.NT
A NOTE ABOUT THE LESSONS in C
.b4-24
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These were written while the author was ~Ilearning~N the language and since
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they are ~Ifree~N ( to copy and/or distribute ) there is a money-back
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guarantee on the accuracy of each and every statement in the lessons (!)
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The ~Idisplay~N program was written ( in C ) in order to provide a vehicle
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for displaying the lessons.
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.B
P.J.Ponzo
.B
Dept. of Applied Math
.B
Univ. of Waterloo
.B
Ontario N2L 3G1
.K16,30
[1mPonzoTUTOR
.WNT
let's try IF FOR a WHILE
.R4C1
~b~Imain() { /* what's in a name ? */ ~N
~b~I int i, j; /* a bunch of integers. */ ~N
~b~I char name[10]; /* an array of 10 chars. */ ~N
~b~I for (i=0; i<25; i++) /* print '\n', 25 times, */ ~N
~b~I printf("\n"); /* to clear the screen! */ ~N
~b~I printf("\n Type your name : "); /* ask for a name. */ ~N
~b~I scanf("%s",&name); /* input the name. */ ~N
~b~I for (i=0, j=0; name[i] != '\0'; i++) { /* a nice for-loop. */ ~N
~b~I if ( name[i] == 'e' ) /* check for an 'e'. */ ~N
~b~I j++; /* if so, increment j. */ ~N
~b~I } /* end of for-loop */ ~N
~b~I printf("\n The letter e occurs %d times in %s",j,name); */ ~N
~b~I} /* end of main() */ ~N
.R17C1
Let's look at this program.
It's supposed to ask for a name, then print out the number of times
the letter ~Ie~N occurs in the name.
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~Vmain() { /* what's in a name ? */ ~N
~V int i, j, num; /* a bunch of integers. */ ~N
~V char name[10]; /* an array of 10 chars. */ ~N
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This part is familiar. Note that we allow for a name of ~I9~N characters
since the 10th will be the terminating ~I'\0'~N (remember?).
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~b~Imain() { /* what's in a name ? */ ~N
~b~I int i, j; /* a bunch of integers. */ ~N
~b~I char name[10]; /* an array of 10 chars. */ ~N
~V for (i=0; i<25; i++) /* print '\n', 25 times, */ ~N
~V printf("\n"); /* to clear the screen! */ ~N
.R17C1
.R17C1
We use a ~Ifor~N loop to print ~I25~N ~b~In~Newlines (thereby clearing
the screen ..not very elegent, but.. ).
The new thing here is ~V i++ ~N which, in ~IC~N, means ~Iincrement i~N.
.WR7C1
~b~I for (i=0; i<25; i++) /* print '\n', 25 times, */ ~N
~b~I printf("\n"); /* to clear the screen! */ ~N
~V printf("\n Type your name : "); /* ask for a name. */ ~N
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.R17C1
We now ask for your name by printing:
~r~I Type your name : ~N
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~b~I printf("\n Type your name : "); /* ask for a name. */ ~N
~V scanf("%s",&name); /* input the name. */ ~N
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Then we wait for the user to type in his/her name (ending with the
~IEnter~N key), and put this ~Istring~N at ~Imemory address~N ~V &name ~N.
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~b~I scanf("%s",&name); /* input the name. */ ~N
~b~I for (~Fi=0, j=0~N~b~I; name[i] != '\0'; i++) { /* a nice for-loop. */ ~N
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.R17C1
Now we go through the ~b~Iname[]~N array, one character at-a-time, starting
with ~b~Ii=0~N (the first character is the ~Izero~Nth!). We will count the
number of times the letter ~Ie~N occurs and store this count in the variable
~b~Ij~N, so we also initialize ~b~Ij=0~N, too! (Note the use of the ~ICOMMA~N
between ~b~Ii=0~N and ~b~Ij=0~N).
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~b~I for (i=0, j=0; ~Fname[i] != '\0'~N~b~I; i++) { /* a nice for-loop. */ ~N
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This ~Ifor-loop~N will continue so long as the ~b~Ii~Nth character in the
~b~Iname[]~N array is not the ~INULL '\0'~N. (Note the construction ~F~I!=~N
which, in C, means ~INOT EQUAL~N).
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~b~I for (i=0, j=0; name[i] != '\0'; ~Fi++~N~b~I) { /* a nice for-loop. */ ~N
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Of course, each time we advance through the characters in the array
~b~Iname[]~N we must increment ~b~Ii~N (until we reach the end).
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~b~I for (i=0, j=0; name[i] != '\0'; i++) ~F{~N~b~I /* a nice for-loop. */ ~N
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...our openers for the ~Ifor~N...
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~V if ( name[i] ~F==~N~V 'e' ) /* check for an 'e'. */ ~N
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And now we check the ~b~Ii~Nth character, ~b~Nname[i]~N, to see if its ~IEQUAL~N
to the letter ~Ie~N. (Note the curious way we check for ~F==~N).
(Had we used ~b~Iname[i]='e'~N it would compile OK, but this actually
~Iassigns~N to ~b~Iname[i]~N the character ~I'e'~N rather than checking
for ~Iequality~I! ...and ~b~Iname~N would be ~IALL e~Ns).
