Exercise 7

1
Exercise 7

• Strings

2
Multi-dimensional arrays

• Array of arrays
• int A23 1, 2, 3,
• 4, 5, 6
• Means an array of 2 integer arrays, each of
  length 3.
• Access j-th element of the i-th array is
• Aij

3
Multi-dimensional arrays

• The size of the array can be determined by the
  compiler
• double B2 1,2, 2,3, 3,4

Cannot skip this!!
4
Example matrix addition
include ltstdio.hgt define SIZE 3 int main()
double ASIZE 1,2,3, 4,5,6,
7,8,9 double BSIZE 1,1,1, 2,2,2,
3,3,3 double CSIZESIZE int i,
j for(i0 iltSIZE i) for(j0 jltSIZE
j) CijAij Bij return
0
5
Strings

• An array of characters
• Used to store text
• Another way to initialize
• char A blabla

6
The Terminator

• Strings terminate with NULL character, signed by
  \0 (ascii code 0)
• This is a convention used to know where the
  string ends
• It means that in order to hold a string of 6
  chars we need an array of length 7
• So the previous initialization is equivalent to
• char A b, l, a, b, l, a,
  \0

7
The fool on the null

• include ltstdio.hgt
• int main(void)
• char str"I'm a full string"
• printf("s\n", str)
• str7'o'
• str8'o'
• printf("s\n", str)
• str11'\0'
• printf("s\n", str)
• str11 s
• printf("s\n", str)
• return 0

I'm a full string
I'm a fool string
I'm a fool
I'm a full string
8
Reading-in strings

• There are several ways of accepting strings as
  input from the user
• The obvious way to go is read character by
  character using getchar()

9
Example

• include ltstdio.hgt
• define MAX_LENGTH 10
• int main(void)
• char sMAX_LENGTH1 / We need one more place
  for the '\0' /
• char c
• int i
• printf("Please enter a string\n")
• for (i0 iltMAX_LENGTH i)
• c getchar()
• if (c'\n')
• break
• si c

10
Reading-in strings scanf

• A simpler way is to use scanf
• To read in a string to a variable str, write
• scanf(s, str)
• Note theres no sign!!!

11
Reading-in strings - scanf

• scanf reads-in letters until a space or newline
  (\n) is encountered
• The maximum length can be stated in the
  parentheses
• scanf(10s, str)
• This will read in 10 letters, plus the \0 sign
  (so str should have place for 11 characters)

12
Reading-in strings - scanf

• scanf reads-in letters until a space or newline
  (\n) is encountered
• scanf(10s, str)
• This will read in 10 letters, plus the \0 sign
  (so str should have place for 11 characters)
• IMPORTANT better always to give maximum length
  to scanf otherwise the program may be aborted
  if the word entered is longer than our string
  array.

13
Example

• include ltstdio.hgt
• define MAX_LENGTH 5
• int main(void)
• char strMAX_LENGTH1 / We need one more
  place for the '\0' /
• printf("Please enter a string\n")
• scanf("5s", str) / Note - no '' sign!!! /
• printf("The string you entered is - s\n", str)
• return 0

14
Example

• include ltstdio.hgt
• define MAX_LENGTH 5
• int main(void)
• char strMAX_LENGTH1
• char c
• scanf("5s", str)
• scanf("c", c)
• printf("str is - s\n", str)
• printf(c - c\n", c)
• return 0

USER INPUT 12345\n
str is - 12345 c \n
15
Example

• include ltstdio.hgt
• define MAX_LENGTH 5
• int main(void)
• char strMAX_LENGTH1
• char c
• scanf("5s", str)
• scanf(\nc", c)
• printf("str is - s\n", str)
• printf(c - c\n", c)
• return 0

USER INPUT 12345\n x
str is - 12345 c x
16
Comparing strings

• We cannot just compare strings contents by
• char A7Hello
• char B7Hello
• if (AB)
• ...
• Because A and B are addresses of A0 and B0
• AB only if A and B are the same string in
  memory
• In order to compare the contents we must scan
  char by char

