

number of elements in the array but it can be less in which case, the remaining elements are initialized to

0 if you like, the array size can be inferred from the number

of initializers by leaving the square brackets empty so these are identical declarations :int array1 [8] = {2, 4, 6, 8, 10, 12, 14, 16};

int array2 [] = {2, 4, 6, 8, 10, 12, 14, 16};

Example

Sort.c

Sort – step by step

#include <stdio.h>

#define SIZE 3

int main(void){

int A[SIZE] = {4,8,2}; int min_index;int i,j,tmp;

4 8 2

A[0] A[1] A[2]

min_index

----

---- ---- ----

i j tmp


for(i=0; i<SIZE-1; i++){

min_index=i;for(j=i+1; j<SIZE; j++)

min_index=(A[min_index]>A[j] ? j : min_index);

tmp=A[i];A[i]=A[min_index];A[min_index]=tmp;

}

4 8 2

A[0] A[1] A[2]

min_index

----

0 ---- ----

i j tmp






}

4 8 2

A[0] A[1] A[2]

min_index

0

0 ---- ----

i j tmp






}

4 8 2

A[0] A[1] A[2]

min_index

0

0 1 ----

i j tmp






}

4 8 2

A[0] A[1] A[2]

min_index

0

0 1 ----

i j tmp






}

4 8 2

A[0] A[1] A[2]

min_index

0

0 2 ----

i j tmp






}

4 8 2

A[0] A[1] A[2]

min_index

2

0 2 ----

i j tmp






}

4 8 2

A[0] A[1] A[2]

min_index

2

0 3 ----

i j tmp






}

4 8 2

A[0] A[1] A[2]

min_index

2

0 3 4

i j tmp






}

2 8 2

A[0] A[1] A[2]

min_index

2

0 3 4

i j tmp






}

2 8 4

A[0] A[1] A[2]

min_index

2

0 3 4

i j tmp






}

2 8 4

A[0] A[1] A[2]

min_index

2

1 3 4

i j tmp






}

2 8 4

A[0] A[1] A[2]

min_index

1

1 3 4

i j tmp






}

2 8 4

A[0] A[1] A[2]

min_index

1

1 2 4

i j tmp






}

2 8 4

A[0] A[1] A[2]

min_index

2

1 2 4

i j tmp






}

2 8 4

A[0] A[1] A[2]

min_index

2

1 3 4

i j tmp






}

2 8 4

A[0] A[1] A[2]

min_index

2

1 3 8

i j tmp






}

2 4 4

A[0] A[1] A[2]

min_index

2

1 3 8

i j tmp






}

2 4 8

A[0] A[1] A[2]

min_index

2

1 3 8

i j tmp






}

2 4 8

A[0] A[1] A[2]

min_index

2

2 3 8

i j tmp

ExerciseWrite a program that gets an input line from

the user (ends with ‘\n’) and displays the number of times each letter appears in it.

Example:For the input line - hello, world!The output should be:d - 1e – 1h – 1l – 2o – 2w - 1

Assume that the input is all in lower-case.

Solution

letter_count.c

The debugger Some programs may compile correctly, yet not produce the desirable results. These programs are valid and correct C programs, yet not the programs.

Documents

c slide

p slide

max slide

s slide

int max

int mainvoid

int a10

int asize