if-else in C, Python and R: Syntax Compared Side by Side

The same grade check in C, Python and R

The logic of an if-else statement is identical in C, Python and R: test a condition, run one block if it is true, otherwise run another. What differs is the syntax: how blocks are marked (braces vs. indentation) and how “else if” is written.

The same program in each language (grade from a score of 85):

C uses parentheses () around the condition and curly braces {} for blocks. Multiple conditions use else if, and every statement ends with ;.

#include <stdio.h>

int main() {
    int score = 85;

    if (score >= 90) {
        printf("Grade: A\n");
    } else if (score >= 80) {
        printf("Grade: B\n");
    } else {
        printf("Grade: C\n");
    }

    return 0;
}

Python drops the braces and the parentheses. A colon : and indentation define the blocks, and “else if” is contracted to a single keyword, elif.

score = 85

if score >= 90:
    print("Grade: A")
elif score >= 80:
    print("Grade: B")
else:
    print("Grade: C")

R looks much like C: parentheses, braces, else if. But it has a formatting quirk: at the top level, else must sit on the same line as the closing brace } of the previous block.

score <- 85

if (score >= 90) {
  print("Grade: A")
} else if (score >= 80) {
  print("Grade: B")
} else {
  print("Grade: C")
}

All three print Grade: B.

Direct syntax comparison

Feature	C	Python	R
Condition wrapper	Requires `( )`	No wrapper	Requires `( )`
Block boundary	`{ }`	`:` + indentation	`{ }`
“Else if” keyword	`else if`	`elif`	`else if`
Statement end	`;`	Line break	Line break
Boolean operators	`&&`, `\\|\\|`, `!`	`and`, `or`, `not`	`&&`, `\\|\\|`, `!`
True / false	`1` / `0` (or `true` / `false` with `<stdbool.h>`)	`True` / `False`	`TRUE` / `FALSE`
Equality test	`==`	`==`	`==`

Gotchas in each language

R: else on a new line fails at top level

This works inside a function or a { } block, but at the console or top level of a script it is a syntax error, because R finishes the if statement at the end of the line:

if (score >= 90) {
  print("Grade: A")
}
else {              # Error: unexpected 'else'
  print("Grade: B")
}

Keep } else { on one line and you never hit this.

C: = instead of ==. if (score = 90) assigns 90 and is always true. Compile with -Wall and the compiler warns you. Python and R reject an assignment in the condition in most forms.
C: no braces means one statement. Braces are optional for a single statement, but adding a second line without them is a classic bug. Always use braces.
Python: inconsistent indentation. Mixing tabs and spaces raises an IndentationError. Use 4 spaces everywhere.
R: the condition must be a single TRUE or FALSE. In R 4.2 and later, if (c(TRUE, FALSE)) is an error. For whole vectors, use ifelse() (below).

What counts as “true”?

C: any non-zero value is true; 0 is false. if (5) runs.
Python: 0, 0.0, "", [], {} and None are false; almost everything else is true.
R: only logical values (or numbers, where 0 is false and non-zero is true) are accepted, and NA in a condition is an error: if (NA) stops with “missing value where TRUE/FALSE needed”.

One-line versions (conditional expressions)

Each language has a compact form that picks a value:

// C: ternary operator
const char *result = (score >= 50) ? "Pass" : "Fail";

# Python: conditional expression
result = "Pass" if score >= 50 else "Fail"

# R: if-else is itself an expression
result <- if (score >= 50) "Pass" else "Fail"

R bonus: vectorized `ifelse()`

Base if works on one value. To apply a condition to every element of a vector, use ifelse():

scores <- c(95, 85, 72, 40)
ifelse(scores >= 50, "Pass", "Fail")
#> [1] "Pass" "Pass" "Pass" "Fail"

The Python equivalent for arrays is numpy.where(scores >= 50, "Pass", "Fail").

Which should you use?

There is no “better” version, only different conventions. If you know one, the others are quick to learn: the concept transfers, and only the punctuation changes. Pay most attention to Python’s indentation and R’s } else rule, since those are the two places where a small formatting slip changes the meaning or breaks the program.