H
e
l
l
o
\0
.
.
.
H
e
l
l
o
\0
.
.
.
B
A
17
Example

• include ltstdio.hgt
• int main(void)
• int i
• char A101
• char B101
• printf("Enter first string\n")
• scanf("100s",A)
• printf("Enter second string\n")
• scanf("100s",B)
• for(i0 Ai!'\0' Bi!'\0' i)
• if(Ai!Bi)
• printf("A is different from B!\n")
• return 0

18
Compare step by step
A
Y
e
s
\0

• for(i0 Ai!'\0' Bi!'\0' i)
• if(Ai!Bi)
• printf("A is different from B!\n")
• return 0
• printf("A and B are the same!\n")
• return 0

B
Y
e
w
\0
i
0
19
Compare step by step
A
Y
e
s
\0

• for(i0 Ai!'\0' Bi!'\0' i)
• if(Ai!Bi)
• printf("A is different from B!\n")
• return 0
• printf("A and B are the same!\n")
• return 0

B
Y
e
w
\0
i
0
20
Compare step by step
A
Y
e
s
\0

• for(i0 Ai!'\0' Bi!'\0' i)
• if(Ai!Bi)
• printf("A is different from B!\n")
• return 0
• printf("A and B are the same!\n")
• return 0

B
Y
e
w
\0
i
1
21
Compare step by step
A
Y
e
s
\0

• for(i0 Ai!'\0' Bi!'\0' i)
• if(Ai!Bi)
• printf("A is different from B!\n")
• return 0
• printf("A and B are the same!\n")
• return 0

B
Y
e
w
\0
i
1
22
Compare step by step
A
Y
e
s
\0

• for(i0 Ai!'\0' Bi!'\0' i)
• if(Ai!Bi)
• printf("A is different from B!\n")
• return 0
• printf("A and B are the same!\n")
• return 0

B
Y
e
w
\0
i
2
23
Compare step by step
A
Y
e
s
\0

• for(i0 Ai!'\0' Bi!'\0' i)
• if(Ai!Bi)
• printf("A is different from B!\n")
• return 0
• printf("A and B are the same!\n")
• return 0

B
Y
e
w
\0
i
2
24
Compare step by step
A
Y
e
s
\0

• for(i0 Ai!'\0' Bi!'\0' i)
• if(Ai!Bi)
• printf("A is different from B!\n")
• return 0
• printf("A and B are the same!\n")
• return 0

B
Y
e
w
\0
i
2
25
Compare step by step
A
Y
e
s
\0

• for(i0 Ai!'\0' Bi!'\0' i)
• if(Ai!Bi)
• printf("A is different from B!\n")
• return 0
• printf("A and B are the same!\n")
• return 0

B
Y
e
w
\0
i
2
26
Equal strings step by step
A
Y
e
s
\0

• for(i0 Ai!'\0' Bi!'\0' i)
• if(Ai!Bi)
• printf("A is different from B!\n")
• return 0
• printf("A and B are the same!\n")
• return 0

B
Y
e
s
\0
i
0
27
Equal strings step by step
A
Y
e
s
\0

• for(i0 Ai!'\0' Bi!'\0' i)
• if(Ai!Bi)
• printf("A is different from B!\n")
• return 0
• printf("A and B are the same!\n")
• return 0

B
Y
e
s
\0
i
0
28
Equal strings step by step
A
Y
e
s
\0

• for(i0 Ai!'\0' Bi!'\0' i)
• if(Ai!Bi)
• printf("A is different from B!\n")
• return 0
• printf("A and B are the same!\n")
• return 0

B
Y
e
s
\0
i
1
29
Equal strings step by step
A
Y
e
s
\0

• for(i0 Ai!'\0' Bi!'\0' i)
• if(Ai!Bi)
• printf("A is different from B!\n")
• return 0
• printf("A and B are the same!\n")
• return 0

