V tej vadnici bomo spoznali izjavo if … else za ustvarjanje programov odločanja s pomočjo primerov.
Pri računalniškem programiranju uporabljamo if
stavek za zagon blok kode le, če je izpolnjen določen pogoj.
Na primer dodelitev ocen (A, B, C) na podlagi ocen, ki jih je pridobil študent.
- če je odstotek nad 90 , določite oceno A
- če je odstotek nad 75 , določite oceno B
- če je odstotek nad 65 , določite oceno C
V if… else
jeziku C ++ obstajajo tri oblike izjav.
if
izjavoif… else
izjavoif… else if… else
izjavo
Izjava C ++ if
Sintaksa if
stavka je:
if (condition) ( // body of if statement )
if
Izjavo ocenjuje condition
med oklepaji ( )
.
- Če
condition
oceni natrue
, se izvede koda znotraj telesaif
. - Če
condition
oceni nafalse
, se koda znotraj telesaif
preskoči.
Opomba: Koda znotraj ( )
je telo if
stavka.
![](https://cdn.wiki-base.com/5344099/c_ifelse_with_examples.png.webp)
Primer 1: Izjava C ++ if
// Program to print positive number entered by the user // If the user enters a negative number, it is skipped #include using namespace std; int main() ( int number; cout <> number; // checks if the number is positive if (number> 0) ( cout << "You entered a positive integer: " << number << endl; ) cout << "This statement is always executed."; return 0; )
Izhod 1
Vnesite celo število: 5 Vnesli ste pozitivno število: 5 Ta stavek se vedno izvrši.
Ko uporabnik vstopi 5
, se stanje number> 0
ovrednoti true
in if
izvrši stavek znotraj telesa .
Rezultat 2
Vnesite številko: -5 Ta stavek se vedno izvrši.
Ko uporabnik vstopi -5
, se stanje number> 0
ovrednoti false
in stavek znotraj telesa if
ne izvede.
C ++, če … drugače
if
Izjava lahko izbirno else
klavzulo. Njegova sintaksa je:
if (condition) ( // block of code if condition is true ) else ( // block of code if condition is false )
if… else
Izjavo ocenjuje condition
znotraj oklepaju.
![](https://cdn.wiki-base.com/5344099/c_ifelse_with_examples_2.png.webp)
Če condition
oceni true
,
if
se izvrši koda znotraj telesa- koda znotraj telesa
else
je preskočena iz izvedbe
Če condition
oceni false
,
else
se izvrši koda znotraj telesa- koda znotraj telesa
if
je preskočena iz izvedbe
Primer 2: Izjava C ++, če … drugače
// Program to check whether an integer is positive or negative // This program considers 0 as a positive number #include using namespace std; int main() ( int number; cout <> number; if (number>= 0) ( cout << "You entered a positive integer: " << number << endl; ) else ( cout << "You entered a negative integer: " << number << endl; ) cout << "This line is always printed."; return 0; )
Izhod 1
Vnesite celo število: 4 Vnesli ste pozitivno celo število: 4. Ta vrstica je vedno natisnjena.
V zgornjem programu imamo pogoj number>= 0
. Če vnesemo število, večje ali enako 0, potem pogoj izračuna true
.
Tu vpišemo 4. Torej, pogoj je true
. Zato se stavek znotraj telesa if
izvrši.
Rezultat 2
Vnesite celo število: -4 Vnesli ste negativno celo število: -4. Ta vrstica je vedno natisnjena.
Tu vnesemo -4. Torej, pogoj je false
. Zato se stavek znotraj telesa else
izvrši.
C ++ če … drugače … drugače če stavek
if… else
Izjava se uporablja za izvršitev blok kode med dvema možnostma. Če pa se moramo odločiti med več kot dvema alternativama, uporabimo if… else if… else
izjavo.
Sintaksa if… else if… else
stavka je:
if (condition1) ( // code block 1 ) else if (condition2)( // code block 2 ) else ( // code block 3 )
Tukaj,
- Če
condition1
oceni natrue
,code block 1
se izvede. - Če
condition1
oceni nafalse
, potemcondition2
se oceni. - Če
condition2
jetrue
,code block 2
se izvede. - Če
condition2
jefalse
,code block 3
se izvede.