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~b~I if ( name[i] == 'e' ) /* check for an 'e'. */ ~N
~V j++; /* if so, increment j. */ ~N
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Now, ~Iif~N we find an ~I'e'~N, we increment ~b~Ij~N.
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~b~I j++; /* if so, increment j. */ ~N
~b~I~F }~N~b~I /* end of for-loop */ ~N
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...and this ~b~I~F}~N ends the ~Ifor-loop~N.
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~b~I for (i=0, j=0; name[i] != '\0'; i++) { /* a nice for-loop. */ ~N
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~b~I } /* end of for-loop */ ~N
~V printf("\n The letter e occurs %d times in %s",j,name); */ ~N
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...and, after leaving the ~Ifor-loop~N, we print the results:
~r~IThe letter e occurs ~N ~r~I times in ~N
~Ij~N ~Iname~N
goes here. goes here.
.WN
~b~Imain() { /* what's in a name ? */ ~N
~b~I int i, j; /* a bunch of integers. */ ~N
~b~I char name[10]; /* an array of 10 chars. */ ~N
~b~I for (i=0; i<25; i++) /* print '\n', 25 times, */ ~N
~b~I printf("\n"); /* to clear the screen! */ ~N
~b~I printf("\n Type your name : "); /* ask for a name. */ ~N
~b~I scanf("%s",&name); /* input the name. */ ~N
~b~I for (i=0, j=0; name[i] != '\0'; i++) { /* a nice for-loop. */ ~N
~b~I if ( name[i] == 'e' ) /* check for an 'e'. */ ~N
~b~I j++; /* if so, increment j. */ ~N
~b~I } /* end of for-loop */ ~N
~b~I printf("\n The letter e occurs %d times in %s",j,name); */ ~N
~b~I} /* end of main() */ ~N
After we ~Icompile~N and ~Ilink~N and ~Iexe~Ncute, we get:
~r~I Type your name : ~N ~IPeter~N
we type this, then press the Enter key, and get:
~r~IThe letter e occurs 2 times in Peter~N
.WK9,30
nice!nice!
.WNT
IF (this ) do that ELSE IF (this) do that ELSE do that
.R4C1
We may wish to check for several characters (not just 'e'), so we could say:
1 ~b~I if ( name[i] == 'e' ) j++; /* increment j if an 'e' */~N
2 ~b~I else if ( name[i] == 'f' ) k++; /* increment k if an 'f' */~N
3 ~b~I else if ( name[i] == 'g' ) l++; /* increment l if an 'g' */~N
4 ~b~I else ; /* else do nothing */~N
In Line 1 we increment the variable ~b~Ij~N (which counts the number of times
an 'e' occurs.
In Line 2 we increment the variable ~b~Ik~N (which counts the number of times
an 'f' occurs.
In Line 3 we increment the variable ~b~Il~N (which counts the number of times
an 'g' occurs.
In Line 4 (which is reached only if the character is none of the above) we
do nothing. (We could have done something interesting, but we should, just
once, demonstrate a DO NOTHING statement .... just the !@#$$% SEMI-COLON!).
(We would, of course, have declared ~b~Ik~N and ~b~Il~N as ~b~Iint~N data types).
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of course!
.WNT
More stuff like i++
Although we could have incremented ~b~Ii~N by using ~b~Ii=i+1~N, we used
the ~Iincrement operator ++~N. There is also (what else?) a ~Idecrement~N
~Ioperator --~N. In fact these can be either ~Ipre-~N or ~Ipost-operative~N.
~b~Ij=i--~N will assign to ~b~Ij~N the value of ~b~Ii~N, ~Ithen~N will
decrement ~b~Ii~N.
~b~Ij=--i~N will first decrement ~b~Ii~N, ~Ithen~N assign to ~b~Ij~N the decremented
value of ~b~Ii~N.
Note the convenience of typing ~b~Iantidisestablishmentarianism++~N and not
~b~Iantidisestablishmentarianism=antidisestablishmentarianism+1~N.
Also, the following ~Iassignment operators~N may be used:
~b~Ix+=5~N instead of ~b~Ix=x+5~N
~b~Ix-=5~N instead of ~b~Ix=x-5~N
~b~Ix*=5~N instead of ~b~Ix=x*5~N
~b~Ix/=5~N instead of ~b~Ix=x/5~N
.WNT
and tests for EQUALITY && INEQUALITY etc.
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To test for equality of, say, ~b~Ix~N and ~b~I5~N we ask if ~b~Ix==5~N.
Had we used something like: ~b~Iif (x=5)~N then ~b~Ix~N would be assigned
the value ~b~I5~N (and, of course, ~b~Ix~N would now BE equal to ~b~I5~N and
the if-statements would certainly be executed).
We also use ~b~Iif (x!=5)~N (for NOT EQUAL) and ~b~Iif (x>5)~N and ~b~Iif (x<5)~N.
We also have: ~b~Iif (x>5 && x!=7)~N where ~b~I&&~N means AND ...so this reads:
if (x is GREATER than 5) AND (x is NOT EQUAL to 7)
.R15C1
We also have: ~b~Iif (x=5 āā x>=7)~N where ~b~Iāā~N means OR ...so this reads:
if (x is EQUAL to 5) OR (x is GREATER or EQUAL to 7)
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We also have: ~b~Iif (x~F=~N
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x==5 !!!
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.T
&& that's all folks!
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au revoir!
.q