B
Y
e
s
\0
i
1
30
Equal strings step by step
A
Y
e
s
\0

• for(i0 Ai!'\0' Bi!'\0' i)
• if(Ai!Bi)
• printf("A is different from B!\n")
• return 0
• printf("A and B are the same!\n")
• return 0

B
Y
e
s
\0
i
2
31
Equal strings step by step
A
Y
e
s
\0

• for(i0 Ai!'\0' Bi!'\0' i)
• if(Ai!Bi)
• printf("A is different from B!\n")
• return 0
• printf("A and B are the same!\n")
• return 0

B
Y
e
s
\0
i
2
32
Equal strings step by step
A
Y
e
s
\0

• for(i0 Ai!'\0' Bi!'\0' i)
• if(Ai!Bi)
• printf("A is different from B!\n")
• return 0
• printf("A and B are the same!\n")
• return 0

B
Y
e
s
\0
i
3
33
Equal strings step by step
A
Y
e
s
\0

• for(i0 Ai!'\0' Bi!'\0' i)
• if(Ai!Bi)
• printf("A is different from B!\n")
• return 0
• printf("A and B are the same!\n")
• return 0

B
Y
e
s
\0
i
3
34
Different length step by step
A
Y
e
s
\0

• for(i0 Ai!'\0' Bi!'\0' i)
• if(Ai!Bi)
• printf("A is different from B!\n")
• return 0
• printf("A and B are the same!\n")
• return 0

B
Y
e
\0
i
0
35
Different length step by step
A
Y
e
s
\0

• for(i0 Ai!'\0' Bi!'\0' i)
• if(Ai!Bi)
• printf("A is different from B!\n")
• return 0
• printf("A and B are the same!\n")
• return 0

B
Y
e
\0
i
0
36
Different length step by step
A
Y
e
s
\0

• for(i0 Ai!'\0' Bi!'\0' i)
• if(Ai!Bi)
• printf("A is different from B!\n")
• return 0
• printf("A and B are the same!\n")
• return 0

B
Y
e
\0
i
1
37
Different length step by step
A
Y
e
s
\0

• for(i0 Ai!'\0' Bi!'\0' i)
• if(Ai!Bi)
• printf("A is different from B!\n")
• return 0
• printf("A and B are the same!\n")
• return 0

B
Y
e
\0
i
1
38
Different length step by step
A
Y
e
s
\0

• for(i0 Ai!'\0' Bi!'\0' i)
• if(Ai!Bi)
• printf("A is different from B!\n")
• return 0
• printf("A and B are the same!\n")
• return 0

B
Y
e
\0
i
2
39
Different length step by step
A
Y
e
s
\0

• for(i0 Ai!'\0' Bi!'\0' i)
• if(Ai!Bi)
• printf("A is different from B!\n")
• return 0
• printf("A and B are the same!\n")
• return 0

B
Y
e
\0
i
1
40
Different length step by step
A
Y
e
s
\0

• for(i0 Ai!'\0' Bi!'\0' i)
• if(Ai!Bi)
• printf("A is different from B!\n")
• return 0
• printf("A and B are the same!\n")
• return 0

B
Y
e
\0
i
1
41
String library

• Like in the case of stdio.h and math.h, we have a
  special library for handling strings
• We should include string.h
• Functions
• strlen(s) returns the length of s
• strcmp(s1, s2) compares s1 with s2
• strcpy(s1, s2) copies to contents of s2 to s1
• and more

42
Example
easy_compare.c
43
If theres time

• Exercise write a program that
• gets an input string from the user (up to 100
  chars, no white spaces)
• gets 2 characters from the user
• replaces each occurrence of the first character
  in the string by the second character, and prints
  the result.
• Example
• input papa, p, m
• output mama

44
Solution
replace.c
45
If theres time

• Exercise write a function
• void my_to_lower(char str)
• The function receives an arbitrary string and
  converts all its letters to lower-case letters
• Demonstrate the use of your function by some
  example that prints the input and output on the
  screen