![](https://cdn.wiki-base.com/5344099/c_ifelse_with_examples_3.png.webp)
Opomba: Izjav je lahko več, lahko else if
pa le en if
in else
izjav.
Primer 3: C ++ če … drugače … drugače če
// Program to check whether an integer is positive, negative or zero #include using namespace std; int main() ( int number; cout <> number; if (number> 0) ( cout << "You entered a positive integer: " << number << endl; ) else if (number < 0) ( cout << "You entered a negative integer: " << number << endl; ) else ( cout << "You entered 0." << endl; ) cout << "This line is always printed."; return 0; )
Izhod 1
Vnesite celo število: 1 Vnesli ste pozitivno celo število: 1. Ta vrstica je vedno natisnjena.
Rezultat 2
Enter an integer: -2 You entered a negative integer: -2. This line is always printed.
Output 3
Enter an integer: 0 You entered 0. This line is always printed.
In this program, we take a number from the user. We then use the if… else if… else
ladder to check whether the number is positive, negative, or zero.
If the number is greater than 0
, the code inside the if
block is executed. If the number is less than 0
, the code inside the else if
block is executed. Otherwise, the code inside the else
block is executed.
C++ Nested if… else
Sometimes, we need to use an if
statement inside another if
statement. This is known as nested if
statement.
Think of it as multiple layers of if
statements. There is a first, outer if
statement, and inside it is another, inner if
statement. Its syntax is:
// outer if statement if (condition1) ( // statements // inner if statement if (condition2) ( // statements ) )
Notes:
- We can add
else
andelse if
statements to the innerif
statement as required. - The inner
if
statement can also be inserted inside the outerelse
orelse if
statements (if they exist). - We can nest multiple layers of
if
statements.
Example 4: C++ Nested if
// C++ program to find if an integer is even or odd or neither (0) // using nested if statements #include using namespace std; int main() ( int num; cout <> num; // outer if condition if (num != 0) ( // inner if condition if ((num % 2) == 0) ( cout << "The number is even." << endl; ) // inner else condition else ( cout << "The number is odd." << endl; ) ) // outer else condition else ( cout << "The number is 0 and it is neither even nor odd." << endl; ) cout << "This line is always printed." << endl; )
Output 1
Enter an integer: 34 The number is even. This line is always printed.
Output 2
Enter an integer: 35 The number is odd. This line is always printed.
Output 3
Enter an integer: 0 The number is 0 and it is neither even nor odd. This line is always printed.
In the above example,
- We take an integer as an input from the user and store it in the variable num.
- We then use an
if… else
statement to check whether num is not equal to0
.- If
true
, then the innerif… else
statement is executed. - If
false
, the code inside the outerelse
condition is executed, which prints "The number is 0 and neither even nor odd."
- If
- The inner
if… else
statement checks whether the input number is divisible by2
.- If
true
, then we print a statement saying that the number is even. - If
false
, we print that the number is odd.
- If
Notice that 0
is also divisible by 2
, but it is actually not an even number. This is why we first make sure that the input number is not 0
in the outer if
condition.
Note: As you can see, nested if… else
makes your logic complicated. If possible, you should always try to avoid nested if… else
.
Body of if… else With Only One Statement
If the body of if… else
has only one statement, you can omit ( )
in the program. For example, you can replace
int number = 5; if (number> 0) ( cout << "The number is positive." << endl; ) else ( cout << "The number is negative." << endl; )
with
int number = 5; if (number> 0) cout << "The number is positive." << endl; else cout << "The number is negative." << endl;
The output of both programs will be the same.
Note: Although it's not necessary to use ( )
if the body of if… else
has only one statement, using ( )
makes your code more readable.
More on Decision Making
V nekaterih primerih lahko ternarni operater nadomesti if… else
stavek. Če želite izvedeti več, obiščite C ++ Ternary Operator.
Če se moramo odločiti med več kot enimi alternativami glede na dani preskusni pogoj, switch
lahko uporabimo izjavo. Če želite izvedeti več, obiščite stikalo C ++.
Oglejte si te primere, če želite izvedeti več:
Program C ++ za preverjanje, ali je število sodo ali liho
Program C ++ za preverjanje, ali je znak samoglasnik ali soglasnik.
Program C ++ za iskanje največjega števila med tremi številkami